Optimization of ethanol yield from cassava

Abstract

The energy crisis and worldwide economic depression has highlighted the production of biofuels from agricultural materials as an important national policy. Cassava, a root plant indigenous to Africa, is not cultivated commercially in South Africa because it is not a staple food source and contains some cyanide components in its raw form. Cassava is mostly grown as a food supplement by informal households. Cassava roots are rich in starch (approximately 80%) and are therefore an excellent candidate for the production of bio-ethanol in South Africa. In this research, Cassava roots, which consist mostly of starch, as well as the peels, which consist of cellulose, were converted to bio-ethanol. As a baseline, the Cassava starch and the peels were converted to ethanol separately by using traditional pretreatment methods and Saccharomyces cerevisiae as yeast. The hydrolysis process for starch was optimized with respect to substrate concentration, enzyme concentration, enzyme combination, treatment temperature and pH of the different process steps. The best fermentation step was determined through fermentation of the optimized starch hydrolysate using the separated hydrolysis and fermentation process (SHF), the simultaneous saccharification and fermentation process (SSF) as well as a direct fermentation (DF) process from the raw starch using Schwanniomyces occidentalis (ATCC 26076). Cassava roots (starch) and peels (cellulose) were then pretreated and fermented simultaneously using different combinations of enzymes. A substrate concentration of 20 wt% biomass gave the highest glucose concentration in the final hydrolysate, while the best enzyme concentration was found to be 0.2% for Termamyl SC, 0.25% for Spirizyme fuel and 0.1% for Celluclast 1.5L. The liquefaction and saccharification treatment temperature that gave the highest ethanol yield were 95°C and 55°C respectively. The best pH for the two hydrolysis steps was found to be 6 and 4.5 for the liquefaction and saccharification steps respectively. The optimum pretreatment conditions with a substrate concentration of 20wt% yielded a final glucose concentration of 141 g.L-1 (Yp/s = 0.7 g.g-1) for Cassava starch, 109 g.L-1 (Yp/s =0.55g.g-1) for Cassava cellulose (peels) and for the simultaneous conversion of both the starch and cellulose, a final glucose concentration of 184 g.L-1 (Yp/s = 0.9 g.g-1) was obtained. It can be concluded from these results that unpeeled Cassava roots (starch and cellulose) yield a higher final glucose concentration in the final hydrolysate than converting the cellulose (peels) and starch (peeled roots) separately. This means that it is more productive and economical to use unpeeled Cassava roots with the correct combination of starch and cellulose enzymes to produce a glucose rich hydrolysate for ethanol production through fermentation. The direct fermentation (DF) process yielded the lowest final ethanol concentration (0.14%) resulting in a yield coefficient (Yp/s) of just 1 %. The SHF process yielded 9.6 % (v/v) (Yp/s = 0.38 g.g-1) ethanol for Cassava starch and 10.6 % (v/v) (Yp/s = 0.42 g.g-1) for both roots and peels (starch and cellulose) after 48 hours fermentation. The SSF process resulted in a final ethanol yield of 7 % (v/v) (Yp/s = 0.3g.g-1) for Cassava starch, 4% (v/v) (Yp/s = 0.16 g.g-1) for Cassava peels (cellulose) and 10.6% (v/v) (Yp/s = 0.42 g.g-1) for unpeeled Cassava roots (starch and cellulose). These results demonstrate that Cassava waste (peels) can be used as an alternative biomass for bio-ethanol production. However, the SSF process for unpeeled Cassava roots results in a higher ethanol yield than processing the peels and starch separately and then combining the hydrolysates only for the fermentation step. It also became evident that Cassava containing approximately 85% (g/g) starch is a good feedstock for bio-ethanol production in South Africa.