Comparative impact of steaming and dry heat on functional components of soy hulls

Soy hulls, a major by-product of the soy processing industry, are composed of varying amounts of cellulose, hemicellulose, and lignin. They also contain significant levels of proteins (up to 15%) and valuable phytochemicals, including polyphenols, trypsin inhibitors, phytic acid, and phytates. Thermal treatment is required prior to their utilisation. In the present work, the effects of two types of thermal treatment steaming and dry heating on the protein contents and compositions, trypsin inhibitor (TI) activity, polyphenol contents, and phytate levels of soy hulls were investigated. Protein fractions were analysed by measuring total and water-soluble protein contents, and performing SDS-PAGE under reducing conditions. The heating method significantly affected protein solubility and TI activity. Moist heat improved protein solubility by 34.5%, while dry heating reduced it by 28.9%. Residual TI activity was 18.5 and 84.8% for steamed and dry-heated samples, respectively. Additionally, steaming enhanced the water extractability of polyphenols and phytates. The heating method did not significantly influence the mineral profile of soy hulls.