Effects of different mill types on ethanol production using uncooked dry-grind fermentation and characteristics of residual starch in distiller's dried grains (DDG)

This study aimed to investigate impacts of milling methods on ethanol production using an uncooked dry-grind (cold fermentation) process and characterize residual starch in the distiller's dried grains (DDG) coproduct. Four corn lines with different chemical compositions were ground with cyclone, ultra-centrifugal, or hammer mills equipped with a screen of 0.5 mm opening and used for the cold fermentation process. Greater starch hydrolysis and ethanol yield were obtained from cyclone-milled corn, resulting from larger damaged starch contents and smaller particle sizes of the ground corn. Corn grains and ground corn after five-month storage showed less starch hydrolysis than the freshly ground counterpart. Residual starch (2.8-8.0%) with large proportions of intact amylopectin contents (up to 42.5%) was found in the DDG from all types of milling. The results suggested that the entrapment of starch granules in ground corn and a low activity of amylolytic enzymes at a high ethanol concentration were accountable for the remaining of starch in the DDG.