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For improved production of ethanol from whole sorghum residues, physico-chemical compositions and fermentation characteristics of the substrates are important factors to consider. In the present study, Nigerian sorghum cultivars SSV2, KSV8 and KSV3 were grown under rain-fed conditions without chemical fertilization in Kano state, Nigeria. On harvest, the whole sorghum residues (bagasse) comprising crushed stalks, leaves, panicles and peduncles were collected for further processing. Bagasse samples, which had different macromolecular composition and carbohydrate pasting properties, were pre-treated with dilute sulphuric acid at 75 o C followed by enzymatic hydrolysis and sequential detoxification by Ca(OH) 2 overliming and charcoal filtration. Hydrolysate samples were subsequently fermented with the yeasts, Saccharomyces cerevisiae and Pachysolen tannophilus. Sugar consumption, carbon dioxide evolution and ethanol production were shown to vary depending on the sorghum cultivar type. While KSV3 yielded most favourable biomass of 37 t ha-1 (dry basis), Bagasse from cultivar SSV2 yielded the most favourable level of sugars (69 g/100g) after enzymatic hydrolysis, and also consistently exhibited improved fermentation performance. Detoxification of pretreated sorghum bagasse to remove potential yeast inhibitors resulted in improvement in ethanol yield, with 23 g L-1 ethanol (representing 72% of theoretical yield) being achieved from SSV2 bagasse following fermentation with P. tannophilus

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Sorghum is the fifth most important cereal worldwide and is a major source of agricultural residues in tropical regions. Bioconversion of whole sorghum crop residues comprising stalks, leaves, peduncles and panicles to ethanol has great potential for improving ethanol yield per sorghum crop cultivated, and for sustainable biofuel production. Effective pretreatment of sorghum lignocellulosic biomass is central to the efficiency of subsequent fermentation to ethanol. Previous studies have focused on the bioconversion of sorghum stalks and/or leaves only to bioethanol, but the current study is the first report dealing with whole crop residues. We specifically focused on the impact of Nigerian sorghum cultivation location and cultivar type on the potential ethanol yield from whole sorghum crop residues. Efficient bioconversion of whole sorghum residues to ethanol provides a sustainable route for utilization of crop residues thereby providing a non-food feedstock for industrial-scale bioethanol production.

Efficient starch saccharification is an essential step towards achieving improved ethanol yields by fermentation. Sorghum grains are important starch sources for bioconversion to ethanol. In the present study, disease degraded (spoilt) husked grains from Nigerian sorghum cultivars were obtained from field sites and subjected to bioprocessing to ethanol. The crude husked grains (comprising husks, spikelet, awn, rachis and pubescence materials) were hammer milled and each meal separately mashed with enzyme cocktails comprising amylase, glucanase and protease enzymes. The saccharified worts obtained were then fermented with the yeasts, Saccharomyces cerevisiae and Pichia stipitis (aka Scheffersomyces stipitis), without exogenous nutrient supplementation. Sugars liberated during mashing were determined and it was found that enzymatic hydrolysis of milled sorghum grains was effective in yielding favourable levels of fermentable sugars up to 70g sugar/100g substrate with one particular cultivar (KSV8). Ethanol and carbon dioxide production was measured from subsequent trial fermentations of the sorghum mash and it was found that S. cerevisiae produced ethanol levels equating to 420 L/t that compares very favourably with yields from wheat and barley. Our findings show that crude degraded sorghum grains represent favourable low-cost feedstocks for bioconversion to ethanol with reduced energy input and without additional costs for nutrient supplementation during fermentation. Consequently, our results suggest some economic benefits could be derived from spoilt or degraded sorghum grains.