Cereal Chem. 85(6):792-798The aim was to study the dual effect of sorghum decortication and protease treatment before liquefaction with α-amylase on the performance of subsequent steps of saccharification and fermentation. A bifactorial experiment with a level of confidence of P < 0.05 was designed to study differences among grains (maize, whole, and decorticated sorghum) and the effectiveness of the protease before liquefaction. Sorghum was decorticated to remove most of the pericarp and part of the germ and increase starch concentration of the feedstock. The decorticated sorghum had significantly higher starch hydrolysis during liquefaction compared with the whole kernel. These hydrolyzates contained ≈50% more reducing sugars than the untreated counterparts. At the end of saccharification, the final glucose concentration in hydrolyzates treated without protease was the highest for maize (180 mg/mL), followed by decorticated sorghum (165 mg/mL), and whole sorghum (145 mg/mL). Decortication and protease treatment had a significant effect on fermentation times. In decorticated sorghum mash treated with and without protease, fermentation times were 22 and 60 hr, respectively. The decorticated sorghum treated with protease yielded similar amounts of ethanol compared with maize and 44% more ethanol compared with the untreated whole sorghum. Both sorghum decortication and protease treatments before hydrolysis with αamylase are recommended to increase ethanol yields, lower yields of distilled grains, and save liquefaction, saccharification, and fermentation times.

The efficiency of ethanol fermentation, as affected by grain source (maize and decorticated red sorghum), total sugar concentration (13 or 20° Plato) and type of microorganism (Saccharomyces cerevisiae or Zymomonas mobilis) was studied. Maize mashes yielded 0.32 l ethanol kg(-1) ground grain whereas mashes prepared with decorticated red sorghum produced 0.28 l ethanol kg(-1). Both microorganisms yielded similar amounts of ethanol. However, high-gravity mashes (20° Plato) yielded lower amounts of ethanol compared to counterparts adjusted to 13° Plato (0.28 vs. 0.22 l ethanol kg(-1) ground grains). In decorticated sorghum mashes adjusted to 20° P, Z. mobilis produced 40 ml kg(-1) more ethanol compared to S. cerevisiae. In addition, Z. mobilis had a lower dependency on nitrogenous compounds.

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BACKGROUND Bermuda grass is an irrigated forage crop adapted to arid and subtropical zones. This grass can be effectively bioconverted into second‐generation fuel ethanol. The aim was to optimize different acid hydrolyses conditions to maximize the release of C5 and C6 sugars from the forage with minimal production of microbial inhibitors. RESULTS The optimum conditions for H2SO4 were 12.5% solid to liquid, an acid concentration of 1.25%, and 75 min hydrolysis. The efficiencies after dilute acid and enzymatic treatments were 92.5, 73.5 and 89.9% for glucose, xylose and arabinose, respectively and the amounts of inhibitors were 8.2 kg m‐3 of acetic acid, 0.5 kg m‐3 of HMF, and 1.7 kg m‐3 furfural. For the HCl pretreatment, the optimum conditions were 12.5% solid to liquid fraction, 1.25% acid concentration and 37.5 min hydrolysis. These conditions released 56.2% glucose, 86.2% xylose and 48.3% of arabinose and generated 9 kg m‐3 of acetic acid, 0.5 kg m‐3 of HMF, and 2.6 kg m‐3 of furfural. CONCLUSION Dual chemical and enzyme conversions are promising for production of second‐generation ethanol from Bermuda grass. © 2013 Society of Chemical Industry