Denaturation of soybean proteins by isoelectric precipitation

Water extracts of soybean meal were acidified with HCl; neutralized; equilibrated with a buffer of pH 7.6, ionic strength 0.5, containing 0.01M mercaptoethanol; and analysed in an ultracentrifuge. Loss of solubility in buffer, as compared to a nonacidified control, served as a criterion of denaturation. Factors causing denaturation were time of acid treatment and extremes of acidity. 2-h acidification of water extracts to pH 4.5 decreased solubility and total ultracentrifuge area of globulin fraction ~12%, with decreases in 2S, 7S, 15S, and >15S fractions. Alkylation of sulphydryl groups did not prevent these losses. When whey was removed before neutralizing, protein solubility was reduced and all ultracentrifugal fractions decreased in area. Even though no loss of protein solubility occurred on pH 4.5 treatment of water extracts that were dialysed to remove phytates, total ultracentrifuge areas decreased. 7S and 11S fractions accounted for most area losses. Although stable on titration of a water extract to pH 4.5, 11S protein showed marked sensitivity to lower pH values. 7S and 11S fractions decreased in water extractability with ageing of the meal.