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United States Department of Agriculture

Agricultural Research Service

Research Project: ENHANCED END USE QUALITY AND UTILIZATION OF SORGHUM GRAIN

Location: Grain Quality and Structure Research Unit

Title: Application of Acetate Buffer in pH Adjustment of Mash and its Influence on Fuel Ethanol Fermentation

Authors
item Zhao, Renyong - KANSAS STATE UNIVERSITY
item Bean, Scott
item Crozier-Dodson, Beth Ann - KANSAS STATE UNIVERSITY
item Fung, Daniel - KANSAS STATE UNIVERSITY
item Wang, Donghai - KANSAS STAT UNIVERSITY

Submitted to: Journal of Industrial Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 9, 2008
Publication Date: January 1, 2009
Citation: Zhao, R., Bean, S., Crozier-Dodson, B., Fung, D.Y., Wang, D. 2009. Application of Acetate Buffer in pH Adjustment of Mash and its Influence on Fuel Ethanol Fermentation. Journal of Industrial Microbiology and Biotechnology. 36:75-85.

Interpretive Summary: Sorghum is a drought resistant, low input cereal grain that grows well under limited rainfall. While primarily used as an animal feed, it is increasingly used for bio-ethanol production. To support such uses, one research need is to identify sorghum hybrids with improved ethanol yields and efficiencies. In order to accomplish this, methods are need to screen a relatively large and diverse set of samples from the huge pool of sorghum breeding lines, evaluate their performance in a bench-scale “dry-grind” conversion of sorghum to ethanol and identify key factors affecting ethanol yield and conversion efficiency. To save time and reduce the risk of contamination, an acetate buffer was quantitatively pipetted to liquefied mashes before inoculation in our SSF procedure. Ethanol yields of 18 sorghum hybrids improved significantly (2.0-5.9% relative increases, 3.9% on average) by changing the method for pH adjustment from traditional HCl to the acetate buffer. Furthermore, ethanol yields obtained using the two methods were highly correlated, indicating the acetate buffer did not influence resolution of the SSF procedure to differentiate grain samples varying in fermentation quality.

Technical Abstract: A 2M sodium acetate buffer at pH 4.2 was used to adjust pH of liquefied mashes in a simultaneous saccharification and fermentation (SSF) procedure. Although 5 mL of the buffer did not bring the pH values of the mashes (~100 mL) from a sorghum hybrid to 4.2, it kept the system stable (pH from 4.7 to 4.5) throughout the fermentation process. Concentration of the undissociated acetic acid in the mashes was about 41-47 mM. Acetate buffer retarded growth of yeast cells. The maximum growth rate constants (µmax) were 0.42 and 0.32 h to the power of -1 for cells grown in mashes with pH adjusted by HCl and acetate buffer, respectively. Viable cell counts (VCC) of yeast in mashes with pH adjusted by the acetate buffer were 36% lower than those in mashes adjusted by HCl during the stationary phase. Compared with pH adjustment by HCl, a 5.3% relative increase in ethanol and a 43.6% relative decrease in glycerol were obtained when acetate buffer was used. Ethanol yields of 18 other sorghum hybrids also improved significantly (2.0-5.9% relative increases, 3.9% on average) by changing from the traditional pH adjustment method (HCl) to adjustment by acetate buffer. Furthermore, ethanol yields obtained using the two methods were highly correlated (R squared = 0.96, p < 0.0001), indicating the acetate buffer did not influence resolution of the SSF procedure to differentiate grain samples varying in fermentation quality.

Last Modified: 9/22/2014