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

Abstract

Bioethanol is one of the most commonly used biofuels in transportation sector to reduce greenhouse gases. S. cerevisiae is the most employed yeast for ethanol production at industrial level though ethanol is produced by an array of other yeasts, bacteria, and fungi. This paper reviews the current and nonmolecular trends in ethanol production using S. cerevisiae. Ethanol has been produced from wide range of substrates such as molasses, starch based substrate, sweet sorghum cane extract, lignocellulose, and other wastes. The inhibitors in lignocellulosic hydrolysates can be reduced by repeated sequential fermentation, treatment with reducing agents and activated charcoal, overliming, anion exchanger, evaporation, enzymatic treatment with peroxidase and laccase, in situ detoxification by fermenting microbes, and different extraction methods. Coculturing S. cerevisiae with other yeasts or microbes is targeted to optimize ethanol production, shorten fermentation time, and reduce process cost. Immobilization of yeast cells has been considered as potential alternative for enhancing ethanol productivity, because immobilizing yeasts reduce risk of contamination, make the separation of cell mass from the bulk liquid easy, retain stability of cell activities, minimize production costs, enable biocatalyst recycling, reduce fermentation time, and protect the cells from inhibitors. The effects of growth variables of the yeast and supplementation of external nitrogen sources on ethanol optimization are also reviewed.

Optimization of Corn Steep Liquor Dosage and Other Fermentation Parameters for Ethanol Production by Saccharomyces cerevisiae Type 1 and Anchor Instant Yeast

“…The increases in concentration of corn steep liquor influenced the concentration of ethanol and brought about a reduction in the use of inoculum size. Tesfaw and Assefa [46] also confirm the importance of the finding that lower inoculum size reduces the cost of production in ethanol fermentation. The mutually significant relationship between inoculum size and temperature was due to the use of a commercial strain (Anchor Instant Yeast) which operated within the specified temperature range (25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35) • C) in the design of the experiment.…”

“…The range and the levels of the independent variables investigated using the Box-Behnken experimental design in this study are shown in Table 8. The variables chosen for the study have been reported as part of the growth variables that support the specific rate of yeast growth and ethanol production, and their values were carefully chosen to fall within the values that were previously reported [46]. …”

Improving ethanol production by co-culturing of Saccharomyces cerevisiae with Candida tropicalis from rice husk hydrolysate media

“…The specific rate of yeast growth and ethanol production were influenced by pH fermentation medium (Tesfaw and Assefa, 2014). In the present work, initial pH of the medium affected ethanol yield and the content of N and C at DDG.…”

“…Initial sugar concentrations before fermentation in the growth media can influence the specific rate of yeast growth and ethanol production (Tesfaw and Assefa, 2014). There are varieties of yeast, which are used to convert molasses into ethanol and CO 2 , such as S. cerevisiae and Klyureomyces marxianus (Parkash, 2015).…”

“…Co-culture is a potential bioprocess whereby, there are no cross-interactions among microorganisms and each microorganism metabolizing its substrate is unaffected by the presence of other microorganism (Park et al, 2012). Co-culture of S. cerevisiae and other microorganism increases ethanol productivity which might be due to enhanced substrate utilization (Tesfaw and Assefa, 2014). Co-culture of S. cerevisiae with other microbes reduces inhibitory compounds in lignocellulosic hydrolysates (Tom´as et al, 2013;Taherzadeh and Karimi, 2011;Wan et al, 2012) which increases ethanol yield and production rate (Singh et al, 2014;Wan, 2012), shortens fermentation time, and reduces process cost (Hickert et al, 2013;Tesfaw and Assefa, 2014).…”