ARS | Publication request: Effect of endosperm hardness on an ethanol process using a granular starch hydrolyzing enzyme

Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 1, 2009
Publication Date: January 1, 2010
Citation: Wang, P., Liu, W., Johnston, D., Rausch, K.D., Schmidt, S.J., Tumbleson, M.E., Singh, V. 2010. Effect of endosperm hardness on an ethanol process using a granular starch hydrolyzing enzyme. 2010. Transactions of the ASABE, American Society of Agricultural and Biological Engineers. 53(1):307-312.

Interpretive Summary: The production of fuel ethanol using enzyme mixtures that can convert the starch in corn kernels to sugars without the need for high temperature cooking can decrease the overall energy needed for processing. Utilizing this type of enzyme preparation requires the corn to be ground into much smaller particles for efficient conversion. Corn kernels also do not grind uniformly due to differences in composition, physical structure and hardness. The research conducted in this study examined how different types of endosperm (the starch containing portion of the corn kernel) performed in ethanol fermentations and how the addition of a protease and/or urea (nitrogen source for yeast) could improve the process. The results showed that the addition of protease could increase the fermentation rates and yield for both hard and soft endosperm; however, the effect was most pronounced with soft endosperm. It was also found that the addition of protease with hard and soft endosperm required little or no additional urea to achieve high ethanol yields. These results will be useful to others studying alternative fuel ethanol processes and producers interested in process modifications as a way of lowering production costs and decreasing energy use.

Technical Abstract: Granular starch hydrolyzing enzymes (GSHE) can hydrolyze starch at low temperature (32 deg C). The dry grind process using GSHE (GSH process) has fewer unit operations and no changes in process conditions (pH 4.0 and 32 deg C) compared to the conventional process because it dispenses with the cooking and liquefaction step. In this study, the effects of endosperm hardness, protease, urea, and GSHE levels on GSH process were evaluated. Ground corn, soft endosperm, and hard endosperm were processed using two GSHE levels (0.1 and 0.4 mL per 100 g ground material) and four treatments of protease and urea addition. Soft and hard endosperm materials were obtained by grinding and sifting flaking grits from a dry milling pilot plant; classifications were confirmed using scanning electron microscopy. During 72 h of simultaneous granular starch hydrolysis and fermentation (GSHF), ethanol and glucose profiles were determined using HPLC. Soft endosperm resulted in higher final ethanol concentrations compared to ground corn or hard endosperm. Addition of urea increased final ethanol concentrations for soft and hard endosperm. Protease addition increased ethanol concentrations and fermentation rates for soft endosperm, hard endosperm, and ground corn. The effect of protease addition on ethanol concentrations and fermentation rates was most predominant for soft endosperm, less for hard endosperm, and least for ground corn. Samples (soft endosperm, hard endosperm, or corn) with protease resulted in higher (1.0% to 10.5% v/v) ethanol concentration compared to samples with urea. The GSH process with protease requires little or no urea addition. For fermentation of soft endosperm, GSHE dose can be reduced. Due to nutrients (lipids, minerals, and soluble proteins) present in corn that enhance yeast growth, ground corn fermented faster at the beginning than hard and soft endosperm.