The application of ultrasound in the enzymatic hydrolysis of switchgrass

Authors: Easson, Michael; Condon, Brian; Dien, Bruce; Iten, Loren; Slopek, Ryan; Yoshioka-Tarver, Megumi; Lambert, Allan; Smith, Jade

Submitted to: Applied Biochemistry and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2011
Publication Date: 9/13/2011
Citation: Easson, M.W., Condon, B.D., Dien, B.S., Iten, L.B., Slopek, R.P., Yoshioka-Tarver, M., Lambert, A.H., Smith, J.N. 2011. The application of ultrasound in the enzymatic hydrolysis of switchgrass. Applied Biochemistry and Biotechnology. 165(5):1322-1331.

Interpretive Summary: As global demand for fossil fuels steadily increases, there is an ever-increasing push for the development of alternative energy sources. The list of promising bioenergy crops encompasses a number of warm seasonal grasses including switchgrass which is considered among the most promising due to its perennial growth, high yield in poor quality soil, ease of cultivation using conventional equipment, and improved soil conservation. Furthermore, switchgrass offers reduced carbon emissions, lower fertilizer and pesticide requirements, and is not a food crop. For the past several years researchers at the Southern Regional Research Center (SRRC) in New Orleans, Louisiana have been investigating the application of ultrasonic energy as a means of improving the biofuels processing of corn stover, cotton lint, cotton trash, and sugarcane bagasse by enzymatic hydrolysis. This biofuels research is based on the theory that there is a synergistic effect when using a combination of ultrasound and enzymes that lowers the diffusion-limiting barrier between the macroscopic enzyme and the cellulosic substrate. According to the theory, during the collapse of the ultrasound-induced cavitation bubbles, powerful jet streams are created in the liquid media which act as a transport mechanism, lowering the diffusion-limiting barrier surrounding the substrate and allowing for an increase in the rate of enzyme-substrate binding. After enzymatic hydrolysis is complete, the ultrasound aids once again in the removal of hydrolysis products away from the substrate using a similar transport mechanism. The present paper focuses on the application of ultrasound in the enzymatic hydrolysis of untreated and ammonium hydroxide pretreated switchgrass and seeks to ascertain the nature of the synergistic effect between ultrasound and enzymes which results in the conversion of cellulose-containing switchgrass to reducing sugars. Several experiments were performed and several scanning electron microscopic (SEM) images were taken. The experimental results indicate that there is a synergistic effect when combining ultrasound and enzymes on untreated and pretreated switchgrass which effectively lowers the diffusion-limiting barrier between enzyme and substrate and results in an increase in the rate of enzymatic hydrolysis. Further experiments indicated that collapsing ultrasonic cavitations created additional surface area in the switchgrass substrate which further promoted enzymatic hydrolysis. Experiments demonstrated that five hours of ultrasonic pretreatment of switchgrass resulted in a higher yield of reducing sugars. This is significant when considering the costs of production of alternative biofuels.

Technical Abstract: In a series of experiments, untreated and ammonium hydroxide pretreated Klenow lowland variety switchgrasses are converted to reducing sugars using low frequency (20 kHz) ultrasound and commercially-available cellulase enzyme. Results from experiments using untreated and pretreated switchgrasses with and without ultrasound are presented and discussed. In untreated switchgrass experiments, the combination of ultrasound and enzymes resulted in an increase of 7.5% in reducing sugars compared to experiments using just enzymes. In experiments using ammonium hydroxide pretreated switchgrass, the combination of ultrasound and enzymes resulted in an increase of 9.3% in reducing sugars compared to experiments using just enzymes. Experimental evidence indicates that there is a synergistic effect from the combination of ultrasound and enzymes which lowers the diffusion limiting barrier to enzyme/substrate binding and results in an increase in reaction rate. SEM images provide evidence that ultrasound-induced pitting increases substrate surface area and affects reaction rate and yield.