Influence of Lactic Acid on the Solubilization of Protein during Corn Steeping

The role of lactic acid (LA) in corn wet-mill steeping is not well understood. Because LA is known to improve wet-milling starch yields and steepwater contains a large amount of proteinaceous material, one of the effects of LA in steeping may be to help break down the endosperm protein matrix. Protein solubilization was studied for four different steeping solutions containing LA, sulfur dioxide (SO2), a combination of LA and SO2, or no added chemicals at temperatures between 44 and 60 °C with steep times of up to 48 h. The accumulation of proteinaceous material in steepwater with time was sigmoidal regardless of the steeping chemicals or temperature. The initial slow rate of solubilization appeared to be due to incomplete kernel hydration. Significantly greater amounts of protein were released in the presence of LA than in its absence, with the greatest amounts found when steeping was performed with both LA and SO2. The increase of proteinaceous material in steepwater containing LA was not due to low pH, because steeping solutions containing other organic and inorganic acids did not increase steepwater protein. The effect of LA concentration was also studied. In the absence of SO2, higher concentrations of LA resulted in higher steepwater protein concentrations. The opposite trend was observed in the presence of SO2. Similar steepwater protein concentrations were obtained with dl-lactic acid and l-lactic acid, indicating that the additional protein release was not sensitive to isomeric effects. Keywords: Corn; lactic acid; steeping; steepwater; wet milling