Chemical, In Vitro, and In Situ Evaluation of Heat-Treated Soybean Proteins

¹ US Dairy Forage Research Center, Agricultural Research Service, USDA and Department of Dairy Science, University of Wisconsin, Madison 53706

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% far 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.

Key Words: rumen degradation • roasted soybeans • available lysine

Submitted on October 25, 1990
Accepted on February 11, 1991