Yeasts in sustainable bioethanol production: A review

Abstract

Bioethanol has been identified as the mostly used biofuel worldwide since it significantly contributes to the reduction of crude oil consumption and environmental pollution. It can be produced from various types of feedstocks such as sucrose, starch, lignocellulosic and algal biomass through fermentation process by microorganisms. Compared to other types of microoganisms, yeasts especially Saccharomyces cerevisiae is the common microbes employed in ethanol production due to its high ethanol productivity, high ethanol tolerance and ability of fermenting wide range of sugars. However, there are some challenges in yeast fermentation which inhibit ethanol production such as high temperature, high ethanol concentration and the ability to ferment pentose sugars. Various types of yeast strains have been used in fermentation for ethanol production including hybrid, recombinant and wild-type yeasts. Yeasts can directly ferment simple sugars into ethanol while other type of feedstocks must be converted to fermentable sugars before it can be fermented to ethanol. The common processes involves in ethanol production are pretreatment, hydrolysis and fermentation. Production of bioethanol during fermentation depends on several factors such as temperature, sugar concentration, pH, fermentation time, agitation rate, and inoculum size. The efficiency and productivity of ethanol can be enhanced by immobilizing the yeast cells. This review highlights the different types of yeast strains, fermentation process, factors affecting bioethanol production and immobilization of yeasts for better bioethanol production.

Stress-Induced Expression is Enriched for Evolutionarily Young Genes in Diverse Budding Yeasts

“…Yeasts in the Saccharomycotina subphylum, "budding yeasts", have proven to be useful platforms for the production of ethanol, flavors, nutritional supplements, biopharmaceuticals, as well as other valuable chemicals 1,2,3 . At present, industrial production using budding yeasts as a platform is dominated by the extensively characterized species Saccharomyces cerevisiae.…”

A carbon footprint assessment of multi‐output biorefineries with international biomass supply: a case study for the Netherlands

“…In contrast, sugar beets (SB) only provide C6 sugars (sucrose), and beet pulp. Currently, industrial fermentation of biofuels and bio‐based chemicals is mainly carried out by employing Saccharomyces cerevisiae to ferment C6 sugars . However, modifications in S. cerevisiae have allowed this yeast to ferment C5 sugars on an industrial level .…”

Ethanol production from cellulosic cassava waste

“…This was similar to that reported by Bayitse et al (2015). Enzymatic hydrolysis of cassava peels with 2.5% (v/w) cellulases (NS22186) at 4 h released 0.9 g/l of glucose which rose to 1.2 g/l in 48 h. When the enzyme loading was increased to 10% cellulase, glucose concentration was doubled from 1.02 g/l at 4 h to 2.17 g/l at 48 h of hydrolysis which increased marginally to 2.6 g/l at 120 h. The amount of fermentable sugar obtained increased as the enzyme load increased while cellulose load decreased (Azhar et al, 2017). Deeprasert et al (2012) can produce 65.80 g/l of reducing sugar from cassava solid waste (100 g/L), using 3 steps of enzymatic hydrolysis, Cellic® CTec2 for 6 h, α-Amylase (Novozyme) for 2 h and Glucoamylase (Novozyme) for 12 h. However, the presence of high sugar concentration in the fermentation medium may lead to substrate inhibition and results in the inhibition of cell growth and ethanol production (Azhar et al, 2017).…”

“…Most of the substrate was immediately converted to ethanol. The common inoculum size employed in bioethanol production is 5% and 10% (Azhar et al, 2017). …”