Protein content of foods
• the molecules of proteins are large and are made
up of amino acids which are linked together.
• indispensable compounds of the animals and
humans are proteins (enzymes, hormones, body
tissues)
• valuable animal products
• protein can be synthesised only from protein or
N-containing foods
• influence on the price of the animal products
• in lots of countries protein sources are not
enough
• crude protein
– based on N-determination
– the average N-content of food proteins is 16%
– crude protein = N x 6,25 (100/16 = 6,25)
• digestible crude protein
– the protein content which is absorbed from the
digestive tract
– species dependent
• real protein
– the protein content from biological point of view
• Non Protein Nitrogen (NPN) or amid materials
– peptides, amino acids, nitrates, amides
Using crude protein in the nutrition of farm animals is the
most common.
Amino acids
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Amino acids are produced when proteins are hydrolysed by
enzymes
Among over 200 amino acids can found in biological materials,
only 20 are found commonly in proteins
The quality of proteins depends on their amino acid
composition.
It is amino acids rather than proteins that are essential to the
body.
Humans, unlike plants, cannot make amino acids from simpler
substances such as nitrates and so they must be supplied in
the diet.
The body is able to convert some amino acids to others
(glycine into alanine for example) and such amino acids are
called nonessential.
Amino acids
• Essential or indispensable amino acids
– can not be synthesized in the body,
– the rate of synthesis is not enough
• Semi-essential amino acids
– the requirement of certain amino acid ids dependent upon
the presence of other amino acids
• Limiting amino acids
– the essential amino acid which is present in the lowest
amount relative to the requirement
• Ruminants are self-sufficient in essential amino acids
Classification of amino acids
essential
partly essential
non- essential
arginine
asparatic acid
histidine
asparagine
isoleucine
alanine
leucine
glutamic acid
lysine
glutamine
methionine
cystein
serine
phenylalanine
tyrosine
proline
threonine
tryptophan
valine
glicine
methionine
lysine
Deficiency and surplus of amino acids
Complementation of amino acids
• Proteins are limited by different amino acids and have a relative
excess of others
– wheat protein provides only 62% of the requirement for lysine
and excess amount of methionine + cystine
– pea protein provides only 49% of the requirement for
methionine + cystine, but more than the requirement of lysine
• Mixing different proteins increase the nutritional value of the
mixture
– mixing equal parts of wheat and pea provides a protein that is
still limited in methionine + cystine, but its rate is already 77%
• Cristallyne amino acids (lysine, methionine, threonine, tryptophan,
arginine)
• Non-essential amino acids must be also available
Some parameters that influence the
utilisation of amino acids
• Energy content of the diet
• Antagonism between amino acids
LEU – ILE; LYS – ARG; ILE – VAL; PHE – VAL; THR –
PHE
• Toxicity of amino acids (crystalline methionine)
• Digestibility
– denaturation improves the digestion
– Anti-nutritive compounds can decrease
• Heat damage
– Maillard reaction, bounds between the amino acids and sugars
• The structure of the protein
• Timing
– all the amino acids must be available at the same time
Digestion and metabolism of protein
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Food proteins are digested into single amino acids in the
gut by the enzymes pepsin (from the stomach), trypsin and
others (from the pancreas) and finally by enzymes in the
brush border of the enterocytes (the cells lining the small
intestine).
The amino acids pass first to the liver and then, in the
blood, to the cells for protein synthesis.
The digestibility of proteins is the percentage of dietary
protein which is absorbed.
Proteins from eggs, meat and other animal sources have a
higher digestibility than vegetable proteins, with a range of
values from 78% for a diet of rice and beans, to 96% for a
typical meat-based diet.
NPN (non protein nitrogen) compounds
• Amines
– they are present in small amounts in plant and animal tissues
– microorganisms can produces amines from amino acids
– lysine – cadaverine
– arginine – putrescine
NH2
urea
– histidine – histamine
O=C
– betain, trimetil amine (fishy taint)
NH2
• Amides
– urea (endproduct of nitrogen metabolism in mammals)
– uric acid (the principal endproduct of nitrogen metabolism in
birds)
• Nitrates
– present in plant materials
– not toxic, but in the rumen it can be reduced to nitrite, which is
toxic
NPN (non protein nitrogen) compounds
• Nitrates
– present in plant materials
– not toxic, but in the rumen it can be reduced to nitrite, which is
toxic
• Alkaloids
– many of them have poisonous properties
– Solanine (unripe potatoes, potato sprouts)
– Lupinin (lupine seeds)
– Nicotine (tobacco)
– Cocaine (leaves of coca plant)
• Nucleic acids (ribonucleic acid; RNA), deoxyribonucleic acid;
DNA)
– high molecular weight compounds
– store of genetic information
– after hydrolysis they yield nitrogenous compounds (purines,
pirimidines), pentose (ribose, deoxyribose) and phosphoric
acid
Qualification of food proteins
Animal experiments
1. Biological value (BV)
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a N balance trial is used
protein intake should be not more than the potential retention
utilised N
BV = ----------------------------digested (absorbed) N
N intake – (faecal N – MFN) – (urinary N – EUN)
BV = --------------------------------------------------------------------N intake – (faecal N – MFN)
MFN = metabolic (endogenous) faecal N
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digestive enzymes
erosion of epithelial cells
microbial N
EUN = endogenous urinary N
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urinary N coming from the breakdown of body proteins
The biological value of some food proteins
Food
Biological value
Egg or human milk
1.00
Skimmed milk
0.85
Meat, fish protein
0.75
Wheat protein
0.50
Peanut
0.40
Gelatine
0.00
2. PER (protein efficiency ratio)
gain in body weight (g)
PER (%) = ---------------------------------------------protein consumed (g)
3. NPR (net protein ratio)
weight gain A – weight loss B
NPR (%) = --------------------------------------------------protein consumed (g)
A = group given the test protein
B = group on protein free diet
Chemical methods
1. CS (chemical score)
– The essential amino acid content of certain food protein is compared with a
standard protein (egg, recommendations of FAO, or WHO).
–The lowest proportion is taken as the chemical score.
2. essential amino acid index (EAAI)
– The geometric mean of the amino acid ratios, used in the CS determination
n ratio * ratio *…ratio
1
2
n
In ruminants there is not a close relation between
the amino acid content of food protein and its
quality
Calculation of chemical score
essential amino
acid
essential amino acids mg/g protein amino acid ratio
(%)
soybean protein
egg protein
histidine
25
22
114
isoleucine
45
54
83
leucine
78
86
91
lysine
64
70
91
methionine +
cystine
26
57
46
phenilalanine +
thyirosine
80
93
86
threonine
39
47
83
thriptophan
13
17
76
valine
48
66
73
The CS value of soybean protein is 46%; its limiting amino acid are MET+CYS
Crude protein content of some feedstuffs
(g/kg dry matter)
alfalfa (early flowering)
170
potatos
90
corn silage
110
grass hay (good quality)
110
wheat grain
124
corn grain
98
soybean
415
lupin seed
440
extracted sunflower meal (decorticated)
430
extracted rapeseed meal
400
Crude protein content of some compound feeds (g/kg)
pigs (20-50 kg)
205
pigs (50-90 kg)
175
broiler chicks (0-3 week)
230
broiler chicks (5-6 week)
180
turkeys (0-6 week)
300
turkeys (12-16 week)
180
laying hens
150
The BSE story
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BSE (bovine spongiphorm encephalomacy)
1980-ies United Kingdom
Feeding meat meal of sheep origin
Not sufficient heat treatment
Infection in cattle, milking cows
pathogen: prion
Results:
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Huge economic losses
Consumption of beef decreased dramatically
the confidence against animal products decreased
Ban for using feeds of animal origin in the EU
Human aspects
Effects of protein deficiency
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A deficiency of protein may occur in two ways:
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through a low intake of protein
• or through a low energy intake.
Some degree of adaptation to reduced intakes does occur.
With very severe protein deficiency, the concentration of the
plasma proteins falls and this causes generalized oedema
(sometimes called hunger oedema).
Prolonged deficiency of protein and energy in children
causes proteinenergy malnutrition, in the forms of
marasmus or kwashiorkor, which occur in some developing
countries.
Effects of excessive intake of protein
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There has been some concern about the health risks of
an excessive intake of protein because it may cause
demineralization of the bone or gout.
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It has also been noted that populations eating vegetarian
diets, with lower average protein intakes than omnivores,
have lower average blood pressures.
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These facts, along with the effect of dietary protein of
increasing excretion in the kidney, it was recommended
for adults to avoid protein intakes of more than twice the
requirement.
Dietary requirements for protein
Age
Children 0-3 months
4-6 months
7-9 months
10-12 months
1-3 years
4-6 years
7-10 years
Males 11-14 years
15-18 years
19-50 years
50+ years
Females 11-14 years
15-18 years
19-50 years
50+ years
Pregnancy
Lactation 0-6 months
6+ months
Weight (kg)
Requirement (g/day)
5.9
7.7
8.8
9.7
12.5
17.8
28.3
43.0
64.5
74.0
71.0
43.8
55.5
60.0
62.0
12,5
12,7
13,7
14,9
14,5
19,7
28,3
42,1
55,2
55,5
53,3
41,2
45,4
45,0
46,5
+6
+11
+8
Protein content of some human foods
Food
Amount
Protein
content (g)
Bread
Six slices
18.9
One, size 2
7.5
200 g
10.2
280 ml
9.0
Two
11.7
Egg
Baked
beans
Milk
Sausages
Total
57.3