Effect of low-shear extrusion on corn fermentation and oil partition

To study oil distribution in fermentation liquid and solids for the purpose of recovering oil from corn stillage by centrifugation, a low-shear single-screw extruder was used to treat corn for dry-grind ethanol fermentation. Five different treatments for corn were used, and their effects on ethanol fermentation, oil distribution, and oil extractability were studied. Extruded corn with different particles sizes had similar ethanol yields (33% based on corn) because the starch was equally gelatinized by extrusion. Pretreatment with larger particle size before extrusion tended to have higher free oil than pretreatment with smaller particle sizes, but the effect was not dramatic, which indicates that manipulating particle size has limited effect on oil distribution in the liquid. Autoclaved flaked corn had lower ethanol yield because autoclaving at 28% moisture did not fully gelatinize the starch. Addition of protease and cellulase significantly increased the ethanol yield by at least 4%. A significant amount of bound oil became more extractable after enzyme treatment. Such oil can be effectively extracted into liquid phase by using a surfactant. In general, oil tended to be strongly associated with the solids in the thin stillage. By enzymatic treatment, 70% oil distribution was achieved in the thin stillage, compared to the conventional fermentation, where only 50% oil goes into the liquid. It was also demonstrated that mass loss after fermentation can be used to accurately quantify ethanol yield.