Value of protein

Osborne and Mendel continued their extensive studies of the nutritive values of proteins while extending the list of foods known to contain vitamines. (4jU 42) They attempted to isolate the growth promoter in butter with no success but they made significant progress in their quest for a totally synthetic diet by developing an excellent salt mixture for the basal dietaries of their rats. (43)... 

The nutritional value of proteins (see p. 360) is decisively dependent on their essential amino acid content. Vegetable proteins—e.g., those from cereals—are low in lysine and methionine, while animal proteins contain all the amino acids in balanced proportions. As mentioned earlier, however, there are also plants that provide high-value protein. These include the soy bean, one of the plants that is supplied with NH3 by symbiotic N2 fixers (A). 

The influence of neutral salts as well as of acids and bases on the swelling of gelatine which we have seen can be attributed to an apparent change in the solvation of the gel fibrils and may be interpreted in the light of Donnan s theory of the effect of a non-diffusible ion on the osmotic pressure differences between the two phases, is likewise to be noted in the alteration of the viscosity and alcohol precipitation values of protein solutions. From the considerations already advanced there should exist two well-defined maxima in the viscosity and alcohol precipitation curves when these properties are plotted as functions of the Ph, the maxima coinciding with the points of maximum dissociation of the salts... 

At low and medium doses, it is well established that the nutritional value of proteins, carbohydrates, and fats as macronutrients are not significantly impaired by irradiation, and neither the mineral bioavailability is impacted. Like all other energy depositing process, the application of ionizing radiation treatment can reduce the levels of certain sensitive vitamins. Nutrient loss can be minimized by irradiating food in a cold or frozen state and under reduced levels of oxygen. Thiamin and ascorbic acid are the most radiation sensitive, water-soluble vitamins, whereas the most sensitive, fat-soluble vitamin is vitamin E. In chilled pork cuts at the 3 kGy maximum at 0-10°C, one may expect about 35 0% loss of thiamin in frozen, uncooked pork meat irradiated at a 7 kGy maximum at —20°C approx., 35 % loss of it can be expected [122]. 

The initial Schiff base is digestible but after the Amadori rearrangement, the products are not metabolically available. Since lysine is the amino acid most likely to be involved and is an essential amino acid, Maillard browning reduces the biological value of proteins. Interaction of lysine with lactose renders the adjacent peptide bond resistant to hydrolysis by trypsin, thereby reducing the digestibility of the protein. 

Friedman, M. 1996. Nutritional value of proteins from different food sources. A review. J. Agric. Food Chem. 44 6-29. 

To study the effect of the Maillard reaction on nutritive value of protein, Patton, et a l. (57) heated purified casein and soybean globulin in 57. glucose solution for 24 hrs at 96.5°C, and found significant losses of lysine, arginine, tryptophan, and histidine (52). 

Zduhczyk, Z., Juikiewicz, J., Flis, M. and Frejnagel, S. (1996) The chemical composition and nutritive value of low-alkaloid varietiesofwhitelupin.2.Oligosaccharides, phytates, fatty acids and biological value of protein. Journal of Animal and Feed Science 5, 73-82. 

Table 1.3 Examples of the p/values of proteins (various sources)...

Increasing Nutritional Value. Few direct attacks have been reported on increasing the nutritional value of proteins by those chemical modifications that incorporate substances in the protein. A successful exception has been the incorporation of amino acids into proteins. Since a separate presentation on this subject is given in this volume (113), only a brief discussion will be presented here. 

The chemical modifications of the tryptophan residues lead to a decrease in the nutritive value of proteins as observed in autoclaved soja meals (124), heated meats (125), heated casein (126), and heated skim milk (122) this last reference is probably the most reliable work published in this field. The nutritional effects and the metabolic transit of heat-treated and oxidized tripeptide (gly-try—gly) have been investigated (123,132,137) recently only the metabolic transit study is related here. 

No, it is inconsistent with this hypothesis, which predicts an H+/e value of 1.0. Such stoichiometries may, however, be explained by proton-pump mechanisms, in which electron transfer is coupled to changes in the pKa values of proteins within Complex I. 

Nutritional Implications. The nutritive quality of any protein depends on three factors amino acid composition, digestibility, and utilization of the released amino acids. Bacemization brought about by processing can impair the nutritive value of proteins by (a) generating non-metabolizable forms of amino acids (D-enanticmers), (b) creating peptide bonds inaccessible to proteolytic enzymes, and (c) toxic action (or interaction) of specific D-enanticmers. Little is known concerning the health consequences of human consumption of racemized proteins. No study has specifically evaluated amino acid losses due to racemization within food proteins. 

Since temperature has a marked effect on the pK of the carrier ampholytes and immobilines, and on the pi values of proteins, IEF gel runs should be thermostatted to achieve reproducible results. Also, for rapid IEF separations, it is necessary to use higher voltages. This produces a lot of heat which is best removed by running the gel on a cooled plate (10 °C) it is also desirable to even out thermal fluctuations by using stabilised power packs such as those used for DNA sequencing. 

More recently, software has been developed that predicts H, C, and N chemical shift values of proteins from either 3D stmcture files, for example, SHIFTS (6), SHIFTX (7), and SPARTA (8), or from the mere amino acid sequence using SHIFTY (9). First results have been reported on the de novo stmcture determination of proteins using fragment-based chemical shift predictions and molecular modeling (10, 11). 

The calculation of pK values of protein molecules thus requires a detailed consideration of the environment of each ionisable group, and is consequently highly complex. An additional complication is that a protein with N ionisable residues has 2 possible ionisation states the extent of the problem is apparent when it is realised that a moderately sized protein may contain as many as 50 ionisable groups. 

All nutrients (except water) may undergo either chemical or physical changes during processing that render them inactive or less bioavailable. This occurs for macro- as well as micronutrients. During browning, the amino acid lysine can react with carbohydrates to lower the biological value of protein. 

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