Composition biological value

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. 

The composition described includes sugar, cocoa powder, stabilizer, Jerusalem artichoke powder, and milk base. The pudding has improved taste, higher biological value, and curative and prophylactic properties. 

The biological value of food proteins is greatly influenced by the ratio of their essential amino acids. Most food proteins, first of all plant proteins, are deficient in some essential amino acids. The limiting essential amino acid in legume and milk proteins is methionine, and in cereal proteins it is lysine. Beside the knowledge of the requirements of quality and composition of amino acids, it is equally important to know whether these amino acids can be best utilized as free amino acids, in the form of peptides or proteins. The following possibilities exist for improving the balance of essential amino acids in the proteins ... 

The amino acid composition of some )S-lg variants is shown in Table 4.4. It is rich in sulphur amino acids which give it a high biological value of 110. It contains 2 moles of cystine and 1 mole of cysteine per monomer of 18 kDa. The cysteine is especially important since it reacts, following heat denatura-tion, with the disulphide of K-casein and significantly affects rennet coagulation and the heat stability properties of milk it is also responsible for the cooked flavour of heated milk. Some /S-lgs, e.g. porcine, do not contain a free sulphydryl group. The isoionic point of bovine j3-lgs is c. pH 5.2. 

Whenever the calculated biological value was compared to experimental determinations of the biological value, the two values were found to be extremely close. Such mathematical formulas are limited by the fact that the proteins are not all digested at equal rates. This difficulty has been overcome by estimating the amino acid composition not directly after total hydrolysis of the protein, but on the product of peptic and tryptic digestion of the protein. Such experiments have revealed that whereas the amino acids of the proteins contained in roast beef or wheat or peanut flour are readily available, the amino acids of cotton seed flour are not [27]. 

The amino acids absorbed by the animal are required for the synthesis of body proteins. The efficiency with which this synthesis is effected depends partly on how closely the amino acid proportions of the absorbed mixture resemble those of the body proteins and partly on the extent to which the proportions can be modified. The biological value of a food protein therefore depends upon the number and kinds of amino acids present in the molecule the closer the amino acid composition of the food protein approaches that of the body protein, the higher will be its biological value. Animals have little ability to store amino acids in the free state, and if an amino acid is not required immediately for protein synthesis then it is readily broken down and either transformed into a non-essential amino acid or used as an energy source. Since essential amino acids cannot be effectively synthesised in the animal body, an imbalance of these in the diet leads to wastage. Food proteins with either a deficiency or an excess of any particular amino acid will tend to have low biological values. 

The nature and extent of the chemical changes induced in proteins by food processing depend on a number of parameters, for example, composition of the food and processing conditions, such as temperature, pH or the presence of oxygen. As a consequence of these reactions, the biological value of proteins may be decreased ... 

The plastein reaction can help to improve the biological value of a protein. Figure 1.52 shows the plastein enrichment of zein with tryptophan, threonine and lysine. The amino acid composition of such a zein-plastein product is given in Table 1.41. 

Membrane Composition. Biological membranes consist mostly of protein and lipid in roughly comparable quantities by weight. The overall composition of a variety of different membranes is given in Table 1.1. Values range from the relatively protein-rich inner mitochondrial and bacterial membranes which contain 70-80%... 

Table II. Carbohydrate compositions (weight percentage) of individual oligomer peaks purified (QAE-Sephadex or HPLC ion-exchange separation, respectively) from mixtures of citrus pectin oligomers or B fruit extracts Compositions shown are for peaks whose biological activity is described in Figure 4. Uronic acid values are based on colorimetric assay. Proportions of neutral sugars were determined by GC and adjusted so that totals equal 100%. In fact, some oligomers (G7 peaks 8, 9 and 10. B extract peak 10) produced small (less than 1 % of the total integrated area), unknown peaks in the GC chromatograms.

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