Modified simultaneous saccharification and fermentation for the production of bioethanol from highly concentrated raw corn starch

Abstract

Bioethanol is one of the main bio-based molecules produced mainly from sugar cane, molasses and corn. Its environmental advantages allow it to be considered as safe and the cleanest fuel alternative. Starch is a widespread renewable carbohydrate conventionally used for bioethanol production via energy demanding liquefaction and saccharification processes. Raw starch hydrolysis using enzymes capable of degrading it below the gelatinization temperature significantly simplifies the process and reduces the cost of starch processing. In this study, an innovative modified simultaneous saccharification and fermentation process is proposed for the production of bioethanol from highly concentrated raw corn starch (30 % w/v). A two-step synergistic hydrolysis and fermentation was carried out in a single bioreactor vessel. To ensure high process efficiency, factors influencing the hydrolysis of concentrated raw corn starch by raw starch degrading α-amylase from Bacillus paralicheniformis ATCC 9945a (BliAmy) and commercial glucoamylase were investigated. Box–Behnken experimental design was used to predict the effects of different ratios of added enzymes, glucoamylase addition time, incubation time, and pH on hydrolysis yield. Optimal conditions for the highest yield of hydrolysis of raw corn starch (90 %) were obtained after 8 h using 5.0 IU BliAmy per mg of starch and 0.5 % (v/v) glucoamylase at pH 4.5 and 60 °C. Obtained glucose was further fermented with Saccharomyces cerevisiae at 30 °C in the same vessel for bioethanol production. Bioethanol concentration at 129.2 g/L, with productivity of 2.94 g/L/h and ethanol yield (YP/S) at 0.50 g EtOH/g total sugar, equivalent to 87.8 % theoretical yield, was obtained by modified simultaneous saccharification and fermentation. This work enriches the information of bioethanol production and offers a novel strategy for raw starch hydrolysis under industrial conditions.