Crop & Food Research Confidential Report No. 143
Nutrition and health qualities of potatoes - a
future focus
C E Lister & J Munro
March 2000
A report prepared for
New Zealand Federation of Vegetable and Potato
Growers
Copy 13 of 13
New Zealand Institute for Crop & Food Research Limited
Private Bag 4704, Christchurch, New Zealand
Circulation of this report is restricted. Consult the author(s)
and the institute’s scientific editor about obtaining further
copies. This report may not be copied in part or full.
© 2000 New Zealand Institute for Crop & Food Research Limited
Contents
1
Executive summary
1
2
Potatoes and their consumption
2
2.1
3
1997 National Nutrition Survey
The composition of potatoes
3
4
3.1
Dry matter and water content
5
3.2
Carbohydrates
5
3.2.1
3.2.2
3.2.3
5
6
7
3.3
Starch
Sugars
Dietary fibre (non-starch polysaccharides)
Nitrogen compounds
3.3.1
3.3.2
3.3.3
3.3.4
Protein
Free amino acids
Inorganic nitrogen
Glycoalkaloids
8
8
10
10
11
3.4
Lipids
11
3.5
Vitamins
11
3.6
Minerals
13
3.7
Phenolic compounds
14
3.7.1
3.7.2
3.7.3
3.7.4
3.7.5
3.7.6
14
15
15
16
16
16
3.8
3.9
Role of phenolics in browning reactions
Antioxidant activity
Antimutagenic and anticarcinogenic effects
Glucose-lowering properties
Cholesterol-lowering effect
Effect on protein nutritional quality
Plant pigments
17
3.8.1
3.8.2
3.8.3
17
17
18
Carotenoids
Anthocyanins
Others
Nutrient losses/changes during cooking
18
4
The health benefits of potatoes
19
5
The future
21
5.1
5.2
Feeding the world in the twenty-first century
21
5.1.1
5.1.2
5.1.3
5.1.4
Protein quality improvement
Provitamin A
Disease and pest resistance
Minerals
22
22
23
23
Delivery of additional health benefits
24
5.2.1
5.2.2
24
26
Increased levels of phenolics and other antioxidants
Recombinant proteins/vaccines
DCom4:Users:dcom4:Desktop:NZ HORT REPORTS:C & F potatoes143.doc
5.2.3
5.2.4
5.3
Modified carbohydrate
Other
27
27
Removal of antinutritional compounds and undesirable chemicals
27
5.3.1
5.3.2
27
28
Glycoalkaloids
Chemical residues
5.4
Browning prevention
28
5.5
Other new and improved potato products
29
5.5.1
5.5.2
29
29
5.6
“Healthier” fried products
Novelty coloured potato products
Utilizing potato waste
30
6
Recommendations/future research
30
7
Acknowledgements
31
8
References
32
Appendix I - the chemical composition of New Zealandgrown potatoes, raw and cooked by a variety of
method
DCom4:Users:dcom4:Desktop:NZ HORT REPORTS:C & F potatoes143.doc
39
1
Executive summary
This report describes the results of a literature review undertaken for the
potato sector of Vegfed to describe the nutritional status and health benefits
of potatoes. It highlights current knowledge and identifies some prospects for
the future. It also explores the next generation of key attributes that may be
used for product development and in marketing strategies. The key points
are noted below.
♣
Potatoes are a vegetable whose nutritional value is underestimated.
They are often believed to be a high energy food that provides little in
the way of nutrients. However, potatoes are a considerably richer
source of nutrients than of energy. In brief, potatoes are:
Potatoes are a
vegetable whose
-
a source of several vitamins, especially vitamin C and some
important B group vitamins,
-
rich in minerals such as potassium and iron,
-
a source of phenolics, compounds that may have an important role
in health,
-
virtually free of fat, although they are easily turned into fatty foods,
-
almost free of soluble sugar,
that provides little in
-
of low energy density - they ‘fill you up’ without providing many
calories,
the way of nutrients.
-
a source of high quality protein, although they are deficient in the
essential amino acid methionine,
-
readily digested but they also have a high water content so weight
for weight there is a relatively low impact on blood sugar.
♣
On the other side of the ledger, potatoes may also contain
glycoalkaloids (often found in green tubers) although the risk that they
contain toxic levels is minimal.
♣
Despite potatoes already having a number of desirable nutritional
qualities there is still considerable scope to improve the composition of
potatoes in a number of areas. These improvements may be achieved
through traditional plant breeding, genetic modification, and/or changes
in agronomic and postharvest handling procedures. They include:
-
boosting nutritional quality by increasing the levels of essential
amino acids, β-carotene (a precursor of vitamin A) or other
antioxidants,
-
removing or limiting the levels of glycoalkaloids,
Nutrition and health qualities of potatoes – a future focus
C Lister, March 2000
Crop & Food Research Confidential Report No. 143
New Zealand Institute for Crop & Food Research Ltd
Page 1
nutritional value is
underestimated. They
are often believed to
be a high energy food
This is simply not
true.
2
-
developing new cultivars that absorb less fat during frying,
-
using potatoes to produce genetically engineered vaccines and
other high value pharmaceutical products,
-
developing novelty coloured potato products that have added
health benefits.
♣
Initially new potato cultivars could be developed followed by new
potato products tailored to meet specific nutritional requirements of
certain sectors of the population.
♣
As new cultivars are developed with additional health benefits there will
be a need to re-evaluate the potential of potato wastes to better utilize
key components with the aim of improving human or animal health.
Potatoes and their consumption
The potato is an extremely important food crop. It is grown in more countries
than any other crop except maize. Its volume of production ranks fourth in the
world after rice, wheat and maize. Potatoes are number one in terms of
household expenditure on vegetables in New Zealand. In 1998 the total
household expenditure on potatoes was $65.07 million, up from $45.8 million
in 1984 (Commercial Grower, March 1999). Daily consumption of potatoes
per capita varies considerably and depends on age, sex, eating habits and
daily activities of consumers. In most countries potato consumption is not
influenced by family income. US per capita consumption of potatoes is about
61 kg/year, for the UK it is 105 kg/year, for the USSR 110 kg/year and it may
even reach close to 150 kg/year in Poland and Germany (Lisiska &
Leszcyski 1989). Figures from the recent National Nutrition Survey (Russell
et al. 1999) show that New Zealand consumers of potatoes eat an average of
180 g of potatoes (in all forms) per day. For females the average is 140 g per
day while for males it is 219 g per day. Thus, potatoes are a very significant
part of the diet in many countries and can make a significant contribution to
human nutrition.
The global production of potatoes has been estimated at 350 million tones
per annum (Friedman & McDonald 1997). New Zealand’s share is
approximately 490 000 tonnes per annum, some of which is used for
domestic seed (Table 1). The countries with the largest production volumes
of potatoes are the USSR, China, Poland, US and Germany (Lisiska &
Leszcyski 1989). In addition to human consumption, a large proportion of
potatoes are used as stock feed.
The nutritional value of potatoes is also important. The higher the nutritional
quality the better the stock feed, lessening the requirement for potatoes to be
mixed with other higher value feeds. Considerable amounts of potatoes are
used for processing. Volumes have been increasing rapidly in the last couple
Page 2
of decades. Nutritional considerations are important when processing
potatoes. Some of the issues are different from those for the fresh market.
Table 1: Potato production in New Zealand
(combined domestic and export use),
volume in tonnes (from Heyes et al. 1997)
Fresh
Seed
Process
Stockfeed
2.1
1995/6
259 000
30 000
200 000
1983/4
194 305
29 262
60 938
10 086
1997 National Nutrition Survey
In 1997 a New Zealand National Nutrition Survey (NNS97) was funded by the
Ministry of Health and conducted by the LINZ® Activity & Health Research
Unit of the University of Otago. The aim of this survey was to collect
information on food and nutrient intakes, dietary habits and nutrition-related
clinical measures of New Zealanders. NNS97 was based on a nationally
representative sample of 4636 New Zealanders living in selected households
and aged 15 years or more. Some of the data from the survey are
summarised below (Russell et al. 1999). Note that in most cases potatoes
also included kumara.
ϒ
Potatoes are the most commonly consumed vegetable in the New
Zealand diet; 95% of males and 93% of females ate potatoes (boiled,
mashed, baked or roasted) at least once a week (for kumara the pattern
was 27% and 32% for males and females respectively) while 54% of
males and 42% of females ate hot potato or kumara chips, French fries
or wedges at least once a week.
ϒ
Older people are more frequent consumers of potato. Provincial people
ate potatoes more often than people living in metropolitan areas. Rice
and pasta consumption showed the reverse trends.
ϒ
7% of energy in the diet was contributed by potatoes and kumara.
ϒ
6% of total fat was contributed from potatoes and kumara, for young
males (19-24 years) the contribution was 10%.
ϒ
Potatoes and kumara were a principal carbohydrate source, contributing
10% of total carbohydrates, this was higher for males (11%) than
females (9%).
ϒ
Vegetables (including potato and kumara) were the greatest source of
dietary fibre in the diet at 28%. For potatoes and kumara alone the figure
was 11%. Again the figures were higher for males (13%) than females
(10%).
ϒ
Potatoes and kumara provided 7% of daily iron intake, 8% of folate, 4%
of protein and 2% of calcium.
Page 3
In terms of the general health of the New Zealand population, 17% of New
Zealanders are regarded as obese (that is a body mass index of greater than
32 for New Zealand M∼ori and Pacific people and greater than 30 for New
Zealand European and others) and an additional 35% were considered
overweight. The obesity rate has risen from 11% in 1989 for both males and
females. About 22% of males and 18% of females had high blood pressure.
However, the mean serum cholesterol in NNS97 (5.7 mmol/L) has decreased
slightly from the 1989 level (5.9 mmol/L), although 23% of the population still
has serum cholesterol greater than 6.5 mmol/L. These figures indicate that
there is considerable room for improvement in the health of New Zealanders.
Diet is one major way to improve health.
3
The composition of potatoes
Numerous books and papers cover the composition of potatoes (for example
Burton 1989; Lisiska & Leszcyski 1989). Table 2 shows the range of
chemical composition of potato tubers using data from around the world.
Data on the composition of New Zealand-grown potatoes are available in the
New Zealand Food Composition Database, and some of these data are
presented in Tables 8-12 in Appendix I. For comparison, the composition of
other foods that may replace potatoes in a meal (e.g. rice, pasta and bread)
are also provided. There is a considerable range in the concentration of each
Table 2: The typical chemical composition of potato tubers (taken
from Lisiska & Leszcyski 1989).
Content (%)
Substance
Dry matter
Starch
Reducing sugars
Total sugar
Crude fibre
Pectic substances
Total nitrogen
Crude protein (total nitrogen x 6.25)
Protein nitrogen in total nitrogen
Amide nitrogen
Amino acid nitrogen
Nitrates
Lipids
Ash
Organic acids
Ascorbic acid and dehydroascorbic acid1
Glycoalkaloids1
Phenolic compounds1
1
In mg/100 g.
Page 4
Range
Mean
13.1–36.8
8.0-29.4
0.0-5.0
0.05-8.0
0.17-3.48
0.2-1.5
0.11-0.74
0.69-4.63
27.3-73.4
0.029-0.052
0.065-0.098
0.0-0.05
0.02-0.2
0.44-1.87
0.4-1.0
1-54
0.2-41
5-30
23.7
17.5
0.3
0.5
0.71
0.32
2.00
54.7
0.12
1.1
0.6
10-25
3-10
-
of the components. The chemical composition of potato tubers is mainly
controlled by genetic factors. In addition, the composition is affected by the
age and maturity of the tubers as well as environmental conditions, i.e.
climate, soil and cultural practices. The composition of potato tubers also
changes during storage and is affected by the way in which they are
processed. The significant potato components, in terms of health benefits,
are discussed in more detail below.
3.1
Dry matter and water content
Dry matter content of potato tubers ranges from about 13 to 37% with an
average of 24% (Table 2). The other 75% of the potato consists of water. Dry
matter content increases during the growing season and is highest in the
vascular system, intermediate in the cortex and lowest in the pith. The dry
mater or ‘solids content’ of tubers is one of the prime characters used by
potato processors to evaluate a crop. Potatoes with high dry matter are most
suitable for the manufacture of dehydrated food products and stockfeed and
are especially good for the production of fried foods. The dry matter of potato
tubers is composed of a number of substances that are either soluble or
insoluble in water. Dry matter is particularly significant when frying as the
greater the dry matter the less fat uptake because there is less water for it to
replace. Table 3 shows the average contents of potato tuber dry matter.
There may be considerable changes in dry matter after cooking potato,
particularly frying (see Table 8 in Appendix I).
Table 3: Components or dry matter of
potato tuber (taken from Lisiska &
Leszcyski 1989).
Component
Starch
Total sugars
Crude fibre
Crude protein
Crude lipids
Ash
3.2
Mean content (%)
75.30
2.10
2.32
7.94
0.50
4.41
Carbohydrates
Carbohydrates make up the greatest proportion of the dry weight of potato
tubers (Table 3). Carbohydrates are present in the form of starch, sugars and
non-starch polysaccharides (cell wall components, often termed dietary fibre).
Available carbohydrate values for New Zealand-grown potatoes are given in
Table 8 in Appendix I.
3.2.1
Starch
Starch is a major component of the potato tuber at approximately 17 g/100 g
FW or 75% of its dry weight (Tables 2 and 3). During the growing season
starch accumulates in the cells of tubers, forming single or complex granules.
Starch, being a carbohydrate, is an important energy source and can affect
quality. Potato starch is an important part of food products and is a raw
Page 5
material for industry. Starch occurs in a number of forms that differ in
molecular structure and their susceptibility to digestion. Starch may be
classified into digestible starch (DS) and resistant starch (RS). From a health
standpoint, susceptibility to digestion is important for several reasons.
1.
Starch is a chain of glucose molecules, and during digestion starch
chains are typically broken down very quickly in the small intestine into
maltose and then glucose. Starch digestion can, therefore, have a
large impact on blood glucose levels, which need to be closely
controlled in people with diabetes.
2.
Starch that cannot be digested before passing from the small intestine
is fermented by bacteria in the colon, with the production of short chain
fatty acids. The short chain fatty acids from starch fermentation
provide the body with about half as much energy as would have been
obtained from the starch if it had been digested in the small intestine.
3.
There is increasing scientific evidence that resistant starch can act as
a prebiotic, increasing the population of beneficial bacteria, such as
bifidobacteria, and having a beneficial effect on the ecology of the
large bowel (Bird 1999).
Before consumption, starch must be gelatinised. Consumption of
ungelatinised potato starch reduces protein digestibility and induces toxicity
(Lisiska & Leszcyski 1989). Resistant starch (RS) in raw potatoes is high.
However, different RS values were obtained when tubers were processed,
ranging from 1.18% in boiled potatoes to 10.38% in retrograded flour (GarciaAlonso & Goni 2000). The starch in cooked potatoes is generally easily
digested, but more so in mature than in new potatoes. In a recent study
comparing three potato varieties, four cooking methods, and two states of
maturity, the impact of potatoes on blood glucose levels, reflected in the
glycaemic index, was exceptionally high, regardless of variety, cooking
method and maturity (Soh & Brand-Miller 1999). New potatoes had lower GI
values than mature potatoes, probably because, with maturation, the amylose
component of starch becomes increasingly branched, and thus increasingly
susceptible to digestion. In cold cooked potato and mashed potato, starch
digestibility is reduced due to its retrogradation. Repeated cooling and
mashing increases the amount of starch that is indigestible by humans
(Englyst & Cummings 1987).
3.2.2
Sugars
The sugar content of potatoes is very low at an average of 0.5% of the wet
weight or just over 2% of dry weight (Tables 2 and 3). However, sugar
content is highly variable, depending on the type, maturity and physiological
state of the potato. In free form the following sugars are found in potato
tubers: the monosaccharides glucose and fructose, which are reducing
sugars, and sucrose, a non-reducing disaccharide. A high sugar content
(especially reducing sugars) renders potato tubers unsuitable for use as raw
material for processing, especially for dehydrated and fried products.
Page 6
3.2.3
Dietary fibre (non-starch polysaccharides)
Dietary fibre consists mainly of the polysaccharides of plant cell walls. The
dietary fibre may “dilute” highly caloric components in food, stimulate
peristaltic movement and retard digestion of some other food components.
The essential non-starch polysaccharides in potatoes are cellulose,
hemicellulose, pentosans and pectic substances. Starch is also a
polysaccharide, but the sugars and the way they are bonded to one another
in cell walls and in starch are different.
The human gut produces enzymes to break down the structure of starch, but
they cannot attack cell wall polysaccharides, which therefore survive the
small intestine as non-digested “dietary fibre”. Potatoes contain a relatively
small proportion of dietary fibre because they are a storage organ in which
starch becomes the dominant constituent in the mature tuber. Most potatoes
contain about 1-3% dietary fibre compared with about 17% starch, on an
edible weight basis (Table 2 and New Zealand data in Table 8 in Appendix I).
Potatoes consist of a mass of thin-walled cells full of starch. As well as being
thin, the cell walls of potatoes are not lignified. Lignin is a resistant material
that encrusts cell walls such as those found in wheat bran, making them
resistant to fermentation in the large bowel and an excellent faecal bulking
agent. Being thin, and not lignified, the cell walls of potatoes are almost
totally fermented by bacteria in the colon. The dietary fibre in potatoes thus
has almost no effect on faecal bulk and does little to prevent constipation.
However, during fermentation of dietary fibre in the colon short chain fatty
acids are produced. One of the short chain fatty acids, butyric acid, is thought
to provide some protection against colorectal cancer by inducing apoptosis,
which is the destruction of abnormal cells. Butyric acid would be most useful
if it were produced where most of the colorectal cancer occurs, that is, in the
distal colon. However, potato cell walls are so thin and fermentable that most
of the butyric acid produced is likely to be in the proximal rather than the
distal colon. The cell walls could be induced to ferment in the distal colon only
if there was some other bulky non-digested material in the diet that increased
movement of gut contents sufficiently for the potato fibre to reach the distal
colon before being fermented.
One portion of the cell wall fraction of potatoes that might have a more
positive impact on health is the potato peel fraction, not because of the cell
walls per se but because they are impregnated with materials that provide the
potato with a resistant coating. The resistant, water proofing material of the
potato “coat” consists largely of suberin, which, because it is non-polar, has
the capacity to bind some of the most potent food carcinogens, the
heterocyclic amines (Harris 1999). Heterocyclic amines are produced during
cooking and charring of proteins, as occurs abundantly during barbecuing. So
eating a whole baked potato with barbequed food may help “mop-up” some of
those carcinogens. Other components of potato peel, such as phenolics, may
also have beneficial effects on human health (see Section 3.7).
Page 7
3.3
Nitrogen compounds
Nitrogen compounds are the second major component in potato tubers, after
carbohydrates. Their amount in conversion to total protein (N x 6.25) ranges
from 2.77 to 14.6% of dry weight (Table 3 and New Zealand data in Table 8
in Appendix I). The content of total nitrogen increases upon maturation.
Approximately 90% of the nitrogen present in potato tubers is in the form of
compounds that are soluble in water. Insoluble nitrogen compounds are
found mostly in the skin. About 50% of the total nitrogen from potatoes is
derived from proteins; the remaining nitrogen consists of free amino acids,
amide nitrogen associated with asparagines and glutamine, nitrogen of other
organic compounds, inorganic nitrogen and alkaloid nitrogen (Lisiska &
Leszcyski 1989).
3.3.1
Protein
The essential nitrogen fraction in a potato tuber is protein nitrogen. Potatoes
are commonly perceived as a carbohydrate source only, but they do contain
high quality protein. On a fresh weight basis they contain only about 2%
protein; the value increases to about 8% when examined on a dry weight
basis (Tables 2 and 3 and New Zealand data in Table 8 in Appendix I). That
makes potatoes comparable to cereals, such as rice or wheat (McCay et al.
1987). In countries where potato consumption is high this vegetable can
make a significant contribution to health as a protein source (Woolfe 1986).
Due to the dietary quality of tuber nitrogen, 100 g of boiled potatoes supplies
8-13% and 6-7% of the FAO-WHO recommended daily allowance of nitrogen
for children and adults, respectively (Horton & Sawyer 1985). The
contribution of protein nitrogen to total nitrogen depends on potato cultivar,
environmental conditions and cultural practices, especially nitrogen
fertilization application rates.
Proteins are important constituents of cellular membranes as well as various
cytoplasmic structures. Also the enzymes present in potatoes are made up of
proteins. The fraction of proteins that is not easily soluble is built up in the cell
walls. The major fraction of proteins present in potato tubers is constituted of
simple proteins. Potato proteins comprise 18-20 amino acids present in
varying quantities (Table 4 and Table 11 in Appendix I). Humans cannot
synthesize some amino acids and so they must to be provided in the foods
we eat. These amino acids are called essential amino acids and include
methionine, threonine, tryptophan, valine, lysine, isoleucine, leucine and
phenylalanine. Deficiency in any of these amino acids limits the quality of the
protein. There are a number of ways to measure the quality of protein
contained in different foods. These include the chemical score (CS), which is
the ratio of limiting amino acids in the tested protein to amino acid content in
standard protein, expressed as a percentage. Protein of the whole hen egg is
usually referred to as standard protein. Other indices of protein nutritional
value are used on the basis of protein conversion rate such as: (a) protein
efficiency ration (PER), which expresses body weight gain ratio to protein
intake; (b) net protein utilization (NPU) or (c) biological value (BV) expressed
in % of nitrogenous substances absorbed by the digestive tract.
Page 8
A summary of the nutritive value of potato protein (Markakis 1975) shows:
1.
on the basis of amino acid composition, the calculated protein quality
is about 70% that of whole egg protein. This is higher than that of most
other food plants,
2.
potatoes provide a good source of lysine but low contents of sulphurcontaining amino acids (methionine, cysteine) limit their nutritive value.
Tuber methionine is the limiting essential amino acid for human
nutrition (Kaldy & Markakis 1972; Rexen 1976),
3.
human feeding trials suggest that potato proteins are of a very high
quality, possibly higher than indicated by their amino acid composition.
This may be because protein utilization is enhanced by the high
content of free amino acids and other metabolites mentioned above.
Since potatoes need to be baked, boiled, fried, or otherwise cooked before
consumption, it is of interest to determine to what extent exposure to heat
affects nutritive value of the proteins. Losses during chipping, canning and
drum-drying are considerable (Jaswal 1973), but appear to be minor during
boiling and frying (Friedman 1992a and b). Heat treatment of potatoes
destined for direct consumption (cooking, baking) does not cause significant
changes in total nitrogen content or protein content, except from losses on
peeling (Toma et al. 1978; Weaver et al. 1983). When potatoes are cooked
some loss of free amino acids and combined amino acids occurs. The losses
are higher when low specific gravity potato tubers are cooked and they affect
non-essential amino acids more than essential amino acids (Jaswal 1973).
Protein concentrates prepared from potatoes have excellent nutritional quality
when measured in terms of protein efficiency ratio (PER = 2.90), biological
value (BV = 79.5), net protein utilization (NPU = 74.2) and nitrogen retention
(Nestares et al. 1993). If potato protein is supplemented with protein that is
rich in methionine the nutritive value of such a mixture is higher than that of
either protein separately. The conversion rate of such a mixture is
considerably higher and thus the amount of protein necessary to satisfy daily
requirements is lower. The amount of potato protein necessary to satisfy daily
requirements is a little higher than that of whole egg but almost the same as
the protein present in whole milk and about 40% lower than wheat protein.
Potato protein in combination with egg protein (rich in sulphur-containing
amino acids, such as methionine) is of high biological value. The combination
of these two proteins alone in the proportion 65:35 potato to egg will furnish
the daily requirements if supplied to the organism in an amount 30% lower
than provided by egg alone (Lisiska & Leszcyski 1989). Thus, potato
protein merits inclusion in various food formulations as a source of high
quality protein.
Table 4: Amino acid composition of potato tubers (adapted from Lisiska
& Leszcyski 1989).
Amino acid
Alanine
Composition of amino acids
in tuber protein (%)
4.62-5.32
Page 9
Free amino acids
(mg/100 g DW)
6-251
Arginine
Aspartic acid
Cysteine
Glutamic acid
Glycine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Proline
Serine
Threonine
Tryptophan
Tyrosine
Valine
3.3.2
4.74-5.7
11.9-13.91
0.2-1.25
10.23-11.8
4.3-6.05
2.06-2.5
3.73-5.8
9.7-10.28
6.7-10.05
1.2-2.15
4.8-6.53
4.7-7.38
4.9-5.92
4.6-6.5
0.3-1.85
4.5-5.68
4.88-7.4
42-736
123-892
0-9
76-766
1-52
15-328
14-256
8-157
9-319
9-128
0-266
0-484
15-197
14-270
1-174
9-319
15-405
Free amino acids
More than two-thirds of the non-protein nitrogen present in potatoes is
present as free amino acids. A number of different amino acids have been
detected; aspartic acid, glutamic acid and valine are generally present in the
highest quantities (Lisiska & Leszcyski 1989). Table 4 shows the free
amino acid composition of potato tubers. Tyrosine is one of the free amino
acids that has attracted a lot of attention. Tyrosine, oxidized by polyphenol
oxidase, changes colour and causes darkening of potato flesh, a process
termed enzymic browning (see Section 3.7.1). A close relationship has been
reported between tyrosine content and discoloration (Stark et al. 1985),
although other researchers have not observed this relationship.
3.3.3
Inorganic nitrogen
The inorganic nitrogen found in potato tubers occurs mainly as ammonia
compounds, nitrates and sometimes nitrites, although the latter are present
only in trace amounts. In recent years some attention has been focused on
the nitrate content of potato tubers. As in all food products this is associated
with the conversion of nitrates to nitrites in all living organisms. Nitrites
subsequently produce carcinogenic nitrosamines. The nitrate content of
potato tubers is generally within the range of 4.0 to 25 mg NO3 per 100 g
fresh weight. Unfavourable conditions can cause the levels to go even higher.
The levels are higher in potato skin than flesh (Lisiska & Leszcyski 1989).
The levels are considered to be low and are similar to those in bananas,
tomatoes, cucumbers or peas although higher than in some vegetables such
as spinach, lettuce and cabbage. Large losses of nitrates occur during
washing, peeling and cooking of potatoes.
Page 10
3.3.4
Glycoalkaloids
Members of the Solanaceae family, and the Solanum genus in particular,
synthesize a variety of alkaloidal compounds. The most frequently
encountered are the commonly named glycoalkaloids, which are nitrogencontaining steroidal glycosides. Potato glycoalkaloids have been the subject
of an extensive recent review (Friedman & McDonald 1997). In commercial
potato cultivars the primary compounds are α-solanine and α-chaconine,
glycosides of the steroidal alkaloid solanidine. These and similar compounds
have toxic effects in humans (Friedman & McDonald 1997). This toxic effect
occurs only when glycoalkaloid intake is very high. At sublethal doses
(3-5 mg/kg body weight) for adults the amount of glycoalkaloid intake would
have to be 200-350 mg. The chances that such a quantity of glycoalkaloid
could be consumed in a potato meal are extremely remote. The
glycoalkaloids produce a tart astringent taste and larger quantities even
impart a sharp bitter taste, so it is a warning against consuming them. The
common current guideline for potatoes establishes an upper level of
20 mg/100 g FW of glycoalkaloids. However, the glycoalkaloid content of the
majority of potato cultivars is between 3 and 10 mg per 100 g of tubers.
The levels of glycoalkaloids present in potatoes may be affected by a variety
of factors, including the particular variety, cultural practices and climatic
conditions at the growing area (Morris & Petermann 1985). Tubers do not
stop producing glycalkaloids after harvest and improper handling and/or
storage conditions may cause a dramatic rise in these compounds (Salunkhe
et al. 1972). Levels can be especially high in green or damaged potatoes.
Processing may also affect glycoalkaloid content. While glycoalkaloids seem
to be largely unchanged by cooking, other types of preparation such as
slicing and peeling may alter the levels (Friedman & McDonald 1997). This is
because the concentrations of glycoalkaloids are highest in the peels and,
since they are soluble in water, they are often leached into the cooking liquid.
3.4
Lipids
Lipids, also called crude fat, are present at very low levels in raw potatoes
(Tables 2 and 3) and they contain no cholesterol. Thus, the energy value
provided by potato fats and their effect on obesity are not important.
However, frying potatoes, roasting or adding milk and butter during mashing
can change this considerably (see details for potatoes cooked by different
methods in Table 8 in Appendix I).
3.5
Vitamins
Potatoes contain some vitamins in significant amounts and are a much better
source than other foods which may replace them in a meal (see Table 10 in
Appendix I). Of all the vitamins, potatoes contain vitamin C in the highest
quantity (Table 5). Vitamin C is a water-soluble vitamin and its main
component is ascorbic acid. However, dehydroascorbic acid has properties
similar to vitamin C if it is reduced to ascorbic acid in the organism. The total
amount of the two acids in potato tubers ranges from 1 to 54 mg per 100 g
Page 11
fresh weight, although most frequently it is between 10 and 25 mg/100 g. The
contribution of dehydroascorbic acid to total vitamin C content averages
12-15% (Lisiska & Leszcyski 1989). The highest concentration of vitamin
C is found in the vicinity of the vascular system and is lowest in the pith and
skin. There has been some breeding of potatoes to increase the levels of
vitamin C.
There may be considerable losses of vitamin C during processing, and the
losses are different for peeled and unpeeled potatoes. Typically, vitamin C
losses from potatoes boiled in their skins are 18-24% while peeled boiled
potatoes lose 35-50% (Lisiska & Leszcyski 1989). Some of the loss is due
to leaching into the cooking water and the rest is decomposed during
cooking. Vitamin C losses due to frying reach 75% while baking losses are
generally lower at 25-40%. Data from the New Zealand Food Composition
tables indicate that microwaving may result in large losses of vitamin C (see
Table 10 in Appendix I). However, the amount of vitamin C remaining in
potatoes after cooking is still high and it makes a considerable contribution to
the recommended daily intake of vitamin C (see Section 4). A typical portion
of 150 g of boiled or baked potatoes will provide about 30% of the daily
requirement of vitamin C.
Potato tubers also contain vitamin B1 (thiamine), but other water-soluble
vitamins are present in fairly low amounts (see Table 5 and New Zealand
data in Table 10 in Appendix I), although some do make a contribution to the
recommended daily intakes of vitamins (see Section 4). There is some loss of
these vitamins during processing.
Fat-soluble vitamins are present in trace amounts. Provitamin A is
contributed by some of the carotenoids. Not all carotenoids have provitamin A
activity, the main one is ∃-carotene (for details on carotenoids see Section
3.8.1).
Table 5: Vitamin content in potato tubers (taken from Lisiska &
Leszcyski 1989).
Content (µg/100 g)
Vitamin
Substance
Water soluble
C
Ascorbic acid + dehydroascorbic
acid
B1
Thiamine
B2
Riboflavin
B6
Pyridoxine
PP
Niacin
Pantothenic acid
Folic acid
H
Biotin
Fat soluble
Provitamin A
K1
Phytonadione
Page 12
Range
Mean
1000-54000
10000-25000
24-180
7-200
9001
360-3300
190-320
5-33
0.6
100
70
1000
-
11-56
60-80 1
-
1
3.6
In dry matter.
Minerals
Mineral substances, called ashes, average 1.1% in potato tubers (Table 2
and New Zealand data in Table 9 in Appendix I). Potassium is found as the
major cation in potato tubers. Higher concentrations of potassium are present
in the skin and directly beneath it than in the interior of the potato tuber.
Potassium content increases during the entire growing season. Potassium
plays an important role in water and ionic supply in humans. In several
disease (e.g. renal insufficiency) a potato diet is recommended due to its high
potassium content along with other desirable nutritional characteristics. Other
minerals present in moderate quantities include phosphorus, chlorine, sulphur
and magnesium. Calcium is present in small quantities, mostly in the skin and
the vascular system. Iron is also present in fairly low amounts but may make
a contribution to dietary intake (see Section 4). Iron, in association with
chlorogenic acid, causes after-cooking darkening of potatoes.
During preparation of potatoes for consumption a portion of the minerals
present in the skins is lost with peeling. Heat treatment on the other hand
does not result in significant mineral losses (True et al. 1979). The potato
itself is low in sodium, which is desirable. However, the amount of sodium is
often increased ten-fold by the addition of salt during or after cooking (Table 9
in Appendix I).
Table 6: Composition of potato tuber ash
(adapted from Lisiska & Leszcyski 1989).
Element
Potassium (K)
Phosphorus (P)
Chlorine (Cl)
Sulphur (S)
Magnesium (Mg)
Sodium (Na)
Calcium (Ca)
Silicon (Si)
Iron (Fe)
Aluminium (Al)
Manganese (Mn)
Zinc (Zn)
Copper (Cu)
Content in tuber dry matter
(mg/100 g DW)
1400-2500
120-600
45-800
40-400
45-220
0-330
10-130
5-89
2.5-72
0.2-35
0.5-8
0.8-2.2
0.06-2.8
Page 13
3.7
Phenolic compounds
Potato tubers contain a number of phenolic compounds, but their percentage
is rather low (Table 2). Some of them appear in a free form while others are
bound. The phenolic compounds present in the tubers include polyphenols,
monohydric phenols, coumarins, anthocyanins, flavones, tannins and lignin.
Phenolic compounds are distributed mostly between the cortex and skin
(peel) tissues of the potato. There are about ten times as much phenolic
compounds in the peel as in the flesh of the potato (Lisiska & Leszcyski
1989). Some substances are only detected in the peel. Only tyrosine (a
monohydric phenol) is highest in the interior of the tuber and lowest in the
external layer. Tannins are mostly localized in the outer layers of the potato,
imparting a tan shade to the skin.
The chemistry, biochemistry and dietary role of potato polyphenols have
been reviewed by (Friedman 1997). About 50% of the polyphenolic
compounds are located in the peel and adjoining tissues while the rest
decreases in concentration from the outside towards the centre of the potato
tuber (Hasegawa et al. 1966). Chlorogenic acid constitutes up to 90% of the
total polyphenolic content of potatoes (Friedman 1997). Other phenolic acids
include protocatechuic, sinapic, coumaric and vanillic acids. Other
polyphenolic compounds present in potatoes include anthocyanins (see
Section 3.8.2), flavanones (naringenin and eriodictyol), flavan-3-ols (catechin
and epicatechin) and flavonols (kaempferol and sometimes quercetin
glycosides) (Lewis 1996). Many of these compounds are present in fairly low
concentrations. Tannins are mostly localized in the outer layers of the potato
and impart a tan shade to the skin.
Polyphenols may have little dietary significance if they are destroyed during
processing (baking, boiling, frying, etc.). Destruction of chlorogenic acid is
least during microwaving but the fate of phenolics during processing has not
been fully studied. For components such as phenolics that are present in
much higher concentrations in the peel the amount of peel consumed is
important. The peel of baked or fried potatoes is the principal source of peel
in the human diet (Bushway et al. 1983; Friedman & Dao 1992).
3.7.1
Role of phenolics in browning reactions
Browning reactions are very common in potatoes and potato products, and in
some cases are undesirable from a visual point of view (although in other
cases they are desirable). Enzyme-catalyzed browning reactions involve the
oxidation of phenolic compounds by the enzyme tyrosinase (polyphenol
oxidase, PPO) to quinines, followed by transformation of the quinines to dark
pigments. Chlorogenic acid seems to be responsible for the bluish-grey
discoloration of boiled or steamed potatoes following exposure to air. This so
called “after-cooking blackening or darkening,” which can occur within a few
minutes after steam peeling, is perceived by many consumers as
undesirable. The enzymatic and nonenzymatic browning reactions of amino
acids and proteins with carbohydrates, oxidised lipids, and oxidised phenolics
will cause food to deteriorate during storage and processing. In addition, a
Page 14
loss in nutritional quality and potentially in safety is attributed to the
destruction of essential amino acids, a decrease in digestibility, the inhibition
of proteolytic and glycolytic enzymes, interaction with metal ions and the
formation of antinutritional and toxic compounds. However, some browning
reaction products may have beneficial effects, including antimutagenic activity
since browning reactions lead to the formation of naturally occurring
antioxidants, antibiotic activity and antiallergenic properties (Friedman
1996b).
The sometimes green colour of potato cooking water appears to be due to a
pigment derived from the reaction of chlorogenic acid and glutamine (Adams
1994). Whether the heat-induced discoloration adversely affects the
nutritional value of boiled and darkened potatoes is not known.
3.7.2
Antioxidant activity
Polyphenolic compounds in potatoes show antioxidative activity in several
systems. In a US study total phenol content of potato (peeled) was 28 mg per
100 g FW and it was ranked twentieth out of 23 commonly consumed
vegetables. However, it was ranked ninth in terms of antioxidant activity
(Vinson et al. 1998). This study gave potatoes as the fourth greatest
contributor to phenolics consumed per day from vegetables. Chlorogenic acid
from potato has been found to be an effective inhibitor of lipid oxidation (AlSaikhan et al. 1995). Extracts from potato peels possess strong antioxidant
activity, attributed mostly to their chlorogenic, protocatechuic and caffeic acid
contents (Onyeneho & Hettiarachchy 1993). Extracts prepared from red peels
have stronger activity than those from brown peels, probably due to the
strong antioxidant activity of the anthocyanins.
New Zealand studies (Lister & Podivinsky 1998; Lister 1999; Lister et al.
2000) have measured the antioxidant activity of New Zealand-grown
potatoes. Red Desiree ranked ninth out of 15 vegetables and Rua was
eleventh. Antioxidant activity was correlated with the phenolic content of the
vegetables, and the two potato cultivars ranked eighth and tenth respectively
when vegetables were compared on an equal weight basis. If the weight of
potato consumed per day is taken into consideration then potatoes move up
the rankings and are better than many vegetables. However, compared to
other rich sources of antioxidants (e.g. red wine, berry fruit) potatoes are a
small contributor to the potential daily intake of antioxidants. Peeling the
potato considerably reduced both the phenolic content and the antioxidant
activity. Other potato cultivars have higher phenolic contents (Lewis 1996)
and thus may have higher antioxidant activity.
3.7.3
Antimutagenic and anticarcinogenic effects
Generally, inhibition of mutagenicity and cancer development by polyphenolic
compounds could be due to their ability to scavenge and trap potentially
DNA-damaging electrophiles, free radicals and toxic metals, inhibiting
enzymes that activate precarcinogens to carcinogens and inducing
carcinogen-detoxifying enzymes. Reported anticarcinogenic or antimutagenic
effects of both free and potato-bound chlorogenic acid include:
Page 15
1.
nitrites in food can react with secondary amines to form mutagenic and
carcinogenic nitrosamines. Chlorogenic acid and other polyphenols
are reported to block nitrosamine formation by competitively reacting
with the nitrite (Kikugawa et al. 1983),
2.
chlorogenic acid and several other simple phenolic acids also
inactivate the mutagenicity of aflatoxin B1 (Stich & Rosin 1984),
3.
chlorogenic acid can bind the carcinogen benzo[α]pyrene (Camire et
al. 1995). Potato peel is better at binding this carcinogen than wheat
bran, cellulose or arabinogalactan.
3.7.4
Glucose-lowering properties
Thompson et al. (1983) reported that the polyphenol content of potatoes,
legumes, and cereals correlated negatively with the blood glucose response
(glycemic index) of normal and diabetic humans consuming them in a
controlled study. The glucose-lowering effect of polyphenols may arise from
their ability to inhibit amylases (which catalyse the hydrolysis of starch to
glucose), phosphorylases (which are involved in starch metabolism), and
proteolytic enzymes (that catalyze the hydrolysis of proteins to free amino
acids in the digestive tract) and/or from their ability to direct complexation
between the polyphenols and starch, preventing digestion. Another possibility
is the prevention of in vivo nonenzymic browning between plasma glucose
and amino groups of hemoglobin, which occurs under physiological
conditions and contributes to the cause of diabetes (Friedman 1997).
3.7.5
Cholesterol-lowering effect
Chlorogenic acid, and other polyphenols, also exhibit strong in vitro
antioxidant activity for heart disease-related lipoproteins (LDL) (Vinson et al.
1995). Since in vivo oxidation of LDL appears to be a major cause of heart
disease, it is possible that chlorogenic acid and other polyphenols may also
lessen heart disease. Lazarov & Werman (1996) found that consumption of
potato peel induced a lowering of cholesterol in rats. They ascribed this to its
fibre content, but it is likely that the polyphenol content and other antioxidants
as well as glycoalkaloids contributed to the observed hypocholesterolemia.
Both tomato and potato glycoalkaloids have a strong affinity for cholesterol
(Friedman et al. 1997).
3.7.6
Effect on protein nutritional quality
Polyphenolic compounds and derivatives (especially tannins) bind to proteins
in the gut, adversely affecting absorption of food (Friedman 1989; Oste 1989;
Duffey & Stout 1996). Rat feeding studies suggest that the reduction in
protein nutritional quality following consumption of polyphenols may be due to
the formation of protein-polyphenol complexes (Spencer et al. 1988) and to
inhibition of the digestive enzymes α-amylases and trypsin (Griffiths 1986).
Polyphenols also induced an increase in lipase activity in the digestive
system. Possible effects of polyphenol-rich diets on protein nutrition need
further study.
Page 16
3.8
Plant pigments
The appearance of food is important to consumers and a major factor
determining appearance is colour. Plant colour is determined by three main
classes of pigments: carotenoids, chlorophylls and flavonoids (anthocyanins).
Some of these compounds have come under increasing scrutiny because of
their possible role in human health. Within potato cultivars there is a
considerable range of colour, including white, yellow, brown, pink, red, purple,
purple-blue to almost black. These colours are due mainly to carotenoids and
anthocyanins.
3.8.1
Carotenoids
The flesh of potato varieties is often tinged with yellow and this is mainly due
to the presence of carotenoids, a class of plastid pigments. The main
carotenoid constituents of potato tubers are the xanthophylls violaxanthin,
lutein and lutein-5,6-epoxide, with small amounts of neoxanthin and
neoxanthin-A present. β-Carotene, a common carotenoid in many other
plants and also present in the aerial parts of the potato plant is absent or
present in only trace amounts in the tubers (Burton 1989). There is a direct
correlation between yellow flesh colour and total carotenoid content, which is
a heritable characteristic. Typical “white” flesh potatoes contain 0.01-0.05 mg
of carotenoids per 100 g FW while varieties with “yellow” flesh contain
0.11-0.34 mg of carotenoids per 100 g FW (Gross 1991). Data on the
carotenoid content (∃-carotene equivalents) of New Zealand potatoes are
give in Table 10 in Appendix I. Even the yellow-fleshed potato varieties are
low compared to many kumara/sweet potato (Ipomoea batatas Lam.)
cultivars where typical carotenoid levels vary between 0.10 and 7.5 mg per
100 g FW, and some very dark orange cultivars even have up to 20 mg of
carotenoids per 100 g FW. It is thought that the tendency for a high
carotenoid content is determined by a single dominant gene, although there
are modifying genes (Brown et al. 1993).
3.8.2
Anthocyanins
The other major group of pigments which occur in some potato cultivars is the
anthocyanins. These have been studied comprehensively by Lewis (1996).
The anthocyanins are vacuolar pigments which give rise to the red to
purple/back colour of tuber skin and sometimes flesh. The degree of tuber
flesh pigmentation can vary considerably from just a slight pigmentation of
the vascular ring to a complete pigmentation of the entire tuber.
Anthocyanins are important antioxidants. Red cultivars of some vegetables
often have stronger antioxidant activity than their non-coloured relatives.
Studies with Red Desiree (red-skinned) potatoes have shown them to have
higher antioxidant activity than Rua (Lister & Podivinsky 1998). However,
since the skin only accounts for a relatively low percentage of the total weight
there were not huge differences (approximately 20% higher), unlike lettuce
where a red cultivar had activity 13 times that of a green cultivar. It is possible
that a red-fleshed potato would have much higher antioxidant activity than
Page 17
those with only red skin. However, there have been no studies of these
aspects to date.
3.8.3
Others
Greening in potatoes is due to chlorophyll formation as a result of exposure to
light. It is usually accompanied by the production of the toxic glycoalkaloids
(see Section 3.3.4). Melanin formation as a result of injury may give rise to
grey or black discolorations, which are also undesirable.
3.9
Nutrient losses/changes during cooking
Some of the effects of cooking on specific nutrients have been discussed in
the previous sections. However, other issues arise. Recent consumer interest
in 'healthy eating' has raised awareness to limit the consumption of fat and
fatty foods. An often asked question is, “What are the relative nutritional
advantages and disadvantages of consuming fried foods?” Are all fried foods
bad for you? The nutrient losses and gains during frying have been reviewed
by Fillion & Henry (1998). Frying generally has little or no impact on the
protein or mineral content of fried food, whereas the dietary fibre content of
potatoes is altered after frying due to the formation of resistant starch. The
high temperature and short transit time of the frying process cause less loss
of heat labile vitamins than other types of cooking. Frying also avoids loss of
water-soluble vitamins through leaching into the cooking water. For example,
vitamin C concentrations of French fried potatoes can sometimes be as high
as in raw potatoes, and thiamine is well retained in fried potato products as
well as in fried pork meat. The nutritive value of the frying media is also
important to take into consideration as it can contain beneficial compounds
such as vitamin E. Although some unsaturated fatty acids and antioxidant
vitamins are lost due to oxidation, fried foods can be a good source of vitamin
E. Of course this depends on the type of oil they are cooked in and only some
of the New Zealand fries had higher vitamin E content (see Table 10 in
Appendix I). It is true that some fat is inevitably taken up by the food being
fried, contributing to an increased energy density. However, this also results
in highly palatable foods with a high nutritional content. It has been concluded
that fried foods certainly have a place in our diets in moderation (Fillion &
Henry 1998). Attempts to reduce the oil content of the potatoes while
retaining the other benefits of frying may be desirable, and it is preferable to
use an oil with a good vitamin E content.
Recent studies have indicated that cooking increases the bioavailability of
some nutrients, especially carotenoids. This may be due to the breakdown
and softening of cell walls, making the cell contents more readily available for
absorption. Since potatoes are always cooked, some components may be
more bioavailable from potatoes than from vegetables eaten raw.
Page 18
4
The health benefits of potatoes
There are numerous misconceptions about the nutritional value of the potato.
It is often believed that the potato is a high-energy food that provides little
else in the way of nutrients. This is partly due to the high satiety value of the
potato and its description in food composition tables and numerous
publications as a “starchy tuber”. It is sometimes regarded that potato should
not be classed as a vegetable in terms of contributing to the 5 plus fruit and
vegetables a day recommendations. However, in view of current
recommendations that the intake of fats in the diet should not exceed about
30% of energy, that free sugar intakes should be reduced, that about 55% or
more of our energy should be derived from carbohydrate, that energy intakes
overall should be reduced, potatoes are an excellent base to the diet. A daily
intake of 150-300 g of potatoes provides 4-8% of the calories required by a
human. Potatoes are a considerably richer source of nutrients than of energy.
Many of the health benefits of the components of potatoes are discussed in
the previous section. The contribution potatoes make to daily energy and
nutrient intakes is shown in Table 7. The benefits of potatoes can be
summarized as:
1.
they are virtually free of fat, although they are quite easily turned into
fatty foods,
2.
they contain very little soluble sugar,
3.
nearly all of the carbohydrate in them is in the form of hydrated starch.
Starch contributes about 17 kJ/g energy compared with 37 kJ/g for fat,
and starch is hydrated because in cooked potato kJ per gram are even
lower. Therefore, the volume that potato occupies per kilojoule is large,
that is, the energy density of potato is low and it is able to displace
fatty materials from the diet, thus reducing energy intake without a
feeling of emptiness. Potato, therefore, has a role to play in managing
the obesity that is contributing to the current burden of disorders such
as diabetes and heart disease,
4.
although the starch in potatoes is readily digested, as indicated by the
high glycaemic index values, the high water content of potatoes means
that on a weight for weight basis they have a relatively low impact on
blood glucose - about one-third that of bread (Relative glycaemic
potency of bread = 30, of potato = 10) (Monro 1999),
5.
potatoes should not be relied on as a source of dietary fibre as they
contain little of it, and it is easily fermented. This does, however, have
some benefits because it means that people with extremely high
energy intakes, such as athletes, do not become overburdened with
the non-digestible food residues, and furthermore the fermentability of
potato fibre means that it can contribute extra energy as short-chain
fatty acids,
6.
they are a good source of several water soluble vitamins. They may
make a significant contribution to the daily intake of vitamin C
Page 19
(approximately 30% of the recommended daily intake). They also
contain important B group vitamins, especially thiamin, and folic acid,
7.
potatoes are rich in minerals, particularly potassium, but are low in
sodium which is a desirable balance for a healthy diet. Other important
minerals include iron,
8.
potatoes contain high quality protein, although they are deficient in the
essential amino acid methionine,
Table 7: Potato consumption and contribution to energy and nutrient
intake (% recommended daily intake) (all data apart from New Zealand
sourced from Lisiska & Leszcyski 1989).
Country
New
Zealand1
Great
Britain
USA
Consumption (g/day)
180
186
150
200
Energy
Protein
Carbohydrates
Dietary fibre
Fat
Vitamin C
Thiamin (B1)
Riboflavin (B2)
Niacin (PP)
Pantothenic acid
Vitamin B6
Folic acid
Vitamin A
Potassium
Phosphorus
Calcium
Magnesium
Iron
Copper
Zinc
Iodine
Fluorine
6.3
8.0
0.03
46.4
13.0
5.2
8.9
12.5
13.7
0.1
27.7
7.3
1.8
10.9
12.7
4.1
-
4.4
3.9
8.7
0.0
24.1
10.1
3.0
8.6
0.0
7.7
-
4.0
4.7
56.6
8.7
3.6
12.1
16.4
4.9
7.3
7.8
5.2
16.9
3.9
15.2
-
5.5-6.6
-2
16.7
0.3
48.6
18.3
6.7
16.3
10.0
26.3
3.5
22.0-29.3
2.4
16.7
8.9
7.5-15.0
2.0
Specification
West
Germany
1
Figures are based on nutrient composition data from the NZ Food Composition Database
(average of potatoes cooked by different methods) and NZ RDI values for adults (average of
male and female requirements and of average weight).
2
No data available.
3
Unless eaten in a fried form which will make some contribution.
9.
phenolics are present in fairly low quantities in typically consumed
potato cultivars but still may have an important role in health because
of the quantities of potatoes consumed compared to many other
vegetables. Not enough is known to be sure of their benefits,
Page 20
10.
5
glycoalkaloids are an undesirable component in potatoes but the risk
of toxic levels causing harm to human health is minimal.
The future
Much of the focus on potato breeding and development in the past has been
focused on increasing yields and developing pest and disease resistance. In
addition to generating new traits that enable the plant to grow better (input
traits), which are useful to farmers, plant breeding and genetic engineering
technology can also generate plants with improved nutritional features (output
traits). Although the nutritional value of the potato is fairly high there is still
plenty of room for improvement. Most other vegetables (e.g. broccoli,
tomatoes, carrots) are regarded as having some health benefits that
outweigh that of potatoes. However, it is much easier to get a child to eat a
potato than say a piece of broccoli. Therefore the question arises, “Can the
health benefits of broccoli be incorporated into potatoes?” Some manipulation
of nutritional characteristics is possible through traditional plant breeding and
cultural practices. However, genetic engineering presents greater
opportunities. Genetic modification of potatoes is relatively easy because
they are amenable to transformation using Agrobacterium tumefaciens, and
plantlets are readily regenerated and clonally propagated.
5.1
Feeding the world in the twenty-first century
One of the major problems facing mankind is the need to feed an increasing
population, particularly in developing countries. As well as gross
undernourishment, lack of protein, vitamins, minerals and other
micronutrients in the diet is also widespread. Although these problems are
mainly evident in developing countries, the health of many New Zealanders
and people in developed countries around the globe is severely compromised
by diseases that afflict affluent societies (e.g. heart disease, cancer). These
disease have increased significantly over the last few decades and are often
associated with high levels of obesity and blood cholesterol. Ironically, large
sections of these populations are "malnourished", not in the classical sense
of having insufficient food but in terms of not eating a balanced diet, with food
of good nutritional quality, to aid in the prevention of disease; the World
Health Organization has reported that more than 30% of non-communicable
disease can be prevented by diet. As a response to this there has been a
huge upsurge in the supplement industry. However, these supplements do
not always deliver the same health benefits as eating foods of high nutritional
quality. Thus, it is important to find ways to improve the nutritional quality of
food that is easily distributed or grown in developing countries and/or
regularly consumed by the majority of the population.
Page 21
5.1.1
Protein quality improvement
When diets are high in carbohydrates and low in protein over a long period,
essential amino acid deficiency results. Many foods, especially those of plant
origin, such as potatoes, have low levels of amino acids which limit their
nutritive value. Inadequacies in nutritional value of protein can be solved in at
least three ways: (a) combining protein sources to create mixtures with an
adequate amino acid balance, (b) fortification of the low quality proteins with
essential amino acids, and (c) developing high quality plant proteins by
breeding or molecular biology techniques. As discussed in Section 3.3.1,
although potatoes contain high quality protein they lack essential amino
acids. Small changes may be possible by selection. However, to make
significant differences strategies for manipulation by genetic engineering are
needed.
Tu et al. (1994) attempted to incorporate the Brazil nut 2S gene into potato.
This gene was expected to express a protein with elevated methionine and
cysteine, but the expression levels observed in tubers of engineered plants
were insufficient to increase their methionine content. With the knowledge
that amino acid synthesis in higher plants is controlled by feedback inhibition,
Widholm (1977) used the appropriate analogue for an amino acid to select
cell lines with relaxed feedback control of amino acid biosynthesis and to
overproduce the protein. For example, a carrot cell line accumulated 10 times
the normal level of free methionine. A similar protocol was used by Langille et
al. (1998) to select protoclones of Russet Burbank potatoes that had up to
three times the free methionine content of the control. It has not been
determined whether this trait remains stable in subsequent generations.
Genes that code for proteins with a high content of the essential amino acids
found to be most deficient in plant-derived proteins have been constructed
and expressed in bacteria (Jaynes et al. 1985). Since then, research has
been directed towards developing potato plants with these synthetic proteins.
There have been some problems with expression levels of the new proteins
(Destefano-Beltran et al. 1991). More recently a US patent has been filed
which describes the insertion of a gene responsible for producing a protein in
potatoes and rice that is relatively high in essential amino acids (Jaynes &
Derrick 1998). Theoretically, proteins could be specifically designed to
supplement any desired animal feed or human food. Insertion of lysine at
frequent intervals in synthetic proteins provides sites for proteolytic attack by
Trypsin (a digestive enzyme), increasing the bioavailability of the
supplemental protein (Destefano-Beltran et al. 1991).
5.1.2
Provitamin A
Vitamin A deficiency is one of the major vitamin deficiencies in developing
countries. A lack of vitamin A can lead to eye damage and even blindness
and also weaken the protective barriers to infection put up by the skin, the
mucous membranes and the immune system (Somer & West 1966). One of
the most exciting recent developments has been the introduction of genes
into rice that result in the production of the vitamin A precursor β-carotene in
the grain (Conway & Toenniessen 1999). β-carotene is a pigment required for
Page 22
photosynthesis and is synthesized in the green tissues of all plants, including
rice and potatoes. However, it is not always present in non-photosynthetic
tissue such as seeds and tubers. No rice mutants have been found that
produce β-carotene in the grain so traditional breeding is not an option.
Therefore, genes encoding for β-carotene synthesis were incorporated into
rice by genetic engineering. The grain has a light golden-yellow colour and
contains sufficient β-carotene to meet human vitamin A requirements.
Although some potato cultivars are yellow they do not produce much
β-carotene and thus have no or low provitamin A content. Strategy will be
different that that used for rice need to alter composition of carotenoids.
Problems with consumer acceptance of yellow potato however those parts of
the world where food is a major doesn’t matter.
5.1.3
Disease and pest resistance
To date the large focus on genetic engineering of potatoes has been to
develop cultivars that are resistant to pests and diseases. The development
of such plants will reduce pesticide usage, benefiting human health. The
volume of chemicals remaining on potatoes at consumption is likely to be
small and in most cases insignificant in terms of human health. However,
there are huge health benefits for growers who do not to have to spray
chemicals.
5.1.4
Minerals
Levander (1990) has discussed the possibilities for developing fruit and
vegetables with altered mineral composition. Fruits and vegetables contribute
relatively little selenium or molybdenum to the total dietary intake. Selenium
may play a protective role against certain human cancers. Should it prove
desirable to increase dietary intake of selenium, vegetables could be an
important component of such a strategy. A variety of vegetables (potato,
tomato, carrots, cabbage, and onion) grown in seleniferous soils may contain
two to three orders of magnitude more selenium than those grown in
nonseleniferous soils. New Zealand tends to have low selenium levels in its
produce because of the low levels in its soils. Currently, potatoes are a very
poor source of selenium; they contain on average 0.63 μg/100 g. A 180 g
serving of potatoes only contributes about 1.5% of the recommended daily
intake of selenium. The desirability of increasing selenium in the food supply
in general is still uncertain. Under some experimental conditions, elevated
intakes of selenium actually increased rather than decreased the incidence of
chemically induced cancer in laboratory animals. Thus, in certain situations
selenium may not only be ineffective, but also harmful. However, the levels
that have adverse effects are far higher than those that may be achieved
from a high intake of fruit and vegetables. Public health authorities in various
countries must exercise caution when deciding whether or not to add
selenium to their national food supplies.
Nutrient elements in other crops have been manipulated. For example, genes
have been added to rice to increase the grain’s available iron content
(Conway & Toenniessen 1999). Such approaches could be applied to
Page 23
potatoes. Potatoes are a significant source of iron in the New Zealand diet.
However, some females do not have adequate iron intake so there is room
for improvement. On the other hand, iron plays a role in after-cooking
darkening so it may not be desirable to increase the level.
5.2
Delivery of additional health benefits
Functional foods are gaining more and more interest from health scientists.
Potatoes may be a good way to deliver health benefits because of their levels
of consumption in most people’s diets.
5.2.1
Increased levels of phenolics and other antioxidants
There have been many promotions to increase fruit and vegetables in the diet
such as the 5+ a day promotion. More recently evidence suggests that
people should be eating at least eight servings a day, especially those with a
predisposition to diseases such as heart disease and cancer. One of the key
components of fruit and vegetables that may help prevent diseases are
antioxidants (e.g. vitamins C and E, phenolics, carotenoids). Although
potatoes have reasonable levels of vitamin C they have relatively low levels
of phenolics and white-fleshed varieties have only traces of carotenoids.
Thus, the antioxidant activity of potatoes is much lower than many other
vegetables. By increasing the levels of antioxidants in commonly eaten
vegetables it may be possible to obtain the health benefits from consuming a
more moderate number of servings which may be achieved by a greater
proportion of the population. Some increase in antioxidant levels is
achievable through traditional breeding and using particular cultural practices.
In addition there are opportunities for the genetic manipulation of antioxidants
in plant foods and some of these have been discussed by Mullineaux &
Creissen (1996).
There has been some traditional breeding of potatoes for increased vitamin C
content. Through an understanding of the biosynthesis of ascorbic acid it may
also be possible to manipulate the levels through genetic engineering. While
the vitamin C content of potatoes is quite reasonable the levels required to
have other beneficial effects on health are likely to be much greater than
those required to avoid vitamin deficiency. High doses probably should be
avoided although any excess of water-soluble vitamins is simply excreted in
urine. Intakes of vitamin C of 1 gram per day have had no adverse effects.
However, it is now thought that the best health benefits, particularly those
from antioxidants, are gained from consuming a range of different
antioxidants rather than large doses of a single compound. Therefore, a
better approach may be to manipulate the levels of other natural antioxidants.
Another way to raise the antioxidant activity of potatoes is to increase the
levels of phenolics. Potatoes with a high phenolic content would have to be
acceptable to consumers with respect to blackspot, after-cooking darkening
and sensory qualities. The consumer may have to choose between perceived
undesirable appearance and real beneficial health effects. Another important
consideration is that some phenolics have a strong taste, mainly bitter, so
Page 24
increasing their levels may have undesirable effects on flavour. Thus, these
factors will need to be carefully balanced.
The following is a list of some aspects of potato phenolics that need
investigation.
1.
Define the relative health promoting properties of the various
polyphenols present in potatoes and determine if there are synergistic
effects.
2.
Examine the antioxidant activity of coloured potato cultivars to see if
these have increased antioxidant activity over non-coloured varieties.
3.
Enhance the content of the most potent antioxidative polyphenols in
potatoes either by plant breeding or molecular biology techniques. It
may be particularly valuable to increase the levels in the flesh so that
peeling does not adversely affect the health benefits. It may also be
advantageous to increase the range of compounds present rather than
just increase the levels of chlorogenic acid.
4.
Define the effects of commercial and home food processing on
polyphenols in potatoes.
5.
Investigate possibilities for enhancing chlorogenic acid content, and
other phenolics, by postharvest exposure to light and other stress
conditions (although care would be required to avoid glycoalkaloid
accumulation).
6.
Develop new, inexpensive, nutritious and health-promoting highchlorogenic acid/phenolic potato tuber, peel and leaf food formulations.
The value of leaves as a possible food source has received some
attention. Edible leaf protein isolates have been prepared previously
and generally major efforts have been made to remove the pigments
(Bickoff et al. 1973; Pirie 1973; Friedman 1996a). Since leaves have a
high polyphenol content and since chlorophyll binds strongly to
carcinogenic heterocyclic amines (Friedman 1996b) it may be
worthwhile to retain both of these leaf constituents in the preparation of
isolates.
7.
Carry out animal and human feeding studies with high phenolic
potatoes to assess whether beneficial effects of chlorogenic acid, and
other phenolics, observed in vitro are confirmed in vivo.
The other group of antioxidants that could be manipulated in potatoes are the
carotenoids. The manipulation of carotenoids has already been discussed in
Section 5.1.2 regarding provitamin A activity. The same principal could be
used to increase total carotenoids (not necessarily just ∃-carotene) in
potatoes for their antioxidant properties. In many European countries yellow
potatoes are quite acceptable and in fact often preferred, but in other
countries, such as New Zealand, a white potato is usually the norm.
Page 25
5.2.2
Recombinant proteins/vaccines
A large number of recombinant proteins are being developed for
pharmaceutical purposes and will have to be produced on a large scale for
clinical studies and pharmaceutical applications. Protein farming in transgenic
plants offers great flexibility at very low primary cost as expensive technical
cell culture facilities are not required (Artsaenko et al. 1998). Furthermore,
contamination of the product by mammalian viruses or bacterial endotoxins is
excluded. Therefore, transgenic plant-based production systems are a
competitive alternative to bacterial or mammalian cell culture-based systems
(Goddijn & Pen 1995). Potato tubers have great potential in biofarming
because those storage organs can be used to accumulate large amounts of
protein.
Vaccination against infectious diseases has revolutionalised human health.
Diseases such as smallpox have been eliminated and others such as polio
have almost disappeared. There has been a significant increase in our
understanding of immunology and in the development of new technologies,
increasing hope that new vaccines will be developed for other diseases.
Recent vaccine developments have focused on finding ways of delivering
various types of vaccines. An approach that has great appeal in relation to
the potential ease of both production and delivery is using transgenic edible
plants to make vaccines. They may provide a more convenient and practical
means of implementing universal vaccination programmes throughout the
world. For example, genes encoding antigens have been cloned into potato
plants. The potato makes the protein. When animals and more recently
humans eat the potato they develop immune responses (Liu 1999). The
drawback of using potatoes is that they are usually eaten cooked, and this
may denature a protein antigen. In addition to having a role in combating
disease, plant-derived vaccines may have potential as a means of
contraception (Smith et al. 1997). The public acceptability of plant vaccines
has been considered by Danner (1997).
Some examples currently being developed using potatoes either as a test
system or as a final delivery system are noted below.
ϒ
A transgenic plant was shown to be capable of synthesizing an
immunogenic form of the diabetes-associated autoantigen, glutamic acid
decarboxylase. Given as a dietary supplement, the transgenic plant
inhibited the development of diabetes in the non-obese diabetic mouse
(Ma et al. 1997).
ϒ
Potato has been used to produce an edible vaccine protecting against
enterotoxigenic E. coli (Mason et al. 1998). The vaccine has been
expressed in leaves and tubers. Trials with mice have shown some
degree of protection against E. coli. However, the rats were fed raw
potato which would not be possible in human trials. A trial with a small
group of humans showed antibody production (Tacket 1998).
ϒ
Transgenic potatoes have been developed in an attempt to protect
against chlorea (Arakawa et al. 1998). Some degree of protection has
been demonstrated in mice.
Page 26
ϒ
5.2.3
Research is currently underway to develop transgenic potato plants for
use as an oral vaccine against cryptosporidiosis, an important cause of
diarrhoea worldwide (Hudspeth 1997).
Modified carbohydrate
One way to improve the nutritional properties of foods may be to modify the
starch component, often included to stabilize the food product. High value
carbohydrates produced in plants include bacterial cyclodextrins that have
been produced in transgenic potato tubers (Oakes et al. 1991). There are
various applications for cyclodextrins, including pharmaceutical delivery
systems, flavour and odour enhancement and the removal of undesired
compounds (such as caffeine) from foods. Modified potato starches have
been tested for their dietary effects (Raben et al. 1997). Two chemically
modified starches—a 1-2% acetylated potato starch and a starch enriched
with 2% beta-cyclodextrin—and a native, unmodified potato starch (control)
have been investigated with regard to 6 hour energy expenditure, substrate
metabolism, hormone concentrations, and subjective appetite sensations. A
flattening of the glucose curve, a lower insulin and gastric inhibitory
polypeptide response, and higher fullness ratings were observed after the
meal with the beta-cyclodextrin starch. Satiety ratings were higher after both
meals with modified starch than after the meal with the control starch. It was
concluded that a minor modification of native potato starch improved the
glycemia, insulinemic, and satiating properties of a meal. Slower gastricemptying rate or delayed intestinal absorption of the modified starch may
explain the observed differences.
5.2.4
Other
A gene encoding the human milk protein beta-casein has been introduced
into potato, and expression of human milk beta-casein was demonstrated in
both leaf and tuber tissues (Chong et al. 1997). These findings open the way
for reconstitution of human milk in edible plants to replace bovine milk in baby
foods for general improvement of infant nutrition and to prevent gastric and
intestinal diseases in children.
The gene for human epidermal growth factor (hEGF) was chemically
synthesized and has been successfully expressed in transgenic potato
(Salmanian et al. 1996).
5.3
Removal of antinutritional compounds and
undesirable chemicals
5.3.1
Glycoalkaloids
Potatoes contain glycoalkaloids which have detrimental effects on human
health. Care must be taken when producing new varieties of potatoes so that
the level of glycoalkaloids does not rise to an unsafe level or that new, more
toxic glycoalkaloids are not introduced into the commercialised germplasm
(Friedman & McDonald 1997). Strategies must be developed to minimize
postharvest glycoalkaloid production. These may include:
Page 27
1.
determining relative susceptibilities to greening and mechanical
damage of current and new cultivars,
2.
evaluating food-compatible enzyme inhibitors, such as citric acid, and
substrate inhibitors for their ability to inhibit enzymes catalysing
glycoalkaloid biosynthesis,
3.
evaluating films with built-in chromophores that absorb light and
protect potatoes against greening, browning and spoilage,
4.
investigating the effects of size and maturity on glycoalkaloid levels
and measuring glycoalkaloid changes during sampling and handling.
In addition, studies are required to reduce preharvest glycoalkaloid formation
by suppressing genes governing their biosynthesis. This will provide a variety
of benefits extending from the growing, processing, shipping and marketing
to the consumption of potatoes and potato products. Reduction of toxicant
levels in selected varieties will allow the introduction of new potato cultivars
that cannot currently be released due to their higher than acceptable levels of
glycoalkaloids. This would enhance the value of potatoes as a high quality
food. Genetic engineering may allow the creation of new cultivars without
altering other desirable characteristics. Approaches may use antisense RNA
and related methods to develop cultivars with low levels of glycoalkaloids
while maintaining acceptable resistance to phytopathogens (Friedman &
McDonald 1997). The gene encoding solanisine glucosyl transferase is one
target. At present it is unclear how lowering glycoalkaloid production would
affect the concentrations of other compounds, such as phytoalexins, which
may have major consequences for pest resistance.
5.3.2
Chemical residues
In addition to possible herbicide and pesticide residues, potato peels have
been shown to harbour significant amounts of chemical residues that may be
mildly toxic. In the USA chemical sprout inhibitors have been found in
significant levels in potato peels, even after cooking (Lang 1992). Sprout
inhibitors are important chemicals that help prevent shrinkage, blackening,
nutrient loss, and susceptibility to bruising, and may even reduce the
accumulation of some natural toxic chemicals that accompany sprouting (see
section on glycoalkoids). Peeling potatoes prior to cooking removes most of
the problems associated with toxic substances in the peel. However, the peel
contains many beneficial components. Strategies for natural prevention of
sprouting by genetic engineering or other means are desirable. Prevention of
sprouting may also help prevent the accumulation of glycoalkaloids during
storage.
5.4
Browning prevention
There are a number of strategies to minimise or prevent browning and these
require continued study. One approach is the breeding of potatoes with an
initial low chlorogenic acid content to minimize both content and rate of
increase, e.g. to minimize after-cooking darkening and/or enzymic browning
during food processing. However, since chlorogenic acid is reported to have
Page 28
beneficial effects on health (as described in Section 3.7), it may be desirable
to breed for high chlorogenic acid varieties. A construct expressing a high
tyrosine protein has been introduced to potato to act as a tyrosine-sink and
decrease the amount of free tyrosine which contributes to browning reactions
(Vayda & Belknap 1992). Other efforts include employing antisense
constructs of potato polyphenol oxidase genes (Bachem et al. 1994).
Understanding the chemical, nutritional and toxicological consequences of
browning reactions and related transformations can lead to better and safer
foods and feeds and improved human health.
5.5
Other new and improved potato products
Over the last few years there has been an increase in the number of potato
products appearing on the market. With the increasing trend to convenience
and processed products there is a need to ensure that nutritional value is not
lost during processing and storage of these foods.
5.5.1
“Healthier” fried products
There is a high consumption of potatoes in the form of chips, French fries,
etc. Yet with the increasing levels of obesity there is a need to reduce
consumption of fatty foods by at-risk sections of the population. Apart from
cutting fried foods out of the diet there are various strategies for achieving a
lower fat content, including altering the cooking media and altering the
composition of the potato. One advancement has been the development of
olestra, a non-absorbable fat substitute. A recent study using this material
showed that substituting fat-free (olestra-containing) potato chips for regularfat chips can help reduce fat and energy intakes in short-term (Miller et al.
1998). However, the use of olestra is being debated and requires further
investigation. Olestra is reported not to be toxic, carcinogenic, genotoxic, or
teratogenic, and is neither absorbed nor metabolised by the body, but may be
associated with gastrointestinal tract symptoms such as cramping or loose
stools (Prince & Welschenbach 1998). In addition, olestra affects the
absorption of fat-soluble vitamins but does not affect the absorption of watersoluble nutrients. The petitioner's studies concluded that when olestra was
consumed with foods containing vitamins A, D, E, or K, the fat substitute
could have an effect on the absorption of these nutrients.
Other strategies for reducing fat intake include modification of starch genes to
produce potatoes with a higher dry matter (Anon. 1992). This results in less
water for oil to replace in processing and thus results in a “healthier” fried
food. Increased solids contents through genetic modification have already
been reported (Fraley 1991; Fraley et al. 1991) and there is potential for
further increases.
5.5.2
Novelty coloured potato products
The array of coloured potatoes that are available, or could be engineered,
presents opportunities for the production of novelty coloured potato products.
These may be red, blue or purple due to anthocyanins or yellow/orange from
carotenoids. In addition to the colour such products may have health benefits
Page 29
due to the antioxidant activity and other modes of action of the anthocyanins
and carotenoids. Details of anthocyanins and carotenoids and possible
manipulation of these attributes have been discussed in earlier sections (3.7,
3.8 and 5.2.1). If coloured potatoes are to be used it is important that they
retain their colour during cooking and that the final product is highly coloured
and retains its nutritional benefits. During normal cooking (boiling, steaming
or crisping) there was no thermal degradation of anthocyanins and intensely
coloured products were obtained (Lewis 1996). The only significant loss of
anthocyanin occurred by leaching from cut surfaces when potatoes were
boiled. Therefore, coloured tubers should be cooked whole and cut up after
cooking. The effects of processing on the antioxidant activity of potatoes have
not yet been evaluated and will need to be if products are to be developed.
5.6
Utilizing potato waste
New opportunities for utilizing potato waste have been explored by Heyes et
al. (1997) and so these are not discussed in any detail here. Uses include:
ϒ
stockfeed: increasing importance particularly if some of the nutritional
qualities are manipulated as discussed above,
ϒ
specialty potato products (e.g. potato mash, flakes and granules for use
in potato novelties or extruded foodstuffs),
ϒ
potato flour improves the nutritional value of bakery products,
ϒ
extraction of potato protein,
ϒ
pigment extraction: natural pigment to replace artificial and also added
health benefits,
ϒ
extraction of nutraceuticals: extraction of potato starch–has potential as a
fat replacer in foods such as sausages making them a healthier option.
Another usage of potato peels in particular could be as a source of a natural
antioxidant preservative with wide food applications. Potato peel extracts
have also been shown to have bacteriocidal and bacteriostatic effects
(Rodriguez de Sotillo et al. 1998).
6
Recommendations/future research
This review identifies a number of potential areas for future research.
Genetic engineering offers huge potential for manipulating many different
aspects of nutritional quality and introducing additional health benefits. In
light of the current debate, and a possible moratorium on genetic engineering,
these will need careful consideration. However, the public may be more likely
to accept a genetically engineered vegetable when they can see a personal
benefit. Despite this there is still considerable scope for improvements
through traditional plant breeding and/or changes in agronomic and
postharvest handling procedures.
Page 30
Future projects could include:
7
♣
developing strategies for improving the antioxidant activity of potatoes.
There are a number of ways to achieve this but first different varieties of
potatoes should be screened and particularly those with coloured flesh.
Levels of phenolics could be increased by traditional breeding and
genetic engineering,
♣
increasing the provitamin A content of potatoes. Some improvements
may be possible through traditional breeding and selection of cultivars,
but to achieve significant increases genetic modification would be
required. Marketing surveys will be required to determine if New Zealand
consumers would accept a yellow potato,
♣
developing potatoes with high anthocyanin and/or carotenoid contents
creates opportunities for the development of novelty coloured potato
products (e.g. extrusion products, crisps), which may have the added
health benefits of the compounds they contain. If such products are to
be developed then the effects of processing must be carefully
investigated to ensure the health benefits are not lost,
♣
producing higher dry matter potatoes for processing to reduce the fat
content of fried products,
♣
reducing glycoalkaloid content of potatoes. In addition it is important to
monitor the glycoalkaloid content of new cultivars as there is a risk of
increased glycoalkaloid biosynthesis, although this risk is very small,
♣
as new cultivars are developed with additional health benefits there will
be a need to re-evaluate the potential of potato wastes to better utilize
the key components either for improvement of human or animal health,
♣
evaluating the possibilities of using genetic engineering to produce
vaccines and high value pharmaceuticals from potatoes. Since we have
very strong programmes for genetic modification of potatoes these could
easily incorporate identified opportunities,
ϒ
developing new potato cultivars with clearly defined nutritional and health
benefits opens up potential for the development of tailored products for
specific sectors of the population (e.g. diabetics, the elderly, athletes).
Market opportunities and potential product development to meet
changing New Zealand and world demands (e.g. as a result of the aging
population) will need to be considered.
Acknowledgements
The authors would like to thank Dr Karen Silvers and Emmeline Taptiklis for
the nutritional information, and Dr Tony Conner for information and literature
on genetic engineering opportunities for potatoes.
Page 31
8
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Page 38
Appendix I - the chemical
composition of New Zealand-grown
potatoes, raw and cooked by a
variety of method
Other foods that may replace potatoes in a meal are also included for
comparison. For details of analytical methods refer to The Concise New
Zealand Food Composition Tables (Burlingame et al. 1997).
Page 39
Water
Energy
Energy
Protein (Nitrogen x 6.25)
Total fat
Carbohydrate, available
Dietary fibre (Englyst)
Ash
Table 8: Proximate composition.
g
kcal
kJ
g
g
g
g
g
Combined cultivars, flesh and skin, raw
78.54
72
299
2.36
0.11
15.5
1.3
1.15
Combined cultivars, flesh, raw
78.17
70
291
2.06
0.06
15.4
1.4
0.97
Fries, fried in beef dripping, salt added
56.44
186
773
3.77
6.26
28.7
1.4
1.69
Fries, fried in butter, salt added
63.33
159
660
2.92
4.71
26.3
1.4
1.65
Fries, fried in peanut oil, salt added
58.18
174
723
3.46
5.34
28.1
1.4
1.35
Fries, frozen, uncooked
73.1
107
445
2.2
3
17.9
1.3
0.6
Fries, independent shops, ready to eat
52.9
241
1002
3.8
14
24.9
1.6
2
Ilam Hardy, flesh, raw
79.07
68
279
1.97
0.05
14.8
1.63 0.93
In skin, microwaved
77.9
74
305
1.94
0.07
16.34
2.7
1.05
Instant powder, prepared with water
79.4
69
283
2
0.2
14.7
1.2
1.1
81
72
297
2.36
0.06
15.5
1.3
1.15
Red King, flesh, raw
79.6
70
291
2.06
0.06
15.4
1.2
0.97
Rua, flesh, baked, salt added
75.36
88
366
2.58
0.15
19.2
1.99 1.28
Rua, flesh, boiled, drained
77.06
83
342
2.1
0.17
18.2
1.73 0.88
Rua, flesh, boiled, mashed with milk, butter and salt added
79.33
95
394
1.84
3.29
14.5
1.7
Rua, flesh, boiled, salt added
77.06
83
342
2.1
0.17
18.2
1.73 0.88
Rua, flesh, microwaved, salt added
72.39
95
394
2.48
0.13
21.1
2.3
Rua, flesh, raw
79.91
70
291
1.78
0.15
15.5
1.56 0.79
Rua, flesh, roasted in beef dripping, salt added
71.82
105
433
2.46
0.64
22.3
1.42 1.43
POTATOES
Red King, flesh and skin, raw
Page 40
0.73
1.34
Dietary fibre (Englyst)
Carbohydrate, available
kcal
73.2
kJ
306
g
g
2.38 0.111
g
15.7
g
g
1.31 1.16
Bread, white, sliced
39
216
904
7.3
0.9
43.4
2.8
Bread, wholemeal
45
198
829
8.1
1.4
37.1
5.7
Macaroni, boiled
78
86
359
3
0.5
16.8
0.9
Rice, brown, boiled
66
141
589
2.6
1.1
29.2
1.8
Rice, white, polished, boiled
70
123
513
2.2
0.3
26.9
1.3
Spaghetti, boiled
74
100
420
3.6
0.3
20.1
1.2
OTHER
Page 41
Ash
Energy
Total fat
Energy
Protein (Nitrogen x 6.25)
Water
g
78.3
Whole, with skin, fried
Sulphur
Chloride
Potassium
mg
35
mg
55
mg
444
2.54 22.9 46.1
32
55
145
33
69
76
Fries, fried in butter, salt added
158
27
57
Fries, fried in peanut oil, salt added
179
31
Fries, frozen, uncooked
25
21
Fries, independent shops, ready to eat
250
27
Ilam Hardy, flesh, raw
In skin, microwaved
Instant powder, prepared with water
Selenium
Fries, fried in beef dripping, salt added
Zinc
Combined cultivars, flesh, raw
Copper
mg
17
Iron
mg
4
Manganese
Combined cultivars, flesh and skin, raw
Calcium
Magnesium
mg
33
Sodium
Phosphorus
Table 9: Nutrient elements.
mg
3
µg
115
mg
0.6
mg
0.093
mg
0.29
µg
1.05
484 4.39
106
0.57
0.186
0.2
0.27
223
881
7
235
1.3
0.25
0.6
0.35
60
244
776
8
176
0.7
0.19
0.4
1.2
69
70
276
770
6
238
1
0.27
0.6
0.35
61
32
45
420
8
169
0.7
0.11
0.3
0.27
70
40
65
650
11
146
1.3
0.12
0.6
0.35
2.43 21.9 44.2 30.7 52.8 464 4.21
161
0.55
0.209
0.39
0.27
1.6
23
48
220
0.83
0.12
0.27
T
POTATOES
42
-
510
6.7
260
15
48
6
380
340
20
79
0.5
0.08
0.2
0.27
Red King, flesh and skin, raw
3
17
5
32
55
484
4
116
0.57
0.086
0.23
0.25
Red King, flesh, raw
3
17
4.6
32
55
484
4
68.2
0.57
0.131
0.22
0.249
104
18
41
40
160
543
4
116
0.3
0.14
0.3
0.98
4
15
32
35
26
332
4
102
0.5
0.15
0.2
0.47
Rua, flesh, boiled, mashed with milk, butter and salt added
130
12
39
30
265
282
21
70
0.3
0.13
0.3
0.4
Rua, flesh, boiled, salt added
127
15
32
35
318
332
4
102
0.5
0.15
0.2
0.47
Rua, flesh, microwaved, salt added
119
22
47
46
183
610
4
134
0.5
0.18
0.4
0.98
Rua, flesh, baked, salt added
Rua, flesh, boiled, drained
Rua, flesh, raw
4
17
33
35
55
444
3
120
0.57
0.14
0.3
0.27
Rua, flesh, roasted in beef dripping, salt added
124
22
44
47
191
589
6
138
0.6
0.17
0.3
0.35
Whole, with skin, fried
4.04 17.2 33.3
-
-
Page 42
448 3.03
120
0.606 0.0939 0.293
1.1
Sodium
Magnesium
Phosphorus
Sulphur
Chloride
Potassium
Calcium
Manganese
Iron
Copper
Zinc
Selenium
mg
mg
mg
mg
mg
mg
mg
µg
mg
mg
mg
µg
OTHER
Bread, white, sliced
666
162
40
1
0.8
4
Bread, wholemeal
641
227
33
1.7
1.3
3.2
Macaroni, boiled
1
25
5
0.4
0.5
0.3
Rice, brown, boiled
1
99
4
0.5
0.7
2.3
Rice, white, polished, boiled
2
38
1
0.2
0.1
2.6
Spaghetti, boiled
T
24
5
0.4
0.5
T
Page 43
Retinol
Beta-carotene equivalents
Total vitamin A equivalents
Thiamin
Riboflavin
Niacin
Vitamin B6
Pantothenate
Biotin
Folate, total
Vitamin B12
Vitamin C
Vitamin D
Alpha-tocopherol
Vitamin E
Table 10: Vitamin composition.
µg
µg
µg
mg
mg
mg
mg
mg
µg
µg
µg
mg
µg
mg
mg
0
6
1
POTATOES
Combined cultivars, flesh and skin, raw
0.09
0.04 1.474
0.07
0.38
0.1
15
0
19
0
0.07
0.08
0.086 0.003 0.887 0.025
0.38
0.1
14
0 12.4
0 0.073 0.083
0.2
0.4
10
T
11
0
0.34 0.09
13
T
10 0.03 -
Combined cultivars, flesh, raw
0
6
1
Fries, fried in beef dripping, salt added
T
34
6
0.13
0.1
0.9
0.15
Fries, fried in butter, salt added
T
71
12
0.13
0.04
0.4
0.13
Fries, fried in peanut oil, salt added
0
23
4
0.14
0.05
0.5
0.14 0.487
0.4
11
0
4
0
Fries, frozen, uncooked
0
T
T
0.08
0.01
1.6
0.28
0.42
0.1
12
0
6
0 0.073 0.083
Fries, independent shops, ready to eat
0
0
0
0.1
0.03
1.1
0.27 0.498
0.4
9
0 -
Ilam Hardy, flesh, raw
0
0
0
In skin, microwaved
0
T
T
Instant powder, prepared with water
0
54
Red King, flesh and skin, raw
0
6
Red King, flesh, raw
0
Rua, flesh, baked, salt added
Rua, flesh, boiled, drained
0.082 0.003
16
0
0.85 0.024
0.39
0.1
14
0 11.9
0
1.04
0.1
0.46 0.46
44
0
0 -
1.2
0.1
0.2
0.1
0.1
0.36
0.07
0.08
0.04
T
9
0.01
0.03
0.18
0.2
0.3
5
0
3
1
0.086
0.03
0.89 0.025
0.38
0.1
15
0
12
0 0.073 0.083
6
1
0.086
0.03
0.89 0.025
0.39
0.1
14
0
12
0 0.073 0.083
0
7
1
0.09
0.04
0.5
0.09
0.34 0.09
13
0
10
0 0.073 0.083
0
7
1
0.07
0.04
0.3
0.07
0.34 0.07
13
0
9
0 0.073 0.083
Rua, flesh, boiled, mashed with milk, butter and salt added
5
39
11.5
0.05
0.05
0.5
0.07
0.33 0.25
12 0.01
Rua, flesh, boiled, salt added
0
7
1
0.07
0.04
0.3
0.07
0.34 0.07
13
0
9
0 0.073 0.083
Rua, flesh, microwaved, salt added
0
13
2
0.09
0.05
0.3
0.1
0.34 0.09
13
0
10
0 0.073 0.083
Rua, flesh, raw
0
6
1
0.07
0.04
0.4
0.07
0.38
14
0
12
0 0.073 0.083
Page 44
0.1
1.5
0.1
0
8 0.01
0.11
0.02 0.021
0.09
0.1
0
0.33
0.09
3.3
Bread, wholemeal
0
0
0.66
0.16
Macaroni, boiled
0
0
0.06
0.02
Rice, brown, boiled
0
0
0.14
Rice, white, polished, boiled
0
0
Spaghetti, boiled
0
0
mg
µg
µg
µg
mg
µg
mg
mg
0.34 0.09 13
0
10
0 0.075 0.085
-
11
0
19
0.02
28
0
0
3.9
0.04
36
0
0
0.7
0.01
3
0
0
0.02
1.9
0.19
10
0
0
0.01
0.01
0.8
0.05
3
0
0
0.02
0.01
0.8
0.02
4
0
0
OTHER
Page 45
Vitamin E
0
Alpha-tocopherol
Bread, white, sliced
1.4 0.067 -
Vitamin D
0.073 0.038
Vitamin C
1
Vitamin B12
6.1
mg
0.11
Folate, total
0
mg
0.8
Biotin
Whole, with skin, fried
mg
0.04
Pantothenate
mg
0.11
Vitamin B6
µg
2
Niacin
Total vitamin A equivalents
µg
10
Riboflavin
Beta-carotene equivalents
µg
T
Thiamin
Retinol
Rua, flesh, roasted in beef dripping, salt added
0 -
0.081
Isoleucine
Leucine
Lysine
Methionine
Cysteine
Phenylalanine
Tyrosine
Threonine
Tryptophan
Valine
Arginine
Histidine
Alanine
Aspartic acid
Glutamic acid
Glycine
Proline
Serine
Table 11: Amino acid composition (g per 100 g FW).
Combined cultivars, flesh and skin, raw
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Combined cultivars, flesh, raw
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
0.158
0.227
0.208
0.06
0.048
0.168
0.119
0.148
0.054
0.198
0.188
0.072
0.139
0.693
0.484
0.129
0.148
0.158
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Fries, fried in peanut oil, salt added
0.15
0.207
0.184
0.055
0.044
0.15
0.105
0.138
0.05
0.173
0.173
0.067
0.127
0.634
0.438
0.115
0.138
0.15
Fries, frozen, uncooked
0.091
0.13
0.12
0.035
0.028
0.095
0.067
0.084
0.032
0.11
0.11
0.042
0.081
0.4
0.28
0.074
0.084
0.091
Fries, independent shops, ready to eat
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Ilam Hardy, flesh, raw
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
In skin, microwaved
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
0.083
0.12
0.11
0.032
0.026
0.086
0.061
0.077
0.029
0.1
0.099
0.038
0.074
0.37
0.26
0.067
0.077
0.083
Red King, flesh and skin, raw
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Red King, flesh, raw
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Rua, flesh, baked, salt added
0.11
0.161
0.141
0.041
0.043
0.111
0.078
0.099
0.037
0.131
0.131
0.05
0.095
0.473
0.332
0.087
0.099
0.111
Rua, flesh, boiled, drained
0.088
0.127
0.114
0.034
0.026
0.091
0.064
0.08
0.031
0.108
0.104
0.041
0.077
0.395
0.263
0.07
0.08
0.088
Rua, flesh, boiled, mashed with milk, butter and salt added
0.076
0.111
0.1
0.029
0.023
0.08
0.056
0.071
0.027
0.094
0.09
0.036
0.067
0.343
0.233
0.061
0.071
0.076
Rua, flesh, boiled, salt added
0.087
0.128
0.114
0.034
0.027
0.091
0.065
0.081
0.031
0.108
0.105
0.04
0.078
0.39
0.269
0.071
0.081
0.087
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
Rua, flesh, raw
0.074
0.109
0.1
0.028
0.022
0.077
0.054
0.069
0.026
0.092
0.092
0.034
0.065
0.326
0.226
0.059
0.069
0.074
Rua, flesh, roasted in beef dripping, salt added
0.105
0.149
0.131
0.039
0.032
0.105
0.075
0.096
0.036
0.122
0.122
0.047
0.087
0.455
0.315
0.083
0.096
0.105
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
POTATOES
Fries, fried in beef dripping, salt added
Fries, fried in butter, salt added
Instant powder, prepared with water
Rua, flesh, microwaved, salt added
Whole, with skin, fried
Page 46
Edible portion
g
W eight
Common Measure
W eight
Common Measure
Table 12: Common serving sizes and edible portions.
g
%
POTATOES
Combined cultivars, flesh and skin, raw
1 potato
150
Combined cultivars, flesh, raw
1 potato
117
96
Fries, fried in beef dripping, salt added
1 cup
60
Fries, fried in butter, salt added
1 cup
Fries, fried in peanut oil, salt added
1 cup
Fries, frozen, uncooked
10 chips
65
100
Fries, independent shops, ready to eat
1 cup
100
100
Ilam Hardy, flesh, raw
1 potato
117
85
Instant powder, prepared with water
1 cup
241
100
Red King, flesh and skin, raw
1 potato
150
99
Red King, flesh, raw
1 potato
117
88
Rua, flesh, baked, salt added
1 potato
90
1 cup
128
100
Rua, flesh, boiled, drained
1 potato
114
1 cup
164
100
Rua, flesh, boiled, mashed with milk, butter and salt added
1 cup
209
Rua, flesh, boiled, salt added
1 potato
114
1 cup
164
Rua, flesh, microwaved, salt added
1 potato
90
1 cup
128
Rua, flesh, raw
1 potato
117
Rua, flesh, roasted in beef dripping, salt added
1 potato
95
1 cup
130
Whole, with skin, fried
1 potato
97
1 cup
134
Bread, white, sliced
1 medium
slice
26
1 thick slice
Bread, wholemeal
1 medium
slice
28
Macaroni, boiled
1 cup
149
Rice, brown, boiled
1 cup
206
Rice, white, polished, boiled
1 cup
216
Spaghetti, boiled
1 cup
148
1 cup, diced
158
84
10 chips
45
100
60
10 chips
45
100
60
10 chips
45
100
In skin, microwaved
100
100
84
100
OTHER
Page 47
36
100