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Determining Optimal Heat Treatment of Soybeans by Measuring Available Lysine Chemically and Biologically with Rats to Maximize Protein Utilization by Ruminants^1,2,

U.S. Dairy Forage Research Center, Madison, WI 53706, U.S. Department of Agriculture, Agricultural Research Service, and Department of Dairy Science, University of Wisconsin, Madison, WI 53706

Soybeans were heated in a forced air oven at 120 and 130°C for 60 and 180 min, and at 140, 150 and 160°C for 10, 30, 60, 90 and 120 min. Two types of measurements were used to determine optimal heat treatment of soybeans to maximize protein utilization by ruminants. One was to estimate the rate and extent of protein degradation in the rumen using an in vitro ruminal system. The second was to determine the nutritional availability of lysine. Methods used to determine available lysine were an indirect fluorodinitro-benzene chemical method and a rat growth assay. The product of undegraded intake protein and available lysine content was used to estimate the amount of lysine that would escape ruminal degradation and be available for intestinal absorption. As heat input increased, ruminal undegraded intake protein increased, and protein degradation rates and total and available lysine decreased. As temperature increased, the time required to maximize post-ruminal available lysine decreased. The optimal heat treatment for soybeans heated in a forced air oven was: 140°C for 120 min or greater, 150°C for 60 min or 160°C for 30 min. A loss of 15–22% of chemically determined available lysine was necessary to achieve the heat treatment that resulted in maximal post-ruminal available lysine.

KEY WORDS: • rat growth • rumen proteolysis • lysine • Maillard reaction

¹ Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suitable.

² Supported in part by the American Soybean Association.

³ Current address: Purina Mills, Inc., 717 S. Hickory Street, Fond du Lac, WI 54935.

⁴ To whom correspondence should be addressed.

Manuscript received 17 January 1991. Revision accepted 20 June 1991.

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