Protease addition to increase yield and fermentation rate in dry grind ethanol production

Authors: Johnston, David; McAloon, Andrew

Submitted to: Bioresource Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/19/2013
Publication Date: 11/27/2013
Citation: Johnston, D., Mcaloon, A.J. 2013. Protease addition to increase yield and fermentation rate in dry grind ethanol production. Bioresource Technology. 154:18-25.

Interpretive Summary: In the United States, fuel ethanol is currently produced almost exclusively from corn. The process has become increasingly efficient over the last decade but incremental improvements can have a substantial impact due to the size of the industry. Using added nitrogen compounds to improve the conversion of corn into ethanol by the yeast is a common practice in the industry. Proteases (enzymes that degrade protein) can also be helpful by releasing nitrogen containing amino acids that the yeast can then use; however, their effectiveness and economics have been questioned. To evaluate if protease can be used effectively and economically, researches at the Eastern Regional Research Center performed a series of laboratory experiments that measured ethanol yields and rates using proteases and urea (a nitrogen compound commonly used in the industry). The data generated showed that proteases were effective at improving fermentation rates and yields when added alone, but were also beneficial when used with added nitrogen (urea). The experimental data generated was used to develop economic models and compared with current fuel ethanol production practices. The economic effect of the protease addition was shown in the models to reduce the overall production cost of ethanol by as much as 4¢ per gallon. If proven on the commercial scale, the addition of proteases could substantially benefit the fuel ethanol industry and lower the production cost and the total amount of corn needed.

Technical Abstract: Using a small scale laboratory procedure (100g shake flasks) for ethanol production from corn, the effects of acid protease addition during the fermentation step were evaluated. The batch fermentations were conducted in duplicate using standard conditions and with protease addition during fermentation. The standard and protease added fermentations were also tested with and without the addition of urea to determine if the protease altered the nitrogen requirements of the yeast. Fermentation rates and yields were evaluated using HPLC and gravimetric techniques. Results show that the addition of the protease had a statistically significant effect on the fermentation rate, yield and the resulting coproduct compositions. Fermentation rates and yields were improved with the addition of the protease over the corresponding controls. Fermentation rates with protease addition but without urea were significantly increased relative to the fermentation without protease or urea; however, they were not as high as with urea added. The addition of protease during fermentation either with or with added urea resulted in a higher final ethanol yield than without the protease addition. The economic effects of the protease addition were evaluated using process engineering and economic models for dry grind ethanol production developed at the Eastern Regional Research Center. The decrease in overall processing costs from the addition of protease was as high as 4¢ per gallon of denatured ethanol produced.