摘要：

Due to issues surrounding the burning of fossil fuels such as the effect of greenhouses gases on the climate and the threat energy security poses to non-producing nations, biofuels are being promoted for their potential local availability and carbon neutrality. Depending on the materials used, biofuels can be qualified as first (edible) or second (non-edible) generation. Whereas second generation technologies are still not economically viable, first generations biofuels (such as bioethanol from starch) will hold a major share of renewable liquids fuels in the short to medium term. The recent commercialization of enzymes with marked activity towards non-gelatinized or raw starch (cold processing), and their subsequent expressions by genetically modified organisms (Consolidated bioprocessing) could potentially cut the costs and energy requirements of the conventional high temperature processing, which involve cooking or gelatinizing starch. Hence, alternatives such as low temperature cold processing are being investigated for industrial application, while processes to improve the performance of the consolidated bioprocessing are being explored. Furthermore given that biofuels production is continuously increasing, the availability of the main co-product of the conversion process known as distillers dried grains with solubles (DDGS), is following the trend. It has been shown that sorghum grains decortication (removal of bran) prior entering the conversion process could significantly improves the DDGS quality, by reducing the fibre content thereof, hence increasing its market value. Furthermore, the bran components in grains have been shown to negatively affect starch hydrolysing enzymes. In this study, three bioethanol conversion processes (conventional warm, cold and consolidated bioprocessing) and the effect of decortication on key performance measures was assessed using sorghum grains. When using whole grains, the cold and conventional processing achieved similar ethanol concentration (130.4 and 132.1 g/L), productivity (1.55 and 1.51 g.L-1.h-1) and ethanol yield as a fraction of the theoretical maximum (89.7 % and 89.03 %). Although a slight decrease in the ethanol yield from consumed glucose was observed in slurries containing decorticated grains, performance of the cold processing was not significantly affected. However, the ethanol productivity of the conventional warm processing decreased with decortication (1.25 g.L-1.h-1). The performance of the cold processing using decorticated grains could match the whole grains process, while using 11.7 wt% less enzymes. The DDGS obtained from decorticated grains had higher average protein content (26%) and lower crude fibre content (30.7 %), compared to DDGS from whole grains processing. The acid and neutral detergent fibres contents in DDGS from both types of grains were on average decreased by 17.6 and 26.7% respectively by the cold processing relatively to the conventional processing. The performance of the consolidated bioprocessing could not match the enzyme-based processing, mostly due to limited production of starch-hydrolysis enzymes. The low ethanol tolerance of the recombinant yeast (approximately 90 g/L) prevented consumption of all of the glucose released in the very high gravity slurry. Furthermore, the CBP yeast inoculum size did not have a significant effect on the rate of starch hydrolysis and ethanol productivity, despite design of a fermentation process with high yeast biomass and yeast-produced enzyme concentrations in the starch slurry. Further improvements to the inoculum production, to increase biomass and enzyme concentrations, can be considered, although CBP yeast still lacks sufficient amylase production to achieve efficient starch grains conversion without supplementation with enzymes.

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