Influence of temperature, pH and ionic strength on the production of isoflavone-rich soy protein isolates

Soy protein isolates (SPI) may present different isoflavone profiles and contents, depending on processing conditions. In the present work, seven different SPI, resulting from changes in the processing steps, were obtained. The best parameters for obtaining isoflavone-richer SPI were: extraction at pH 9 and temperature of 55 °C, acid precipitation performed at pH 4.5, acid-washing of the precipitate and mild centrifugation conditions for the separation of acid-precipitated proteins. Isoflavones were soluble in aqueous solution in the pH range 2–10 (73–93% of the amount solubilized in 80% methanol). The profile of isoflavones was dependent on the temperature used for the aqueous extraction. Temperatures below 50 °C caused hydrolysis of β-glucosides with increase of aglucones, by endogenous β-glucosidase activity.