Pretreatment of dried distillers grains with solubles by soaking in aqueous ammonia and subsequent enzymatic/dilute acid hydrolysis to produce fermentable sugars

Authors: Nghiem, Nhuan; MONTANTI, JUSTIN - Former ARS Employee; TAE, HYUN KIM - Kongiu National University

Submitted to: Applied Biochemistry and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2016
Publication Date: 1/21/2016
Publication URL: http://handle.nal.usda.gov/10113/62615
Citation: Nghiem, N.P., Montanti, J., Tae, H. 2016. Pretreatment of dried distillers grains with solubles by soaking in aqueous ammonia and subsequent enzymatic/dilute acid hydrolysis to produce fermentable sugars. Applied Biochemistry and Biotechnology. 179(2):237-250.

Interpretive Summary: Dried distillers grains with solubles (DDGS) is the most important co-product of corn ethanol production in the dry-grind process, which normally is sold for use in animal feed supplements. DDGS contains significant amounts of carbohydrates, and therefore, is a potential source of sugars that can be used for production of additional ethanol and industrial chemicals in fermentation processes. In the present investigation a bio/chemical processing scheme was developed for production of fermentable sugars from DDGS. The developed process included pretreatment of DDGS with aqueous ammonia to render the cellulosic components more accessible to enzyme action, which released glucose, a readily fermentable six-carbon sugar, and an acid treatment, which released xylose and arabinose, the major five-carbon sugars. The fermentability of the sugar solution was demonstrated with a microorganism capable of producing succinic acid at high rates and yields. Succinic acid was chosen for this study because it is a feedstock for production of many important industrial chemicals and consumer products.

Technical Abstract: Dried distillers grains with solubles (DDGS), a co-product of corn ethanol production in the dry-grind process, was pretreated by soaking in aqueous ammonia (SAA) using a 15% w/w NH4OH solution at a solid:liquid ratio of 1:10. The effect of pretreatment on subsequent enzymatic hydrolysis was studied at two temperatures (40 degree C and 60 degree C) and four reaction times (6, 12, 24 and 48 hours). Highest glucose yield of 91% theoretical was obtained for the DDGS pretreated at 60 degree C and 24 hours. The solubilized hemicellulose in the liquid fraction was further hydrolyzed with dilute H2SO4 to generate fermentable monomeric sugars. The conditions of acid hydrolysis included 1 wt% and 4 wt% acid, 60 degree C and 120 degree C, and 0.5 and 1 hour. Highest yields of xylose and arabinose were obtained at 4 wt% acid, 120 degree C and 1 hour. The fermentability of the hydrolysate obtained by enzymatic hydrolysis of the SAA-pretreated DDGS was demonstrated in ethanol fermentation by Saccharomyces cerevisiae. The fermentability of the hydrolysate obtained by consecutive enzymatic and dilute acid hydrolysis was demonstrated using a succinic acid-producing microorganism, strain Escherichia coli AFP184. Under the fermentation conditions, complete utilization of glucose and arabinose by E. coli was observed, whereas only 47% of xylose was consumed. The succinic acid yield was 0.60 g/g total sugar consumed.