Interactions between isoflavones and soybean proteins: Applications in soybean-protein–isolate production

The effects of several modifications to soybean protein isolate (SPI) production on genistein series composition and content were analyzed. Combined variations of protein extraction pH and protein precipitation pH revealed that the maximum content of total genistein series was obtained with protein extraction at pH 8.0 and protein precipitation at pH 3.5. When protein extraction was carried out at pH 11.0, the saponification of malonylgenistin, resulting in genistin production, led to a decrease in isoflavone content. This fact was attributed to the low affinity of genistin for β-conglycinin. Malonylgenistin exhibited a high level of association with both glycinin and β-conglycinin at precipitation conditions. When temperature was reduced from 20 to 0 °C during protein precipitation, the genistein series content increased if the β-glucosidase activity was inhibited. This increase was at the expense of glucosilated forms. The nature of the interaction between soybean proteins and malonylgenistein and genistin seemed to be – at least in part – enthalpic in nature. In particular, malonylgenistin at pH 3.5 establishes another kind of interaction with protein, besides enthalpic ones. Glycinin exhibited higher affinities for genistein series components than β-conglycinin, the interactions being more stable at acidic than at alkaline pHs.