Direct biotransformation of oil palm frond juice to ethanol and acetic acid by simultaneous fermentation of co-cultures and the efficacy of its culture filtrate as an antifungal agent against black seed rot disease

Abstract

With the enormous amount of oil palm frond (OPF) left unused in an oil palm plantation, this study proposed to develop the process for ethanol and acetic acid production from OPF juice (pH 4.83) using Saccharomyces cerevisiae TISTR5055 and Acetobacter aceti at room temperature (30±2 °C) for 72 h. Among three different fermentation modes tested, single- and two-stage fermentation produced only ethanol, while simultaneous fermentation exhibited both ethanol and acetic acid production. The yields of 0.39 g ethanol/g glucose and 0.88 g acetic acid/g ethanol were achieved with 76% and 68% of the theoretical yields. Simultaneous fermentation of OPF juice under different shaking speeds and aeration rates revealed no shaking (static) and no aeration were the optimum conditions for ethanol production (19.59 g/L). Shaking at 180 rpm and aeration at 0.5–1.0 vvm exhibited the highest acetic acid production with yields of 0.95–1.16 g acetic acid/g ethanol (73–89 % of the theoretical yields). In addition, the aeration rate had a profound effect on acetic acid production rate with 9.0-fold increase as the aeration rate increased from 0 to 0.5 vvm. The culture filtrate from acetic acid production was analyzed to contain 15 compounds that could be chemically grouped into alcohols, alkenes, aromatic hydrocarbons, and alkanes, with acetic acid as the most abundant (54.6%). Interestingly, the culture filtrate had the efficacy to suppress the growth of Ceratocystis paradoxa TT1, causing black seed rot disease in oil palm with the effective dose (100% inhibition) at 0.20% and 0.50% (v/v) acetic acid in liquid and solid culture, respectively. Therefore, OPF juice without nutrient supplementation could be used as an alternative fermentation medium for ethanol and acetic acid production with high potential exploitation of the acetic acid culture filtrate as an antifungal agent against black seed rot disease.