Effects of Propionic Acid and pH on Ethanol Fermentation by Saccharomyces cerevisiae in Cassava Mash

Abstract

The effects of propionic acid on ethanol and glycerol production by Saccharomyces cerevisiae in cassava mash were examined along with the influence of pH (4.0, 5.0, and 6.0) and of dissolved solids content (22%, 25%, and 27%). Inhibition by propionic acid increased as solids content increased and medium pH declined. Complete inhibition of ethanol fermentation was observed in mashes at pH 4.0 (60 mM propionic acid for 22% solids and 45 mM for 25% and 27%). Glycerol production linearly decreased with increased undissociated propionic acid concentration in all mashes at all pH levels, which partly contributed to increased final ethanol production when propionic acid concentration in mashes was low (≤ 30 mM).

Current Trends in Bioethanol Production by Saccharomyces cerevisiae: Substrate, Inhibitor Reduction, Growth Variables, Coculture, and Immobilization

“…Third, chemicals like D-lactic acid [ 46 ] and inulin [ 47 ] have been utilized by S. cerevisiae and the yeast has produced promising amount of ethanol titer though producing lactic acid by itself is expensive for industrial level ethanol production. Fourth, the cheapest and lignocellulosic agricultural residues such as coffee pulp [ 16 ], coffee husk [ 25 ], corn stover [ 17 , 19 ], sugarcane leaves [ 18 ], Jerusalem artichoke [ 47 ], rice hull [ 48 ], decorticated sorghum mash [ 49 ], cassava mash [ 50 ], cashew apple bagasse [ 26 ], mahula flowers [ 30 ], floriculture waste ( Dendranthema grandiflora ) [ 11 ], oil palm empty fruit bunches [ 12 ], oil seed rape straw [ 38 ], root biomass of Coleus forskohlii [ 51 ], mission grass ( Pennisetum polystachion ) [ 8 ], and rapeseed straw [ 52 ] were recently investigated to optimize lignocellulosic ethanol production.…”

“…Kitchen wastes and ethanol stillage [ 50 ] served as substrate for S. cerevisiae after chemical and anaerobic microbial treatment, respectively. However, the organic acid particularly lactic acid present in kitchen wastes and anaerobically treated stillage hinders ethanol fermentation [ 50 ].…”

“…Kitchen wastes and ethanol stillage [ 50 ] served as substrate for S. cerevisiae after chemical and anaerobic microbial treatment, respectively. However, the organic acid particularly lactic acid present in kitchen wastes and anaerobically treated stillage hinders ethanol fermentation [ 50 ]. The problem was circumvented by using lactic acid as substrate for S. cerevisiae NAM34-4C and 2.7 g ethanol/L is produced from lactic acid at pH 3.0 and temperature 35°C [ 46 ].…”

“…Currently, stillage (a waste after ethanol production) is commonly reused for yeast substrate to make the ethanol production more efficient; however, stillage contains more organic acids than expected. The organic acids present in the stillage elongated the ethanol fermentation time [ 50 ]; ethanol fermentation from cassava mash using S. cerevisiae was more inhibited by propionic acid as medium pH decreased, undissociated acid being the effective inhibitory form, whereas glycerol production decreased as propionic acid increased irrespective of solids in cassava mash and pH condition. The plasma membrane allows the easy entrance of undissociated acids, dissociating intracellularly and thus cytoplasm could be acidified.…”

“…The plasma membrane allows the easy entrance of undissociated acids, dissociating intracellularly and thus cytoplasm could be acidified. At the same time, the proton must be transported by membrane ATPase to maintain intracellular pH and thus it results in increased ATP consumption and decreased biomass yield [ 50 ].…”