Integrated process for extraction of wax as a value-added co-product and improved ethanol production by converting both starch and cellulosic components in sorghum grains

Authors: Nghiem, Nhuan; O'CONNOR, JAMES - Former ARS Employee; Hums, Megan

Submitted to: Fermentation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/2018
Publication Date: 2/13/2018
Publication URL: https://handle.nal.usda.gov/10113/5935356
Citation: Nghiem, N.P., O'Connor, J., Hums, M.E. 2018. Integrated process for extraction of wax as a value-added co-product and improved ethanol production by converting both starch and cellulosic components in sorghum grains. Fermentation. 4:1-12.

Interpretive Summary: Grain sorghum is a potential feedstock for fuel ethanol production due to its high starch content, which is equivalent to corn. Grain sorghum has been successfully used in several commercial corn ethanol plants in the United States when the price of corn was high. Sorghum grains contain wax, which has been reported to have negative effect on fermentation. On the other hand, wax can be recovered as a valuable co-product of ethanol and as such can help improve the overall process economics. A process has been developed where in the first step, wax was extracted with boiling ethanol, which is readily available since it is the final product of the intended ethanol plant. The wax-free grains were either used directly in a fermentation process for ethanol production or subjected to further treatment with dilute sulfuric acid under relatively mild conditions prior to mashing and ethanol fermentation. The cellulose component in the hulls of the acid-treated grains was hydrolyzed with a commercial cellulase to generate glucose for additional ethanol production. Final ethanol concentration in a fermentation process using 25 wt% solids (dry basis) of raw sorghum grains was 86.1 g/L. When the grains were subjected to wax extraction followed by acid treatment and the commercial cellulase enzyme product was added to the mash, the final ethanol concentration increased to 117.8 g/L or 36.8 % improvement over the raw grains.

Technical Abstract: Grain sorghum is a potential feedstock for fuel ethanol production due to its high starch content, which is equivalent to that of corn, and has been successfully used in several commercial corn ethanol plants in the United States. Some sorghum grain varieties contain significant levels of surface wax, which may interact with enzymes and make them less efficient toward starch hydrolysis. On the other hand, wax can be recovered as a valuable co-product and as such may help improve the overall process economics. Sorghum grains also contain lignocellulosic materials in the hulls and the cellulose component can easily be converted to additional ethanol. An integrated process was developed, which consisted of the following steps: 1. Extraction of wax with boiling ethanol, which is the final product of the proposed process; 2. Pretreatment of the de-waxed grains with dilute sulfuric acid; 3. Mashing and fermenting of the pretreated grains to produce ethanol. During the fermentation, commercial cellulase was also added to release fermentable sugars from the hulls, which then were converted to additional ethanol. The advantages of the developed process were illustrated with the following results: 1. Wax extracted (determined by weight loss): ~0.3 wt% of total mass. 2. Final ethanol concentration at 25 wt% solid using raw grains: 86.1 g/L. 3. Final ethanol concentration at 25 wt% solid using de-waxed grains: 106.2 g/L (23.3 % improvement). 4. Final ethanol concentration at 25 wt% solid using de-waxed and acid-treated grains (1 wt% H2SO4) plus cellulase (CTec2): 117.8 g/L (36.8 % improvement).