Inactivation kinetics study of the Kunitz soybean trypsin inhibitor and the Bowman-Birk inhibitor

The inactivation of trypsin inhibitors (TIs) in soy flour exhibits a two-phase inactivation behavior. It is sometimes assumed that this behavior is caused by a difference in the heat stabilities of the Kunitz soybean trypsin inhibitor (KSTI) and the Bowman-Birk inhibitor (BBI). Kinetics studies with KSTI and BBI in soy flour showed that this two-phase inactivation behavior of TIs could not be explained by the difference in the heat stabilities of KSTI and BBI. Inactivation of KSTI and BBI in an aqueous solution and in a starch matrix followed a first-order reaction. KSTI and BBI in a starch matrix with added cysteine showed a two-phase inactivation behavior. The existence of thiols in soy flour seems to be responsible for the two-phase inactivation of TIs in soy flour. It is suggested that TIs in soy flour are inactivated by sulfhydryl-disulfide interchange during the first inactivation phase and by heat during the second phase.