Evaluation of the contents and ratios of five fractionated proteins to elucidate the protein composition in soybean seeds

Abstract

Selecting suitable soybean cultivars is important for food processing and exploring their endowment with desirable physiological functions. We applied the fractionation method previously established to compare soy protein composition among cultivars to promote such a selection. More than 95% of the proteins in soybean cotyledons were extracted from 13 soybean cultivars using a high‐concentration salt solution. The extracted proteins were fractionated into five fractions, namely oil body‐associated protein (OBAP), polar lipid‐associated protein (PLAP), globulins (11S and 7S), and whey by centrifugation after tuning the solubility behavior of the proteins with various solutions. Protein species in each fraction were analyzed by SDS‐PAGE. Protein content in the total extract and five fractions was quantified to characterize the protein composition of soybean cultivars. The correlation between the protein content of each fraction and the total protein in cotyledon was investigated. A strong positive correlation was found only for the 11S fraction (r = 0.82), followed by a positive correlation in the 7S fraction (r = 0.65). Thus, we surmised that the increased protein content in soybean was due to increased globulin content. Furthermore, the calculation of the average ratio of protein content in each fraction indicated the globulin fraction (7S and 11S) to be 52%, the lipophilic protein fraction (OBAP and PLAP) to be 33%, and the whey fraction to be 13%. The preparation method employed in this study is a promising tool for efficiently comparing the protein composition of soybean cultivars to evaluate the potential use of cultivars for food production.