Evaluation of protein extraction methods for β-conglycinin quantification in soybeans and soybean products

β-Conglycinin, one of the main proteins found in soybeans, improves lipid metabolism and influences the processing properties of soybeans. Recently, we developed a method to determine β-conglycinin content quantitatively via the use of liquid chromatography/tandem mass spectrometry (LC/MS/MS) and stable isotope-labelled internal standard (SIIS) peptides. In this study, we compared the efficacy of different protein extraction methods for β-conglycinin by comparing the yields of β-conglycinin extracted by each of these methods, using the developed LC/MS/MS analysis. The method using trichloroacetic acid (TCA) and acetone for protein extraction (TCA/acetone method) produced the highest yields of β-conglycinin from the soybean cultivars ‘Enrei’, ‘Fukuyutaka’, ‘Ryuho’, and ‘Satonohohoemi’, all of which were evaluated by LC/MS/MS analysis. We then investigated the applicability of each of these protein extraction methods to kinako (Japanese name, roasted soybean flour), natto (fermented soybeans), soybean milk, and tofu (soybean curd) to obtain information about these soybean products. The TCA/acetone method is judged to be an appropriate method of protein extraction for these products as well as for raw soybeans.