Pretreatment of guinea grass Panicum maximum Jacq. cv. 'TD 53' to enhance cellulase hydrolysis for ethanol fermentation by Saccharomyces cerevisiae

Abstract

Panicum maximum cv TD 53 (purple guinea grass) consisting of 41.7%(w/w) cellulose, 27.1%(w/2) hemicelluloses, and 10.4%(w/w) lignin was oven-dried, cut and Hammer milled to 20-40 mesh particle size, and used as substrate for ethanol production by S. cerevisiae. Optimal condition for pretreatment process was (6% w/v) substrate loading, 1.5 g substrate/g Ca(OH)₂ heating at 121°C, 15 lb/inc² for 5 min. Then the pretreated purple guinea grass was hydrolyzed with Accellerase [Subscript TM] 1000 (45 FPU/g DS or 400 unit of β-glucosidase /g DS)) using 53 FPU/g(DS) substrate (471 unit of β-glucosidase /g DS) at 50°C, 120 rpm for 6 hours. Maximum reducing sugars liberated was 11.9 g/l or 0.25 g glucose/g (DS). The glucose solution (11.9 g/l) obtained after cellulose hydrolysis of the pretreated purple guinea grass was used as substrate for ethanol fermentation by separate hydrolysis and fermentation (SHF) using S. cerevisiae. After 48 hours, ethanol (5.24 g/l or 0.087 g/g (DS) purple guinea grass or 0.44 g/g glucose) was produced. Ethanol fermentation of the pretreated purple guinea grass by simultaneous saccharification and fermentation (SSF) method using S. cerevisiae, maximum ethanol (4.45 g/L) (0.074 g/g (DS) purple guinea grass) was produced after 96 hours. The result indicated that the SHF process gave higher ethanol yield than SSF in term of g/g (DS) purple guinea grass. Scaling up of the SHF process to 3L working volume in 5L jar fermenter using glucose solution (12.0 g/l) yielded ethanol 5.92 g/l or 0.497 g/g glucose of 0.099 g/g (DS) purple guinea grass. An ethanol production yield increased about 13% from 0.44 g/g glucose in flask scale to 0.49 g/g glucose in 5L fermenter scale.