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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
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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 • 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
methionine lysine
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 • 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 • histidine – histamine • betain, trimetil amine (fishy taint) • 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 NH2 urea O=C NH2
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 • Biological value (BV) • 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 • digestive enzymes • erosion of epithelial cells • microbial N EUN = endogenous urinary N • urinary N coming from the breakdown of body proteins
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 In ruminants there is not a close relation between the amino acid content of food protein and its quality n ratio1* ratio2*…ration
Calculation of chemical score The CS value of soybean protein is 46%; its limiting amino acid are MET+CYS
The BSE story • 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: • 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 • A deficiency of protein may occur in two ways: • 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 • There has been some concern about the health risks of an excessive intake of protein because it may cause demineralization of the boneor gout. • It has also been noted that populations eating vegetarian diets, with lower average protein intakes than omnivores, have lower average blood pressures. • These facts, along with the effect of dietary protein of increasing excretion in thekidney, it was recommended for adults to avoid protein intakes of more than twice the requirement.
