Soybean whey protein/chitosan complex behavior and selective recovery of kunitz trypsin inhibitor

Proteins in soybean whey were separated by Tricine-SDS-PAGE and identified by MALDI-TOF/TOF-MS. In addition to β-amylase, soybean agglutinin (SBA), and Kunitz trypsin inhibitor (KTI), a 12 kDa band was found to have an amino acid sequence similar to that of Bowman-Birk protease inhibitor (BBI) and showed both trypsin and chymotrypsin inhibitor activities. The complex behavior of soybean whey proteins (SWP) with chitosan (Ch) as a function of pH and protein to polysaccharide ratio (RSWP/Ch) was studied by turbidimetric titration and SDS-PAGE. During pH titration, the ratio of zeta potentials (absolute values) for proteins to chitosan (|ZSWP|/ZCh) at the initial point of phase separation (pHφ1) was equal to the reciprocal of their mass ratio (SWP/Ch), revealing that the electric neutrality conditions were fulfilled. The maximum protein recovery (32%) was obtained at RSWP/Ch = 4:1 and pH 6.3, whereas at RSWP/Ch = 20:1 and pH 5.5, chitosan consumption was the lowest (0.196 g Ch/g recovered proteins). In the protein-chitosan complex, KTI and the 12 kDa protein were higher in content than SBA and β-amylase. However, if soybean whey was precentrifuged to remove aggregated proteins and interacted with chitosan at the conditions of SWP/Ch = 100:1, pH 4.8, and low ionic strength, KTI was found to be selectively complexed. After removal of chitosan at pH 10, a high-purity KTI (90% by SEC-HPLC) could be obtained.