CHEMICAL, INVITRO, AND INSITU EVALUATION OF HEAT-TREATED SOYBEAN PROTEINS

The effect on protein degradability of roasting soybeans and soybean meal and holding them at elevated temperatures for 0, .5, 1, 1.5, 2, 2.5, and 3 h was assessed using in vitro, in situ, and chemical techniques. Ruminal in vitro degradation rates decreased with initial roasting for both soybeans and soybean meal from .165 to .065 h-1 and .155 to .092 h-1, respectively. Roasting and holding for 3 h resulted in the lowest degradation rates, .037 and .029 h-1 for soybeans and soybean meal, respectively. In situ degradation rates decreased from .174 to .090 h-1 and from .069 to .012 h-1, respectively, for soybeans and soybean meal roasted and held 3 h. There were no increases in ADIN with heat processing of soybeans. However, ADIN increased from 2.3 to 4.1% when soybean meal was roasted and held 3 h. In contrast, nutritionally available lysine decreased dramatically during holding with a loss after 3 h of 22% for soybeans and 17% for soybean meal. Roasting and holding significantly increased estimated postruminally available lysine (g/kg DM) of soybeans from 6.5 to 10.5 (0 h) and 11.2 (mean, .5 to 3 h) and of soybean meal from 7.6 to 10.5 (0 h) and 14.2 (mean, .5 to 3 h). Holding soybeans and soybean meal at an elevated temperature resulted in a more thorough and extensive heat treatment than typically is obtained in commercial processing.