SECTION 5: FOOD COMMODITIES
Section 5: Food Commodities
Resource management
Content
Elaboration
Food commodities
Composition and properties of the following goods in
a raw and cooked state:
1. Fruit and vegetables
 Structure and texture
 Changes during ripening and cooking
 Plant pigments and enzymic browning
 Sensory qualities
 Relationship to health
2. Meat and fish
 Structure and texture
 Post-mortem changes
 Changes during cooking
 Meat and fish colour
 Meat tenderness
 Sensory qualities
 Relationship to health
3. Dairy foods, milk and milk products and eggs
 Constituents
 Uses in food preparation
 Changes during cooking
 Sensory qualities
 Relationship to health
4. Cereals and baked goods
 Types
 Function and uses of rice and pasta, flour, fats and
shortening, sugar, raising agents
 Changes during cooking
 Sensory qualities
 Relationship to health
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Vegetables
Vegetable structure
Vegetables have no common biological structure. They are obtained from
different parts of many plants. They can be categorised as follows:
leaves
roots
fruits
stem, stalk
flower
seeds
eg
eg
eg
eg
eg
eg
cabbage, lettuce
carrots, radishes, turnip
tomatoes, cucumber
celery
cauliflower
peas, beans, lentils
Key facts
 Vegetables usually have a high water content.
 Their content of indigestible carbohydrate (mostly cell ulose) makes them
important in the diet due to the production o f non-starch polysaccharides
(NSP).
 Many vegetables are good sources of vitamin C, beta -carotene and mineral
elements, particularly iron.
Texture of vegetables
The crunchy texture of vegetables adds variety to the diet. The texture of
vegetables (and fruits) is often an indication of quality and it depends on the
turgor of the cells and the presence of supporting tissues and the cohesiveness
of the cells. Plant tissues assume a characteristic water content and at this
level the cell is described as a turgid cell or in a complete state of turgor. A
plant cell is like a balloon blown up with water – this internal pressure is
called turgor pressure. When the water is cut off to the cell, water is
gradually lost from the plant. This means that the turgor pressure cannot be
maintained and the cell walls start to collapse. This spreads throughout the
plant, which ‘wilts’. Leafy vegetables such as lettuce remain turgid for only a
short time before becoming limp.
Key facts
 The wall of cells in plant tissue serves as the structure of the plant. They
are distributed to provide support for the plant.
 Crunchiness in vegetables is caused by the pressure that turgid cells exert
on one another.
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 Cementing material (pectin substances and hemi-cellulose) between cells
makes them stick together. This makes raw vegetables resist the pressure
of the teeth chewing.
Changes during cooking
Some vegetables are almost inedible until they are cooked and for this reason
many are cooked before they are eaten. Those vegetables which are
acceptable raw are termed ‘salad vegetables’.
Key facts
 Cooking will soften vegetables as the pectins and hemi -cellulose dissolve
and the starch gelatinises.
 Uncooked starchy vegetables are difficult to digest because the starch
granules resist the action of enzymes produced during digestion.
 Cooking destroys a number of micro-organisms that are present on the
surface of vegetables.
 Vegetables should be cooked in boiling water (usually smal l quantities).
When crisp vegetables are immersed in boiling water, heat denatures the
cytoplasm and cell membranes. The result is that the cells no longer retain
water. Water is lost by diffusion through the cell walls , which become
permeable at this point. It is this loss of water that makes boiled cooked
vegetables become limp (they are very often overcooked).
 Vitamin C may be destroyed by an enzyme called oxidase , which is found
naturally in vegetables and fruit. This enzyme destroys vitamin C by
oxidation when the cell walls become weakened as the plant wilts or when
cut or bruised during preparation. Oxidase can be destroyed by plunging
the vegetable into boiling water, so conserving vitamin C.
 Ideally, cooking should stop when vegetables are still sl ightly crisp (al
dente) with the exception of starchy vegetables, eg potatoes.
 Cooking dulls the bright colour of vegetables, particularly that of green
vegetables. When a green vegetable is immersed in boiling water it
becomes brighter green. However, as cooking continues, the colour
changes to a more olive green. This result is due to acids in plants that
affect the chlorophyll (which gives plants their green colour).
 To preserve the bright green of vegetables when cooking, it is necessary to
cook in a minimum amount of water for a short time with the lid on.
 Carotenoids are yellow-, orange- and red-coloured fat-soluble pigments
found in vegetables, eg carrots and peppers. Colour loss is not as
noticeable in these vegetables. Colour loss will occur only af ter lengthy
cooking at high temperatures.
 Anthocyannins belong to a group of compounds known as flavanoids ,
which give the red, blue and purple colours to a range of vegetables and
fruit. They are soluble in water and leach out easily into cooking water. I n
other methods of cooking they are relatively stable.
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Yellow and red vegetables
The pigment responsible for the colour of these vegetables is a precursor to
the vitamin A that they contain. These pigments are known as carotenoids and
they are responsible for imparting the colour to most yellow, orange or red
foods. Important carotenoids include:

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
the
the
the
the
orange carotenes of carrots, apricots and peaches
red lycopene of tomatoes, watermelon and apricots
yellow-orange xanthophylls of sweetcorn, peaches and paprika
yellow crocetin of the spice saffron.
Seeds
Pulse vegetables are the seeds of leguminous plants and include peas, beans
and lentils.
They have a high protein content but are low in one essential amino acid,
methionine, but have a plentiful supply of lysine. Apart from being a good
source of cheap protein, especially when eaten in combination to provide all
essential amino acids, pulses are a particularly useful addition to our diet for
two main reasons:
1.
2.
They have the lowest fat content of any protein foods.
Their dietary fibre content is extremely high.
Beans exist in over 200 different varieties but the most popular in the UK is
baked beans. The oil in soya beans is rich in polyunsaturated fatty acids and
are very often used in the manufacture of margarine.
Care must be taken when cooking pulses as the addition of salt, vinegar,
lemon juice or tomatoes can cause the outside of the pulses to toughen,
preventing them from cooking properly.
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Fruits
Structure and texture
The structure and texture of fruits is very similar to that of vegetables. Most
fruits consist of the pulpy, edible material that develops around and adheres
to the seeds of the plant after it has flowered. The taste of fruit is a blend of
the sweetness and acidity present, complemented by the flavour of that
particular fruit. Some fruits, for example tomatoes, peppers and cucumbers ,
are eaten as vegetables. However, they are the fruits of the plant.
Changes during ripening
Unripe fruit is practically inedible. It is often green in colour, but during
ripening the green colour may be replaced by a yellow or reddish colour.
Harvested fruit is kept in cool storage until it is sold to the consumer. At
these cool temperatures fruit should have an extended storage life because the
organic acids of the fruit tend to decrease during storage and ripening at
lower temperatures. Acids disappear during ripening and this affects both the
colour and taste of the fruit. The flesh softens and becomes sweeter and
juicier, and a characteristic ‘ripe’ flavour and odour develop.
The changes that occur during ripening are caused partly by enzyme
conversion of complex substances to simpler substances.
These changes include:
 Hard starch – packed cells are softened by conversion of starch to sugar,
which dissolves and enhances the sweetness and juiciness of the fruit.
 Insoluble protopectin, which cements plant cells closely together, is
converted to soluble pectin. In this soluble state, the pectin connects the
cells more flexibly.
 Fruit is cut off from its supply of nutrients when it is harvested. This
results in the halt of growth. However, ripening may continue during
storage and sometimes the fruit ripens more rapidly than if it had been
allowed to continue growing.
 Citrus fruits and other fruits that do not store starch obtain their sugar
from the leaves of the plant on which they grow. They do not, therefore,
become sweeter after picking.
 Some fruits produce minute amounts of ethylene during their growing
period. Larger amounts are produced when the fruit is passing through the
critical ripening period. At this time, cell activity is at a maximum.
Ethylene activity promotes ripening and is used in commercial fruit
production to accelerate the ripening of fruit.
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Ripening
Ripeness marks the end of the fruit’s growth and the beginning of its death.
Fruits which have soft flesh and thin skin pass rapidly from ripeness to
rottenness. Harder fruits and those protected by a hard skin can remain in
good condition for a lot longer. Undamaged fruit may remain edible for some
time but eventually will decay as a result of the continued activity of its own
enzymes and attack by micro-organisms.
Sensory qualities of fruit and vegetables
 Fruit is refreshing to eat and adds colour and flavour to diet. T he taste of
fruit is a subtle blend of sweetness and acidity. The relative amounts of
sugar and acids present determine whether a particular fruit is sweet or
sour.
 Vegetables do not have such pleasant tastes, smells and flavours as fruit in
general. However, vegetables do have distinctive flavours, tastes and
smells. Many vegetables are less attractive to some people, especially
children, because of the smell given off by sulphur compounds.
Relationship of fruit and vegetables to health
Contribution to general good health
 There is growing awareness that vegetables and fruit provide materials
besides vitamins and minerals that are important for long -term health.
 Both groups constitute such diversity that it is difficult to generalise about
their relationship to health. In general, green leafy vegetables, potatoes
and fruits are good sources of vitamin C.
 Some vegetables contain some iron, folate and vitamin A.
 Citrus fruits, eg oranges, lemons and grapefruit , are high in vitamin C, and
provide an acceptable amount of folic acid and thiamine, although they are
low in protein and fat.
 Legumes are fairly high in protein, good sources of calcium and iron , and
contain some of the B group of vitamins. Soya beans are a particularly
good source of protein.
Bowel disorders
Non-starch polysaccharides
Dietary fibre (non-starch polysaccharides, NSP) in fruit and vegetables
combines with cholesterol and bile salts, preventing cholesterol from being
absorbed. Lack of fruit and vegetables may increase the risk of coronary heart
disease (CHD). NSP can bind with the bile salts to lower cholesterol levels.
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 Fruit and vegetables are important sources of NSP and so reduce the risk
of constipation and bowel disorders.
 Fruits and vegetables are high in NSP and promote a feelin g of fullness,
reducing the risk of snacking on high fat/sugar snacks between meals ,
which can contribute to a higher risk of obesity, type 2 diabetes, CHD,
tooth decay and certain cancers.
Cancer and heart disease prevention
Antioxidants
 A diet lacking in fruit and vegetables may result in a deficiency in
antioxidants which counteract free radicals reducing senility and early
ageing. Antioxidants are vital to health as they are substances that act in
the body’s first line of defence against unwanted da mage to cells.
 If there is an imbalance between antioxidants and free radicals, high levels
of free radicals can attack proteins, fatty acids and DNA, which can result
in chronic illness, eg. CHD. cancer and degeneration of the eyes, so it is
vital to have a good supply of antioxidants in the diet. Studies consistently
show that people who have a high intake of fruit and vegetables (and
antioxidant vitamins) have lower levels of cancer and CHD.
 A diet lacking in fruit and vegetables increases the risk of c ancer, in
particular of the mouth, pharynx, larynx, oesophagus, lung, stomach and
pancreas.
 Carotenoids function as antioxidants and can be converted into retinol
(vitamin A) in the small intestines.
 Red and yellow vegetables and fruit are a good source of beta-carotene.
Phytochemicals
 Vegetables and fruit contain phytochemicals, which are known to help
prevent certain cancers. Citrus fruits are particularly high in a class of
phytochemicals known as limonoids, which can have a protective effect
against variety of cancers. A diet lacking in fruits and vegetables prevent
this benefit.
 A diet lacking in fruit and vegetables may not contain enough
phytochemicals, which are not considered as nutrients but can reduce the
risk of cardiovascular disease, cancers and problems associated with
ageing.
 Cruciferous vegetables consist of cauliflower, brussels sprouts, cabbage
and broccoli. They contain a number of beneficial phytochemicals ,
including indoles, which are nitrogen compounds that may offer some
protection against cancer because they help to prevent carcinogens from
damaging DNA. Phytochemicals are not considered essential nutrients, yet
their consumption can have long-term effects on reducing the risk of
cardiovascular disease, cancers and problems associate d with the ageing
process.
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 Brussels sprouts may help to protect against some types of breast cancer
which are linked to high levels of the hormone oestrogen. The indoles
contained in Brussels sprouts stimulate the liver and break down the
hormone.
Flavonoids
 Fruit and vegetables will also supply flavonoids , which are cardioprotective and are powerful antioxidants and scavenge free radicals.
 Plants, including vegetables, produce flavanoids. These are a broad family
of substances that can function as antioxidants to block oxidative damage
to cells due to free radicals.
 Vegetables produce flavonoids, which act as antioxidants to block damage
to cells by free radicals. Lack of vegetables can lead to a deficiency of
flavonoids.
Refer to Nutrients and their Effect on the Health and Well-being of
Individuals
Meat
Structure and texture of meat
Meat is the flesh or muscle of the animal. It is composed of muscle fibres,
each of which is an elongated cell. The fibres are held together by connective
tissue. Fatty deposits known as ‘marbling’ and blood vessels are found
between bundles of fibres. The fibrous nature of the muscles gives rise to
what is known as the ‘grain’ of the meat. It is easier to cut or chew meat with
the grain, ie in the direction of the fibr es rather than across the fibres.
Structure of meat-muscle bundles
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Composition
The composition of meat is very variable. The fat content varies from 10 to
50% depending on the animal and the part from which it comes. Meat with a
high fat content has a low water content and vice versa.
Protein
The cells of the muscle fibres contain two soluble proteins: myosin, which is
thicker filaments, and actin, which is thinner. These proteins are responsible
for the contraction of muscle and for rigor mortis.
The connective tissue surrounding the muscle fibres is mainly collagen,
whilst the walls of the muscle fibre are mainly elastin. Collagen is gradually
converted into gelatine during cooking. Elastin is a tough, insoluble protein
and is not affected by cooking. It is commonly known as ‘gristle’.
Fat
In addition to the deposits of fat called marbling, fat is also stored under the
skin of animals and around some of the internal organs , such as suet around
the kidney. Meat requires some fat to prevent it dr ying up during cooking but
consumers today favour meat with a reduced fat content.
Post-mortem changes
When an animal is slaughtered, various changes take place. After death
adenosine triphoshate is broken down. Myosin and actin combine to form
rigid chains of actomysin, otherwise known as rigor mortis. At this time the
meat becomes rigid and tough, and requires time, known as conditioning,
when the stiffness diminishes and the tenderness and flavour improve.
Conditioning is also called hanging, ageing or maturation.
Conditioning is a complex process and can be affected by three main factors:
1.
2.
3.
temperature
pH
length of storage.
After an animal’s death the glycogen present in the muscular tissues is broken
down by stages into lactic acids, the pH falling. During conditioning proteins
of the muscular tissue are denatured, ie there is an unfolding of the protein
molecule, which is usually irreversible, although the proteins of the
connective tissue are not. Denaturation results in an increase in tenderness , as
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does the effects of enzymes such as calpains and cathepsins, which are
naturally present in the muscle.
Changes during cooking
There are four main reasons why we cook meat:




to
to
to
to
make
make
make
make
it
it
it
it
easier to chew
easier to digest
safe to eat
more flavoursome.
It is important to choose a cooking method appropriate to the cut of the meat.
The changes which take place are as follows:
 There is a change in colour from red to brown. This is due mainly to
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changes in the pigment myoglobin, which occur at 63°C.
Muscle fibre proteins coagulate. The texture becomes firmer as the
proteins myosin and actin, in the muscle fibres, coagulate above 50 °C.
Meat juices are squeezed out as the collagen and elastin contract at 60 °C.
This causes the meat to shrink and reduce in weight. In the moist methods
of cooking these juices will pass into the cooking liquid and be eaten as
gravy.
Collagen in the connective tissue is converted into gelatine , which makes
the meat more tender. Tougher cuts of meat , which contain more
connective tissue, are best cooked by moist methods of cooking such as
stewing, where greater breakdown of the connective tissue will result due
to the added moisture and the longer length of cooking time.
Fat melts, which helps to keep the lean part of the meat moist by reducing
water loss. Some of the fat runs out to form dripping.
If cooked properly, meat becomes more tender as the collagen is softened
at 80–100°C and dissolves in the presence of water to form gelatine.
Meat colour
Consumers recognise fresh meat as having a bright red colour and hence
colour has a major influence on the visual appearance of meat.
The colour of meat is primarily dependent on the concentration and chemical
state of the major meat pigments in the muscl e and cell tissue, namely
myoglobin and haemoglobin, which are collectively known as haem pigments.
When an animal is bled after slaughter, the blood is drained from the body,
leaving myoglobin as the primary pigment responsible for the meat colour.
When meat is exposed to the oxygen in the air, the purple -red myoglobin
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absorbs oxygen and is converted to bright pink oxymyoglobin. After
prolonged exposure, oxymyoglobin is chemically oxidised to brown
metmyoglobin. Metmyoglobin is the major pigment noticed on discoloured
meat and results from the oxidation of the iron within myoglobin. It is at this
stage that the consumer finds the product unacceptable.
Meat tenderness
The following factors determine the tenderness of meat:
 The size of the muscle fibre – meat composed of small, narrow fibres is
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more tender than meat composed of larger fibres.
The amount of connective tissue – tough meat contains more connective
tissue than tender meat. The older the animal and the greater its activity
during life, the greater the amount of connective tissue.
The activity of the animal before death – the animal must be rested before
slaughter. If it is not, the supply of glycogen in the muscle tissue is
reduced and less lactic acid is produced during hanging.
The length of hanging after slaughter. Meat is hung for several days to
make it more tender. During hanging, glycogen, which is present in muscle
tissue, is converted to lactic acid. The pH of the meat falls from about 7.4
to 5.5. The reduction of the pH brings about p artial denaturation of the
fibre proteins, which increases tenderness.
Meat may be further tenderised before cooking by physical and chemical
means. Physical means may be pounding with a meat hammer, cutting or
mincing. Chemical means are using acid marinades, eg lemon juice,
vinegar or wine. These products help to coagulate proteins.
Sensory qualities
The flavour and the smell of meat are the main attraction of it. The presence
of a variety of substances known as meat extractives , which are soluble in
water ,contribute to the flavour. Fat also contribute s to the flavour of meat.
Relationship to health
 Meat is a valuable protein in food of high biological value , assisting in the
growth, repair and maintenance of body cells.
 Meat is an important source of vitamins, particularly thiamin, riboflavin,
niacin, B6 and B12.
 Meat is one of the best sources of haem iron, with liver being a
particularly rich source that will prevent anaemia.
 Meat can enhance the absorption of non-haem iron from plant sources
when both are eaten together.
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 Meat is a useful source of zinc, in a bioavailable form, and of the trace
elements copper, manganese and selenium.
 Meat is relatively low in fat if the fat is trimmed off before cooking.
 Liver and kidney contain vitamins A and D.
Refer to Nutrients and their Effect on the Health and Well-being of
Individuals
Fish
There are many species of fish, both sea and freshwater varieties, which are
used for human consumption.
There are three main categories of fish:
1.
2.
3.
oily fish – for example herring, salmon, sardines, tuna, sometimes
called ‘pelagic’ as they are found in the middle and surface waters
white fish – for example haddock, halibut, cod, sometimes called
‘demersal’ fish as they are found at the bottom of the sea
shellfish – for example prawns, lobster, mussels, crab, sometimes called
‘invertebrates’ as they do not possess an internal skeleton.
Like meat, fish is the flesh or muscle of the animal. One of the main
differences between fish and meat is that fish deteriorates rapidl y and should
be eaten as soon as possible after being caught. Other variables affect the
quality and freshness of fish:

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
the difference in tissue composition of species
the influences of the season
the difference between salt and freshwater fish
the difference in procurement and holding practices on board fishing
vessels.
Structure and texture
 The flaky texture of fish is due to fish muscle, which consists of blocks of
short fibres called myotomes. The structure makes the texture very
different from meat. The myotomes are separated by thin sheets of
connective tissue.
 Fish has less connective tissue than meat and no elastin. As a result, fish is
more tender than meat.
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Post-mortem changes
There are several reasons for the rapid spoilage of fresh fish:
 microbiological
 physiological
 chemical.
Microbiogical – Bacteria exists on the surface of the fish and in its intestinal
tract. When the fish is killed, these bacteria attack the fish tissue. These
bacteria are resistant to low temperatures and therefore c ontinue to grow even
when fish are kept in refrigerated conditions.
Physiological –. Glycogen in the muscle of fish is used up as the fish
struggles when caught. This means that there is little glycogen left to be
converted to lactic acid after death. Lactic acid acts as a preservative by
slowing bacteria growth.
Chemical – As fish spoils rapidly a number of methods of preserving it have
developed, eg smoking, salting and drying. Freezing and canning are the most
common methods used nowadays to preserve f ish.
Changes during cooking
When fish is cooked, the muscle fibres (collagen) alter their structure and
coagulate to form the characteristic firm flakes of fish. The small amount of
connective tissue is easily denatured and converted to gelatine, at a lo wer
temperature than meat, and dissolves, therefore it cooks quickly and is easily
digested.
It is important not to overcook fish as this will result in a tough, dry texture
because of the hardening of the muscle fibres.
Fish colour
Fresh white fish should have a white translucent colour and a firm texture to
the fillet of the fish. Oily fish will have a pinkish tinge to the flesh and a firm
texture to the fillet of the fish.
Relationship to health
 All fish is a good source of high biological value prot ein, assisting the
growth, repair and maintenance of body tissues.
 Oily fish is a good source of polyunsaturated fats. This type of fat can
assist in the prevention of heart disease and cholesterol reduction, and
reduce the risk of strokes.
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 Oily fish contains between 10 and 20% unsaturated oil: n -3(omega-3).
 Fish is a rich source of omega-3 (a long-chain polyunsaturated fatty acid,
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PUFA), which has an important role in the diet to help lower the effects of
blood fats, decreasing the chances of the blood ve ssels being clogged up
with cholesterol. Omega-3 can also help the blood flow more easily around
the body by making it less ‘sticky’, therefore reducing the risk of heart
disease.
Omega-3 can help to reduce inflammation and may help ease the pain of
sufferers of rheumatoid arthritis.
Omega-3 has also been linked to improved brain function and may
improve concentration and the ability for children to learn.
White fish contains less than 2% oil.
All fish are a good source of some of the B vitamins – thiamine,
riboflavin, niacin and vitamin B6. These vitamins are essential for the
conversion of food to energy (preventing tiredness) and for healthy nerve
tissue (preventing impaired nerve function).
Oily fish are an important source of vitamin A, which is requir ed for
vision in dim light, normal growth in children and protection for surface
tissues.
Oily fish is a good source of vitamin D, which aids the absorption of
calcium, helping the development and maintenance of strong bones and
teeth and preventing osteoporosis.
Oily fish tends to be high in iron, which is required for the formation of
red blood cells, therefore it helps to prevent anaemia.
Fish is a good source of calcium, particularly when the bones are eaten, eg
tinned sardines. This assists the development and maintenance of strong
bones and teeth, and prevents osteoporosis.
Fish is an important source of phosphorus and magnesium and of the trace
elements iodine, fluorine and zinc.
Fish oils may possibly help prevent cancer cells progressing to the st age
where the person develops a tumour.
Fish oils can help some skin conditions, eg psoriasis.
A good intake of oily fish during the last 3 months of pregnancy is
believed to assist in the development of the child’s brain and retina.
The sodium content of oily fish can be high, which may lead to
hypertension, strokes and heart disease.
Refer to Nutrients and their Effect on the Health and Well -being of
Individuals
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Milk and milk products
The milk that is usually consumed in the UK is cows ’ milk. However, other
types are available, for example goats’ milk and ewes’ milk.
Composition
Milk is composed of a variety of nutrients either dissolved in water or
dispersed in a colloid. The colloidal system is complex but in simple terms is
a fat-in-water emulsion. The composition of milk varies depending on the age
and breed of cow, the animal feed given and the time of year.
Protein
The most important proteins in milk are caseinogen and the whey proteins
lactalbumin and lactoglobulin.
Fat
The fat in milk is easy to digest as it is very finely emulsified.
Carbohydrate
The type of carbohydrate in milk is called lactose , which is a disaccharide. In
sour milk the lactose has been changed by the action of certain bacteria to
lactic acid.
Uses in food preparation
Increasingly, milk is consumed in semi -skimmed and skimmed forms; each of
these is manufactured by centrifuging whole milk to remove the butterfat as
cream. By law, semi-skimmed milk must contain 1.5–1.8% fat and skimmed
milk 0.3% fat.
As milk is a rich source of nutrients it provides an ideal medium for the
growth of micro-organisms. It is therefore important that milk is heat treated,
for example pasteurised or treated at ultra-high temperatures (UHT milk), to
ensure that harmful organisms are destroyed before consumption.
Various types of milk are available, such as pasteurised, sterilised, spray
dried, evaporated and condensed. The greater the severity of the heat
treatment the greater the loss of vitamins. Dried skimmed milk contains just a
trace of fat and so lacks vitamins A and D. As a result it should not be used to
feed babies but it does provide a good source of protein, calcium and
riboflavin.
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The following substances are produced from milk and are used in a wide
variety of food production, both domestically and commercially.
Cream
 Cream is separated from milk by centrifugal force. This is a process which
involves spinning milk in a centrifuge so that heavier particles are forced
to the outside and the lighter particles, which make the cream, remain
towards the centre. The fat content of cream is regulated by law.
 Single cream must contain a minimum fat content of 18%.
 Double cream must contain a minimum fat content of 48%.
 Whipping cream must contain a minimum fat content of 35%.
 Clotted cream can have a fat content of up to 55- 70%.
 Cream contains all the fat and a proportion of the protein and lactose in
milk.
 Double cream has a higher energy value that single cream since it has a
higher fat content.
 All types of cream contain vitamins A and D.
Butter
 Butter is made by churning cream.
 Butter is a source of vitamins A and D. However, the vitamin content will
vary as in the winter months cows are fed differently so the vitamin
content is lower.
 Butter has a high fat content.
Crème fraiche
 This made from pasteurised cows’ milk to which a lactic acid bacteria
culture has been added.
 This thickens the cream and gives it a distinctive sharp flavour.
 A half-fat version is available.
 It has a longer shelf life than double cream so it can be sto red in the fridge
for a couple of weeks.
Yoghurt
 This is made by souring milk by using a pure culture of bacteria.
 The bacteria convert the lactose in milk into lactic acid.
 The acid brings about the coagulation of the milk proteins and helps
preserve the yoghurt.
 Since it is made from milk, the nutritional value of yoghurt is similar to
that of milk.
Cheese
 There is a vast range of cheeses available to the consumer. Regardless of
type, they are manufactured by milk being coagulated by a lactic acid
starter culture and then the solid is cut into small pieces to allow the whey
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to drain off. The solid curd is dried off, salt is added and the cheese is
pressed, moulded or ripened.
 Cheese is a more concentrated source of nutrients than milk due because of
its lower water content.
 Cheese does not contain carbohydrate, as milk does. The lactose is partly
converted into lactic acid and the remainder is lost in the whey.
Fromage frais
 This is a fresh, low-fat curd cheese (similar to cottage cheese but
processed until the texture is smooth and free from lumps) made from
pasteurised cows’ milk.
 Fromage frais has very little fat but there are varieties that have cream
added, which makes them better for cooking.
Changes to milk during cooking
 Boiling milk spoils the flavour and slightly reduces the food value of milk.
 When milk is heated, the whey proteins coagulate and a skin forms on the
surface. The skin holds steam and is responsible for the ease with which
milk boils over.
 During prolonged cooking, caramelisation of the milk sugar, lactose, may
occur. This contributes to the flavour of sterilised milk.
 Cooking destroys some of the thiamine and vitamin C in milk.
 Heat-sensitive vitamins (thiamine and vitamin C) are lost in processing.
Relationship to health
 Milk makes a contribution to the intake of a range of nutrients, especially
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protein, so assisting the growth, repair and maintenance of body tissue and
calcium for bone development and maintenance.
Milk contains a moderate amount of vitamin A and vitamin s B1, B2 and
nicotinic acid.
Milk is, however, deficient in iron and vitamins C and D.
Semi-skimmed and skimmed milk are not suitable for babies or infants
under the age of 5 as these groups require the energy from the fat in milk.
Cheese is a good source of calcium, phosphorus, protein and vitamin A.
Cheese also contains the B group of vitamins for energy release and
vitamin D to assist the absorption of calcium.
Cheese is high in saturated fat because of its concentrated nature so
consumption should be monitored to prevent the development of obesity.
A reduction in fat intake can be achieved by selecting reduced -fat versions
of all dairy products.
Refer to Nutrients and their Effect on the Health and Well-being of
Individuals
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Eggs
The new egg of a hen weighs approximately 60 g and comprises three major
parts: the shell, the white and the yolk.
The porous shell is composed mainly of calcium carbonate. The colour does
not indicate the quality of the egg. Inside the shell there are two thin
membranes, which separate the shell from the white. The white is divided
into regions of thick and thin white, and is held in position by a string of
protein called the chalzae. The air space in the egg determines the age and
quality of the egg. The air space becomes l arger as air enters through the
porous shell and collects in the air space and moisture evaporate s through the
porous shell. Bacteria as well as air and odours can pass through the shell and
therefore eggs deteriorate on storage.
Diagram of an egg
Composition
 The white of an egg is a colloidal solution of protein (mainly albumen) in
water together with small quantities of vitamins and minerals.
 The yolk is a fat-in-water emulsion and is approximately one -third fat,
one-third water and one-third protein.
Uses of eggs in food preparation
 Eggs increase the nutritional value of dishes.
 Eggs are an inexpensive ingredient with many properties so are an
invaluable component of many products.
 Thickening agent – egg protein coagulates, enabling mixtures to thicken,
for example egg custard.
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 Binding agent – coagulation of protein enables mixtures to hold together
on cooking, for example rissoles.
 Emulsifying agent – egg yolk contains lecithin, which is an emulsifying
agent used in the production of mayonnaise.
 Foaming – when egg whites are beaten, air is incorporated and the protein
partially coagulates and forms a foam. Meringues are prepared in this way.
 Aeration – eggs are used in creamed mixtures to produce lightness in
products.
 Flavour and colour – eggs provide a rich colour to some products and also
add flavour to otherwise insipid end products.
 Coating – eggs can be used in conjunction with breadcrumbs to coat food
to protect it whilst cooking, for example fish and Scotch eggs.
Changes during cooking
The following changes take place when eggs are cooked:
 When eggs are heated the protein in the white and the yoke coagulate s.
 Egg white proteins coagulate first at about 60 C. The white becomes
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opaque and forms a gel.
Yolk protein coagulates at 66°C and the yolks thicken.
The rate of coagulation is increased by the presence of salts and acid. Salt
and vinegar can be added to the water used for poaching eggs to bring
about rapid coagulation of the white.
Iron sulphate is formed in eggs during cooking and cau ses a black
discolouration around the yolk of eggs that have been hard boiled. This
reaction occurs more readily in stale eggs. Discolouration can be reduced
by placing eggs in cold water immediately after cooking.
There is a slight loss of vitamins, especially the vitamin B group, as a
result of cooking eggs.
Refer to: Food Science: Properties of food and their uses – protein
Relationship to health
 Eggs are an inexpensive food. Battery farming methods have reduced costs
but it has been agreed under an EU directive that barren battery cages will
be banned throughout the UK from 2012. Free-range eggs and organic eggs
are more expensive but because of animal welfare concerns sales of these
are increasing.
 The yolk contains two proteins – vitellin and livetin – containing all the
essential amino acids. This makes eggs a very valuable protein food to
assist the growth, maintenance and repair of body cells.
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 The fat in eggs is very finely emulsified and in an easily digested form so
it provides a source of energy for the elderly or convalescents.
 Eggs contain valuable amounts of iron, to prevent anaemia, and the fat-
soluble vitamins A and D.
 Eggs contain small amounts of vitamin D to assist calcium absorption,
riboflavin and thiamine for release of energy.
 Eggs also provide vitamin B2. Smaller amounts of the other B group
vitamins are also provided.
Refer to Nutrients and their Effect on the Health and Well-being of
Individuals.
Cereals and baked goods
Structure of cereal
Cereals are cultivated grasses, the seeds of which are used as a food source.
The most commonly used cereals or grasses are:
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wheat
rye
oats
barley
rice
maize.
The structure of all cereal grains is similar, with each type differing in detail
only. The structure of the grain can be divid ed into three main parts:
1.
2.
3.
the endosperm
the bran
the germ.
Diagram of a wheat grain
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The endosperm is the food reserves on which the young plant lives until it
has developed a root system. It consists of tightly packed cells containing
mainly starch with some protein and vitamin B complex.
The bran is a tough skin that protects the inner seed from soil organisms that
may attack it. It is an excellent source of non -starch polysaccharides, vitamin
B complex and minerals such as calcium, iron and phosphorus.
The germ is an embryo plant with a radical, which can grow into the root
system, and a plumule, which can subsequently develop into stems, leaves
and ears. A new plant can therefore grow from the germ under ideal
conditions. The germ is a very good source of vitamin B complex, especially
thiamine and vitamin E but this can be readily lost by oxidation.
Types of cereals
In Britain, the most commonly used cereal is wheat. There are two main
growing seasons for which different types of wheat are used:
1.
Winter wheat (soft wheat) is grown in Britain and is sown in autumn
and harvested the following August. It contains less than 10% protein
and therefore is low in gluten and gives a ‘weak’ flour that produces a
close texture. It is therefore best used for cakes and biscuits.
2.
Spring wheat (hard wheat) is sown and harvested in the same year in
countries like Canada which have a very severe winter. This wheat
contains 12–24% protein and is therefore higher in gluten. It produces a
‘strong’ flour that forms a strong elastic dough more suitable for bread making.
Semolina means semi-milled. It is the grains which have not been ground to a
fine flour. Semolina from hard, durum wheat is used for traditional pasta
making. The semolina is mixed with water and dried. It is produced in variety
of shapes by the extrusion process. Pasta provides starch, protein, B vitamins
and minerals. Wholemeal pasta provides a good source of dietary fibre.
Semolina from softer wheat is used in the UK for puddings and c akes.
Self-raising flour is made by adding an acidic raising agent and sodium
bicarbonate to soft flour to allow carbon dioxide to be formed during the
cooking process, which aerates the dough.
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Milling of flour
Milling is the process of converting grains into flour. In wholemeal flour all
parts of the grain are used, whilst in white flour only the endosperm is used.
When grain arrives at the flourmill it will be dirty and may contain other
particles, eg dust, mud and insects etc. All of this material m ust be removed
before milling can take place. This process is called cleaning. Many methods
and combinations of methods of cleaning are employed, eg sieves, magnets or
air currents, to ensure that the wheat is free from other particles.
The grain is then conditioned, which ensures that all the grains have the same
moisture content and distribution of moisture within each grain. Conditioning
assists in toughening the bran so that it can be easily separated from the
endosperm at the next stage.
Wheat grains are almost always milled into flour before being eaten. Some
flours are still produced using the traditional milling method where the wheat
is ground between two circular grooved stones, the upper of which moves
whilst the lower remains stationary. In thi s process, all the wheat grain can be
ground, producing wholemeal flour as the flour contains all the components
of the grain. This flour is described as stone -ground flour.
The modern method of producing flour is roller grinding. It can be divided
into two distinct stages:
1.
Breaking – The wheat grains pass between grooved rollers which
operate in pairs, the top one rolling faster that the lower. These rollers
are set to shear open the wheat grain to expose and remove the white
endosperm.
2.
Reduction – This stage reduces the endosperm to a fine flour. The
endosperm passes through a set of smooth reduction rollers , with each
pair being set more finely than the previous pair. After passing through
each set of rollers the product is sieved and coarse particles are passed
to the next set of rollers to continue the reduction process until the flour
granules are of the necessary size.
Changes during bread making
 Bread is made from a dough of wheat flour, water, salt and yeast.
 When the flour is kneaded with water, the two proteins present in flour,
gliadin and glutenin, become hydrated and form an elastic complex called
gluten. The manufacture of bread is possible due to this protein complex.
 Kneading develops a network of gluten in the dough .
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 The dough is left to rise. The gas produced stretches the gluten in the
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dough, producing little bubbles that become trapped, forming the
characteristic framework of the loaf. The dough increases in size due to
the production of carbon dioxide. This happens as a result of th e
fermentation of sugar by the yeast present in the dough.
When the dough is baked, the increase in temperature causes the carbon
dioxide bubbles to expand within the dough, thereby causing a further rise
in the volume of the bread.
During baking, the expansion of the carbon dioxide causes the bread to
rise rapidly and the alcohol is driven off.
The heating also causes the protein to ‘set’ , turning the dough into bread.
At a temperature of approximately 54 °C the yeast is inactivated.
The action of heat and steam on the outside of the bread forms dextrin ,
which converts to caramel, giving the crust its brown colour.
Sensory qualities
The character of a dough depends on the type of flour used to make it. Strong
flour contains more gluten and is therefore used to make bread as the dough
must be able to expand to a greater degree and yield baked products of
particularly light density. Weaker flours contain less gluten and are therefore
used to make cakes and biscuits.
Relationship to health
All varieties of bread make a valuable contribution to a healthy diet,
providing the following.
Starchy carbohydrates
The Scottish Diet Action Plan has recommended that the intake of complex
carbohydrates should increase, thereby replacing some of the more fatty and
sugary foods in our diet. Eating a diet rich in complex carbohydrates such as
bread, pasta, rice and potatoes is an easy and convenient way to fulfil this
aim.
Non-starch polysaccharides
All breads are a good source of non-starch polysaccharides, but wholemeal
bread is an especially good source because it contains a larger amount of non starch polysaccharides due to the inclusion of the bran of the wheat grain in
the flour.
Protein
Protein in bread comes mainly from gliatin and glutenin present in flour.
When a liquid is added to the flour, they combine to form gluten. Bread is a
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protein of low biological value as it contains little of the essential amino
acids required by the body.
Vitamins
Wholemeal bread is an especially good source of thiamine and ni cotinic acid
as both the bran and the germ sections of the wheat grain are rich sources and
are included in this type of bread. However, it is a legal requirement that
white flour is fortified with thiamine and niacin.
Folate is also located in the bran and the germ sections of the wheat grain and
therefore wholemeal bread is a good source. Some white bread is also an
excellent source of folate due to fortification.
Minerals
Iron and calcium are found in the germ and bran sections of the wheat grain
and therefore wholemeal bread is a good source of these minerals. However,
white bread is also fortified with both minerals. Studies have shown that iron
can be poorly absorbed from bread as it can pass through the body
unabsorbed, therefore iron in bread may have a low bioavailability.
Coeliac disease arises in people who are sensitive to gluten. It is thought to
occur in 1 in 2000 people in the UK, although in western Ireland the
incidence is much greater: 1 in 300. The coeliac condition can develop at any
age in any person. In infants the symptoms usually develop shortly after the
introduction of solid foods, containing gluten, to the diet. The symptoms
usually are:
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loss of appetite
weight remains stable or decreases instead of increasing
irritability
listlessness
abdominal swelling
diarrhoea.
Adult coeliacs are usually diagnosed from the symptoms of the effects of
gluten on the small intestine, namely:
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diarrhoea
abdominal swelling
discomfort
pain
vomiting.
In many cases diagnosis is difficult due to the many similarities to other
diseases.
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The damage which occurs prevents the small intestines from fully absorbing
nutrients from digested foods. This can lead to severe illness similar to
malnutrition.
The only effective treatment is the exclusion of gluten from the diet. A
gluten-free diet always excludes wheat, rye and barley. Oats may have to be
excluded if the sufferer has an intolerance to them also. This diet needs to be
strictly adhered to for life to reduce the risk of further symptoms and
associated conditions.
Bakery products
Wheat flours find their principle applications in the production of bakery
products. Most bakery products are leavened, ie they are raised by some
means to yield baked goods of low density. Baking strictly refers to heating
dough products in an oven but there are many steps that must take place
before that if the product is to be successful.
A general classification for baked goods is as follows:
 Yeast-raised goods. These include breads and sweet doughs leavened by
carbon dioxide from yeast fermentation.
 Chemically-leavened goods. These include cakes and biscuits raised by
carbon dioxide from chemical agents, eg baking powders. Baking powders
used in cake-making contain particles of sodium bicarbonate as a source of
carbon dioxide when water and heat are supplied.
 Air-leavened goods. These include sponges whisked to include air, which
is then used to raise the product.
 Partially leavened goods. These include pie crusts, certain crackers and
other items where no intentional leavened agents are used yet a slight
leavening occurs due to the expanding steam and other gases during the
baking process.
Other ingredients that play a role in baked products include sugar, fats and
shortenings.
Sugar
 Sugar provides sweetness to baked goods.
 Sugar helps to colour baked goods. When it is heated in a liquid it begins
to caramelise at 154°C.
 When sugar and amino acids interact together in baked products then non enzymic browning takes place (also called the Maillard reaction).
 Sugar also helps to aerate mixtures and so makes them light and risen.
When fat and caster sugar are creamed together, the air sticks to the sugar
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crystals. The fat surrounds the air bubbles and traps them in the mixture
and so the mixture rises when cooked.
Sugar increases the coagulation temperature of eggs and gluten in a
mixture. The air bubbles in the mixture have more time to expand before
the mixture sets, making the product lighter and well risen.
Sugar helps foams such as meringues to remain stable so that the air which
has been whisked in is not easily lost. The foam is strengthened by the
sugar, allowing the meringue to be piped or spread without bursting the air
bubbles.
Sugar is attracted to water. This property helps products to remain moist
and helps to maintain the texture and mouth feel of the product.
The water-attracting property of sugar also helps to make baked products
tender. Sugar takes up some of the water that would be taken up by the
protein (gluten) in the flour. Gluten development is reduce d and so a softer
and more tender crumb is produced.
Refer to: Food Science: Properties of food and their uses – carbohydrates
Fats and shortenings
 Fats such as soft margarine trap air bubbles when creamed with sugar. For
this to be successful, the fats should have a mouldable, pliable property
called plasticity.
 Fats have a shortening effect in pastry and biscuits. The fat coats the
particles of flour and reduces the amount of water that can be absorbed by
the flour due to the waterproof coating of fa t. The plasticity of the fat
allows it to surround and coat the flour particles.
 The shortening effect results in a crumbly short texture. When only a little
water is absorbed by the flour, less gluten is developed that would make
the mixture elastic and stretchy, so the mixture is ‘shortened’.
Refer to: Food Science: Properties of food and their uses – fats and oils
Other types of cereals
Rice
 A grain of rice has the same structure as a grain of wheat.
 Milling then ‘polishing’ the rice removes the br an and germ and so also
removes most of the vitamin B complex Where rice is a staple food,
thiamine deficiency is common and a disease called beri beri is caused.
 Par-boiling of rice prior to milling means that the majority of thiamine,
instead of being lost in the bran, migrates into the grain so it is not lost
during milling.
 Brown rice is not polished but contains the whole grain. It is higher in
dietary fibre as well as thiamine.
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Oats
 Oats are fairly rich in protein but do not contain gluten.
 Oats contain more fat than other cereals (about 7%) so care has to be taken
during storage to prevent rancidity.
 Oats can contribute to health as they have been shown to lower blood
cholesterol levels.
 Oats contain phytic acid but only in extreme cases would thi s affect the
absorption of calcium.
Rye
 The rye protein contains some gluten but it lacks elasticity so rye bread
lacks volume and is heavy in texture.
 Doughs made from rye are sometimes soured with starter cultures , which
give acidity and a characteristic flavour to the bread.
Maize
 Maize contains some protein but it is poor quality and as it contains no
gluten it cannot be used for bread.
 Most maize is eaten as sweetcorn or corn on the cob.
 Cornflour is made from maize and is almost pure starch. It is used for
thickening sauces and soups.
Barley
 This is a very hardy cereal and is used in the brewing industry .
 It contains less protein and less fat than wheat and because of its low
gluten content is not used in baking.
 ‘Pearl’ barley has had most of the bran and germ removed and is used in
broths and barley water.
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