Solubilization of Rapeseed, Soy and Sunflower Protein Isolates by Surfactant and Proteinase Treatments

Abstract

Surfactant and proteinase treatments were used to improve the dispersibility of rapeseed, soy and sunflower protein isolates. Of the different surfactants tested, only the anionic surfactants, i.e. sodium and potassium salts of myristic, oleic and linoleic acids and sodium dodecylsulfate (SDS) were effective for solubilizing rapeseed protein prepared from commercial oil-extracted meals. The solubilization was more effective at higher reaction pH and temperature. Addition of sodium hexametaphosphate enhanced the solubilization effect of SDS which was, however, annulled during storage.Potassium linoleate at a level of 8–10% of protein increased the dispersibility from 55% to over 90% when the mixture was treated at 65°C, pH 10 for 15min or at 22°C, pH 12 for 5min. This latter temperature had to be increased, however, to 32–42°C to obtain stable dispersibility during storage.Enzymatic treatment of rapeseed protein with B. subtilis protease, trypsin and pepsin were effective while bromelain, papain and α-chymotrypsin were ineffective. However, due to high salt content of the treated protein isolates resulting from neutralization after reaction at pH 2, the pepsin treatment was impractical.The subtilis protease and pepsin treated protein isolates after drying retained high dispersibility at all storage temperatures of -20, 4, 22 and 37°C, whereas the control as well as the SDS, oleate and linoleate treated protein isolates slowly decreased in dispersibility during storage; the surfactant treated protein isolates still maintained 80–90% dispersibility after storage at 22°C for 6 months compared to 55–65% for the untreated protein isolates. Solubilization effects of palmitate and myristate were about the same as that of oleate and linoleate.The protein suspensions resuspended from isoelectric precipitates showed almost perfect dispersibilities, 92–100%, when the supernatants from centrifugation of the protein suspensions were spray dried or freeze dried. This implies that the isoelectric precipitation reaction, rather than the drying process, is considered a critical factor of low dispersibility of the protein isolates, especially for rapeseed protein.