Bioethanol production from residual starch of sago (Metroxylon sagu rottb.) hampas

Bioethanol has received renewed attention recently due to the uncertainties of oil price, elevated greenhouse gas emission, and the need for increased energy security and diversity. In the state of Sarawak, Malaysia, the waste from sago starch extraction process known as sago hampas is a starchy waste material which is commonly discharged directly into the river. It was revealed about 50 – 60% of residual starch is still trapped within sago hampas during starch production process from sago pith. It was estimated that one ton of sago hampas is formed from every ton of sago starch (dry weight basis) produced. Hence this study is carried out targeting the residual starch of sago hampas as a source for glucose production which can be used as the carbon source for the production of environmental friendly bioethanol. Initially, attempts to extract the starch were made through various approaches such as steeping, retrogradation, steaming and boiling. Boiling was preferable due to the condition of the starch which is ready to be used for saccharification process and less process time was needed for glucose production. The study was then focused to maximize the recovery of glucose (80 g/L) from residual starch in sago hampas through enzymatic hydrolysis process utilizing the commercial enzyme, Dextrozyme. The load of sago hampas at 7% (w/v) was seen to be suitable for the hydrolysis process. However lower glucose concentration (27.79 g/L) as well as less hydrolysis yield (35.73%) was obtained. The amount of substrate loading during the enzymatic hydrolysis seems to be the main obstacle to obtain high glucose yield. Thus, an alternative method (cycles I, II and III) which involved reusing the hydrolysate for subsequent enzymatic hydrolysis cycles was introduced. Greater improvement of glucose concentration (138.45 g/L) and higher conversion yield (52.72%) was achieved after completing cycle III of the hydrolysis process. The capability of CBY to metabolize glucose from SHH for generating high concentration of ethanol was observed to be suitable at 9 h of pre-germination time. Results showed that 40.30 g/L of ethanol was produced, whereas 0.48 g/g of ethanol yield was generated and 93.29% of fermentation efficiency was achieved at an initial glucose concentration of 84.57 g/L after 16 h of fermentation. In the study on effects of various initial glucose concentrations, highest ethanol fermentability with respect to ethanol yield (0.50 g/g) and fermentation efficiency (98.00%) were obtained at 100 g/L glucose. By-products such as acetic acid, lactic acid and glycerol were also produced. Higher concentration of glycerol (9.98 g/L) was observed when initial glucose concentration was increased up to 250 g/L. Yeast-based bioethanol production always considers the supplementation of nitrogen to enhance the fermentation reaction and yeast extract was commonly used. Thus in order to increase the capacity of SHH as an alternative substrate for bioethanol production, utilizing other type of nitrogen source such as urea and ammonium sulfate was also carried out in this study. Urea and ammonium sulfate were found to be feasible in using CBY for bioethanol production. The bioethanol yield was found to be 0.44 g/g and 0.42 g/g when urea and ammonium sulfate was applied as the nitrogen source with fermentation efficiency at 86.27% and 82.35%, respectively. Supplementation of selected metal ionic compounds such as calcium, magnesium and zinc for CBY in SHH was found to be unnecessary as nonsignificant profile of ethanol production and glucose consumption was observed. The recycle strategy for conducting enzymatic hydrolysis process shows advantage for generating high glucose concentration due to its ability to accommodate up to 21% (w/v) of substrate load. The glucose in hydrolysate was metabolized efficiently by CBY during ethanol fermentation, thus exhibits the capability of sago hampas to be an alternative cheap substrate for the renewable bioethanol production.

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