1. The available methionine and cysteine contents of proteins were determined by chemical methods after preliminary enzymic hydrolysis.2. The values for the available methionine and cysteine contents of pure proteins (casein and bovine serum albumin) estimated by chemical methods were similar to those for the total content determined by the method of Moore, Spackman & Stein (1958).3. Reductions of 15 and 11% respectively, when compared with unprocessed samples, were found in the available methionine contents of sweetened and unsweetened, condensed milks; of roller-dried milk and whey powders, and of mackerel sterilized at 126°, the reductions were 22, 14 and 19% respectively.4. The available cysteine content of sweetened, condensed milk was reduced by about 32%, whereas for mackerel sterilized at 115 and 126° it was reduced by 64 and 75% respectively.5. The contents of total sulphur amino acids for these food products did not differ from those for the unprocessed samples.6. Values obtained for available S amino acid contents by rat bioassay confirmed the results of the in vitro estimations.
The authors in previous work have shown that the NPUop of the same diet, determined parallely for children and for rats, did not give identical results. This was interpreted as being due to differences in the rate of N growth of the tissues related to endogenous N exchange by compared organizms. The purpose of the presented paper was to elucidate, in the model experiment (using the rat of different ages), the influence of the growth rate of animals on the value of the NPUst and NPUop coefficients of a few chosen proteins. The diets containing 4, 10, and 20% of protein as egg albumen, casein or gluten (with a small addition of albumen) were tested. The NPUst, NPUop, and urea N to creatinine ratio (Nu/Nc) were determined in the experiments with rats of 25, 45, 70, and 135 days of age. It was found that, for all tested proteins, NPUop coefficient was modulated by different degrees by the growth rate of the animals; parallel changes of Nu/Nc were also observed. The NPUst coefficient did not show distinct changes under the influence of the different growth rates of rats and keeping the constant relationship with the chemical score of the tested proteins. The authors are concluding that the protein of the singular product, as well as a protein in the mixed human diet, should be evaluated on rats in the same conditions of low (about 4 NDp Cal %) protein level in the diet.
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