Effect of protease treatment of soybean meal on content of total soluble matter and crude protein and level of soybean trypsin inhibitors1

An in vitro study was undertaken to determine the effect of treating soybean meal with protease at different temperature and pH conditions on the content of total soluble matter and crude protein (CP) and the level of soybean trypsin inhibitors (SBTI). Soybean meal (5 g), weighed into 25×150 mm culture tubes, was suspended in 20 ml of distilled water, adjusted to pH 3, 4.5 or 6, with Bacillus subtilis subtilisin-protease (Finnfeeds International) at 0, 0.2 or 1.0 mg g−1 soybean meal. The culture tubes were incubated for 16 h in a temperature-controlled water bath at 40 or 50°C. The soluble and insoluble fractions of soybean meal were separated by centrifuging the culture tubes and decanting the supernatant into separate conical tubes. All tubes were freeze-dried and the contents analyzed for CP. The content of total soluble matter and CP, on a dry matter basis, were determined as the difference between the beginning and end weight of the culture tubes. The content of total soluble matter and CP and the level of SBTI were usually similar (P>0.05) for soybean meal treated with protease at 0 or 0.2 mg g−1. Protease pretreatment at 1.0 mg g−1 soybean meal increased (P<0.05) the content of soluble CP and, with the exception of incubation at 50°C and pH 6, lowered (P<0.05) the level of SBTI compared with soybean meal pre-treated with protease at 0 or 0.2 mg g−1 under all conditions of temperature and pH. Based on the results of this in vitro trial, protease pretreatment at 1.0 mg g−1 soybean meal has the potential to improve the availability and digestion of soybean meal protein in diets for pigs.