Feasibility study on fermentative conversion of raw and hydrolyzed starch to hydrogen using anaerobic mixed microflora

In this work, H2 was produced by anaerobic mixed microflora with phosphate-buffered medium containing starch or enzyme-treated starch hydrolyzate as the carbon substrate. The effect of pH on H2-producing performance was examined for cultures converting raw starch or hydrolyzed starch into H2. Response surface methodology was utilized to determine the best condition (41∘C, pH 5.2, 2.1% (v/v) enzyme dosage, 27 h reaction time) for starch hydrolysis with concentrated crude amylase obtained from Bacillus subtilis ATCC 21332. The mixed culture was able to produce H2 at an optimal pH of 7.0 irrespective of raw or hydrolyzed starch. Direct starch fermentation attained a highest maximum H2 production rate (Rmax)(Rmax), overall H2 production rate (Roverall)(Roverall), and H2 yield (YH2)(YH2) of 25.6 ml/h, 88 ml/h/l, and 5.28 mmol H2/g starch (4.64 mmol H2/g COD), respectively. In contrast, using hydrolyzed starch as the substrate gave rise to much better H2 producing performance, as the highest RmaxRmax, RoverallRoverall, and YH2YH2 values increased to 43.1 ml/h, 210 ml/h/l, and 6.1 mmol H2/g COD, respectively. This clearly demonstrates the advantage of using hydrolyzed starch for fermentative H2 production. The soluble metabolites consisted primarily of acetate (HAc), ethanol (EtOH), butyrate (HBu), and 2,3 butandiol. The amount of H2 produced from raw and hydrolyzed starch (especially, raw starch) could be estimated from formation of HAc and HBu known to stoichiometrically correlate with H2 production.