Protein Solubility, Amino Acid Composition, and Biological Value of Regular and Heat-Treated Soybean and Sunflower Meals

Abstract

Commercially available soybean and sunflower meals were subjected to moist heat in a cooker extruder. Heat treatment reduced solubility of nitrogen of soybean and sunflower in diluted Burroughs mineral solution 27 and 35%. Amino acid composition of the heated soybean and sunflower meals were similar to composition in the unheated meals. Essential amino acids tended to be slightly more insoluble than nonessential amino acids. Heat treatment tended to reduce solubility of all amino acids in soybean meal to about the same extent whereas the relative solubility of several amino acids differed substantially between sunflower meal and heated sunflower meal. Comparisons of essential amino acid compositions to the composition of milk indicated that methionine was the first limiting amino acid in soybean meal and remained first limiting in heated soybean meal as well as in the insoluble fraction of both meals. Lysine was first limiting in sunflower meal and its insoluble fraction but was second limiting after threonine in heated sunflower meal and its insoluble fraction. Growth rates of mice fed heated protein supplements were the same as those fed regular soybean and sunflower meals, indicating that the heat treatment did not reduce protein quality. This heat treatment may be an appropriate method for reducing the solubility of ration proteins in the cow's rumen and yet allowing the proteins to be digested and utilized in the intestines.