The Effect of Amino-Acid Supplements and of Variations in Temperature and Duration of Heating Upon the Biological Value of Heated Casein

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Abstract
The biological value of casein heated at 140° for 30 minutes alone and with supplements of lysine, cystine and tyrosine was in all cases inferior to that of the corresponding raw protein both with and without the amino acids except in the case of the lysine-supplemented heated casein. This protein yielded an average biological value of 64 compared with 69 for the raw and 57 for the unsupplemented heated diet. The digestibility of the heated casein was only slightly lower than that of the raw and was decreased in all cases by the amino-acid additions even when the biological value was increased. The value for growth of the heated casein alone and supplemented by lysine, cystine, tyrosine, tryptophane and histidine was likewise inferior to that of the raw casein at all levels both with and without the amino-acid supplements except when the lysine and histidine supplements were used. The lysine supplement made up about two-thirds, and the histidine about one-half of the 14 to 19% loss of value caused by the heat treatment. Further growth tests upon young rats showed that the decrease in nutritive value caused by heat is proportional to the temperature and time of heating. Cold or hot water washing of raw casein decreased its growth value by 15 to 20% but similar washing of heated casein had no effect nor did the latter washings appear to contain any toxic substances. Heating casein at 100° up to 8 hours did not decrease its growth value. Heating at 120° up to 2 hours had no effect but heating for 8 hours or more caused increasing amounts of damage. Heating casein at 130° for 30 minutes caused perceptible loss of growth value which loss was progressively increased by lengthening the heating period to 2, 8 and 24 hours.