Effects of heating time and sodium metabisulfite on the nutritional value of full-fat soybeans for chicks.

Two 21-d trials were conducted to evaluate the effect of heating time and sodium metabisulfite (SMBS) on the nutritional value of full-fat soybeans for chicks. In Trial 1, four pen-replicates of eight chicks each were fed corn-based diets (19% CP; 3,167 kcal of ME/kg) containing either 44% CP soybean meal or full-fat soybeans. The soybeans either were unheated or were autoclaved at 121 degrees C for 10, 20, 30, 40, 60, or 90 min. Soybean oil was added to the soybean meal diet to make it isoenergetic with the soybean diets. Trypsin inhibitor, urease activity, and the solubility of protein in the soybeans decreased as heating time increased. Weight gain increased and feed:gain and pancreas weights decreased quadratically (P less than .01) with heating time. Rate and efficiency of gain were maximized when the soybeans were heated for 40 min; further heating for 60 or 90 min reduced performance. In Trial 2, SMBS was added at levels of 0, 1, or 2% to full-fat, unheated soybeans or to soybeans before autoclaving at 121 degrees C for 10, 20, or 40 min. Four pen-replicates of seven chicks each were fed corn-soybean diets (19% CP; 3,144 kcal of ME/kg) with 12 treatments in a factorial arrangement of heating times and SMBS levels. The rate and efficiency of chick weight gain improved linearly (P less than .01) and pancreas weights decreased linearly (P less than .01) as heating time increased. Less heating time was required to maximize performance and minimize pancreas weights when SMBS was added, resulting in a heating time x SMBS interaction (P less than .05). Under the conditions of this research, chicks fed full-fat soybeans achieved maximum performance when the soybeans were heated at 121 degrees C for 40 min, and SMBS decreased by one-half the heating time required to inactivate the trypsin inhibitors. Trypsin inhibitor activity in soybeans was more closely related to their nutritional value than was urease activity.