Protein relative nutritive value

Dietary Protein Relative Nutritional Value %Protein Required to Alleviate 90%... 

Hegsted, D. M., Neff, R., and Worcester, J. (1968). Determination of the relative nutritive value of proteins. J. Agric. Food. Chem. 16,1%-195. 

Protein Content. The protein content of milk can be determined using a variety of methods including gasometric, Kjeldahl, titration, colorimetric, and optical procedures (see Proteins). Because most of the techniques are too cumbersome for routine use in a dairy plant, payment for milk has seldom been made on the basis of its protein content. Dye-binding tests have been appHed to milk for determination of its protein content these are relatively simple to perform and can be carried out in dairy plant laboratories. More emphasis will be given to assessing the nutritional value of milk, and the dependence on fat content as a basis for payment will most likely change. 

Nutritive value of foods and feedstuffs depends to a large degree on protein level and quality, i.e., the relative amounts of the component amino acids compared to the requirements of the animal for various metabolic functions. The cereal grains are notoriously low in certain essential amino acids. Usually lysine is the first or second limiting amino acid. The grain of rye (Secale cereale L.) exhibits an amino acid profile superior to that of other cereal grains, especially wheat (1,2,3,4,5). Despite this fact, lysine is still the first limiting amino acid in rye in most instances (6,7). 

Zduhczyk et ol. (1996) studied the nutritive value of low-alkaloid varieties of white lupin and found that the lysine contents were relatively low (4.70-5.25g/16g N), with methionine as the limiting AA. Roth-Maier and Paulicks (2004) studied the digestibility and energy contents of the seeds of sweet blue lupins (L. angustifolius) and found digestibility coefficients of 0.43-0.5 for organic matter, 0.36-0.43 for protein, 0.69-0.83 for fat, 0.46-0.58 for NFE and an ME concentration of 7.54-8.22 MJ. 

Our results have been recapitulated with other proteins of varying nutritional value to S. exigua and H. zea they include soy protein, tomato foliar protein, corn gluten and zein. In all cases, more than 2.5% dietary protein was required to alleviate antinutritional effects, because these proteins are less nutritious than casein (Table III). The ability of a protein to alleviate the toxicity of o-quinones is proportional to its nutritional value to the insect (Table III). The proteins ability to function as an alkylatable sink (alleviation of antinutritive effects) is correlated with the relative amounts of alkylatable amino acids (e.g., lysine, cysteine, histidine, methionine Felton and Duffey, unpublished data). 

In some products, however, even large losses in nutritional value would be inconsequential. This would be the case when the chemically modified product would be used to supply certain physical properties, and the amounts of proteins supplied by the product would be relatively small compared with the total protein composition. In other instances, only a very small amount of the total protein content might be modified, although that small amount would be modified extensively. Such an example will be discussed below in the chemical treatment of certain grains or seeds, wherein apparently only a small percentage of the total proteins (the surface proteins) are modified. 

From the differences observed in the products of hydrolysis of the various albuminoids, can we draw any practical conclusions as to the nutritive value of these same proteins In particular, as regards gelatin, which cannot be regarded as an albuminoid food properly speaking, should we seek the cause of its nondigestibility in the excess of glycin formed, or in the relatively... 

Amino acids arising from protein hydrolysate This analysis is used often in food industry as well in basic biochemistry. It is important in determination of the structure of protein and in the assessment of the nutritional value of different proteins. Here the analysis of about 20 amino acids is sufficient. This type of analysis is relatively more difficult than that for a single amino acid. 

Table II. The Relationship Between the Nutritive Value of Various Proteins, as Indexed by Relative Growth Rate, to Larval SoodoDtera exigua and Heliothis zea as a Function of Alkylatable Amino Acids...

The amino acid composition of the protein fraction is very similar between different microalgae and relatively unaffected by light and nutrient conditions (Brown et al, 1997 Daume et al., 2003). On this basis, it is unlikely that protein quality is a major factor contributing to differences in the nutritional value of microalgae. 

Since biological value is dependent primarily upon essential amino acid constitution, it would seem logical to assess the nutritive value of a protein by determining its essential amino acid constitution and then comparing this with the known amino acid requirements of a particular class of animal. Application of modern chromatographic techniques coupled with automated procedures allows relatively quick and convenient resolution of mixtures of amino acids. However, the acid hydrolysis used to produce such mixtures from protein destroys practically all the tryptophan and a considerable proportion of the cystine and methionine. Tryptophan has to be released by a separate alkaline hydrolysis, and cystine and methionine have to be oxidised to cysteic acid and methionine sulphone to ensure their quantitative recovery. Losses of amino acids and the production of artefacts, which are greater with foods of high carbohydrate content, are reduced if the hydrolysis is carried out in vacuo. Evaluations of proteins in terms of each individual amino acid would be laborious and inconvenient, and several attempts have been made to state the results of amino acid analyses in a more useful and convenient form. 

Dried yeast is a protein-rich concentrate containing about 420 g crude protein/kg. It is highly digestible and may be used for all classes of farm animal. The protein is of fairly high nutritive value and is specially favoured for feeding pigs and poultry. It is a valuable source of many of the B group of vitamins, is relatively rich in phosphorus but has a low calcium content. Other yeasts are now available as protein concentrates these are described in Chapter 23. 

The nutritional value of a protein also depends on its digestibility here again, plant proteins tend to be inferior to animal proteins. Their poor digestibility results from (1) the presence of peptide bonds which are relatively resistant to the digestive enzymes, (2) the presence of enzyme inhibitors such as the trypsin inhibitor present in soya beans (page 177), and (3) the presence of fibre which hinders the access of proteolytic enzymes (page 130). 

Although protein quality is important when considering individual dietary proteins, it is not particularly relevant when considering total diets, because different proteins are limited by different amino acids, and have a relative excess of other essential amino acids. This means that the result of mixing different proteins in a diet is to give an unexpected increase in the nutritional value of the mixture. Wheat protein is limited by lysine and has a protein score of 0.6 pea protein is limited by methionine and cysteine and has a protein score of 0.4. A mixture of equal amounts of these two individually poor-quality proteins has a protein score of 0.82 - as high as that of meat. 

Whey provides food processors with an inexpensive product that can be used as a source of lactose, milk solids, milk proteins, or total solids. Additionally, whey, which is relatively high in digestibility and nutritive value, is commonly used as a livestock feed. 

V. R. Young and N. S. Scrimshaw, Clinical Research Center and Department of Nutrition and Food Science, Massachusetts Institute of Technology, make the following authoritative statement relative to the nutritional value of processed soy protein in human nutrition ... 

---