The Nutritive Value of Animal Tissues in Growth, Reproduction nd Lactation

Abstract

There was little difference in the growth rate of rats fed beef heart, kidney, round and liver at a 15 per cent level of protein in an otherwise apparently complete ration; pork liver was practically equal to beef liver in its capacity to promote growth. When these tissues were extracted in a continuous extractor for 60 hours with hot 95 per cent alcohol growth was less rapid and was especially slow on extracted liver. Except with liver, whose proteins are altered by such extraction, the difference between the extracted and the unextracted meats can be ascribed solely to the removal of nutrients not adequately supplied by the yeast and cod liver oil supplements. When the ration contained 130-hour alcohol-extracted liver in place of the 60-hour extracted it did not support growth and the animals died. In contrast to similarly extracted round, heart, and kidney such extracted liver is only slightly digestible and has a low biological value. Although liver heated at 100°C. for 2 weeks no longer possessed its original capacity to support rapid growth in a balanced ration, its digestibility and the biological value were not lowered appreciably. Above 100°C. there was progressive lowering of the digestibility of the protein as the prolonged heating (3 days) became more intense. Heart and round were resistant to change at 120°C., while kidney was still digestible after heating at 130°C. At higher temperatures these meats also became refractory to digestive enzymes in the animal body as well as in vitro. These changes in digestibility probably play no role in the domestic cooking of meats, but are of importance in the preparation of pure proteins for nutrition studies.