Substrate and Product Inhibition on Yeast Performance in Ethanol Fermentation

Abstract

A batch fermentation utilizing Saccharomyces cerevisiae BY4742 was conducted to determine the inhibitory effects of highly concentrated substrate and product levels on yeast. Experiments were performed to determine the largest dosage of substrate and the largest product concentration that the yeast could tolerate in a very high gravity fermentation process. The yeast's growth and fermentation activities were characterized by changes in the biomass and ethanol yield under different substrate and product concentrations during fermentation. All of the experiments were performed at a pH of 5.0 and a temperature of 35°C with a stirring rate of 180 r/min and a fermentation time of 96 h. Furthermore, five cycles of acclimatization were conducted to improve the yeast's tolerance to ethanol. Ethanol yield was maximized at 95% with a product concentration of 39 g/L and substrate dosage of 80 g/L. The system exhibited an obvious increase in cell growth and ethanol production with increasing substrate dosage up to a critical point of 160 g/L glucose (53 g/L ethanol fermented and an ethanol yield of 65%). Above this point, cell growth and ethanol production were inhibited with the final product concentration increasing only slightly with an increase in the initial substrate concentration. The end product (ethanol) was shown to be the primary factor inhibiting yeast growth and fermentation activity because the yeast would completely stop growing and fermenting when the initial exogenous ethanol concentration exceeded 70 g/L. The endogenous ethanol exerted a greater impact on yeast performance during anaerobic fermentation than exogenous ethanol. Five cycles of acclimatization significantly improved the yeast density, cell morphology, and ethanol production during very high gravity fermentation. The ethanol yield increased from 6% to 30% under an initial exogenous ethanol concentration of 60 g/L.

Determining butanol inhibition kinetics on the growth of Clostridium pasteurianum based on continuous operation and pulse substrate additions

“…29 Traditionally, microbial inhibition kinetics are determined through a series of batch experiments at elevated concentration of the inhibitory compound by monitoring the rate of microbial biomass growth and substrate consumption. 15,30,31 However, this methodology can be hardly applied in cases of intense inhibition as this might cause irregular and very long lag-phases, especially when sporulating microorganisms are used. Moreover, determining the microbial growth kinetics on a highly inhibitory feedstock like crude glycerol can be equally challenging.…”

Inhibition of fermentation evolution in bread doughs for aroma analyses

“…Several chemical substances have been reported as inhibitors of yeast fermentation in other matrices like wine. Their action is mainly based on: (i) stoppage of cellular replication, with compounds like furfural; (ii) inhibition of sugar metabolism, normally with weak acids like formic, acetic, butanoic or propanoic acids; (iii) disruption of membrane integrity, with ethyl alcohol and with octanoic and decanoic acids or their corresponding esters; (iv) osmotic stress, increasing the sugar substrate concentration . However, furfural, formic acid, acetic acid, propanoic acid, butanoic acid or ethyl alcohol could not be used for fermentation inhibition in aroma analyses since they are compounds commonly analysed in the dough aroma profile .…”

Bioethanol potential of raw and hydrothermally pretreated banana bulbs biomass in simultaneous saccharification and fermentation process with Saccharomyces cerevisiae

“…As other pretreatment processes, LHW is known to generate fermentation inhibitors during the process. However, S. cerevisiae, the most commonly used microorganism for ethanol production [9,23], has been found to resist such fermentation inhibitors [5,6,[24][25][26][27][28]. After hydrothermal pretreatment, the fiber fractions left are mainly composed of cellulose, hemicellulose, and starch that can be enzymatically hydrolyzed to release monomeric sugars, which subsequently become available for microbial conversion into ethanol.…”