Isolation and Evaluation of Xylose fermenting Thermotolerant Yeasts for Bioethanol Production



Xylose is the main fermentable sugar obtained by hydrolysis of hemicellulosic fraction of lignocellulosic materials. Xylose-fermenting microorganisms are essential for the economic conversion of lignocellulose to ethanol.

The aim of this work was therefore to isolate and evaluate thermotolerant xylose fermenting yeasts. Natural habitats of yeasts were examined for the presence of thermotolerant strains able to produce ethanol from xylose.

Soil, wood and fruit (pawpaw, orange, mango, pineapple and cashew) samples were screened by enrichment in 2% xylose-yeast extract liquid medium. Among the 320 thermotolerant yeasts isolated, 45 produced more than 1g/l of ethanol from 20g/l of xylose.

When their fermentation ability was tested in 3% xylose, only three isolates (Pa27, Ma9, and Pi131) produced more than 7g/l of ethanol after 72 hours and were selected for further studies.


Molecular identification was carried out using Internal Transcribed Spacer (ITS) resulting in determination of the species. Isolate Pa27 was Pichia kudriavzevii strain DBMY82, Ma9 was Candida tropicalis strain m56a and Pi131 was Pichia kudriavzeviistrain H156A.

Optimization studies were carried out to check the effects of different process parameters such as initial pH, inoculum size, temperature, concentration of xylose, and xylose-glucose ratio on ethanol production and yield. 


In recent years, there has been increasing interest on sustainable energy resources, such as biofuels, due to the fluctuation in petroleum prices, decreasing petroleum reserves and environmental problems related to green-house gas (GHG) emissions (Elbehri et al., 2013; Wise et al., 2014).

One particular area of interest has been the production of liquid transportation fuels from lignocellulosic biomass with the main focus on alcohols as alternative fuel sources (Uihlein and Schbek, 2009).

Since ethanol is an oxygenated fuel, it can be easily blended with gasoline and can be widely used for transportation purposes across the globe. The use of biofuels will significantly reduce the emission of exhaust gases thereby resulting in a clean and eco-friendly environment.

Ethanol is one of the best tools to fight vehicular pollution, contains 35% oxygen that helps complete combustion of fuel and thus reduces harmful tailpipe emissions (Balat and Balat, 2008).

The motivation for increased biofuel production varies with regions, but the core reasons were: to reduce fossil fuel consumption, to increase energy diversity in transport, to enhance energy supply security, to reduce greenhouse gas (GHG) emissions and to support farmers (Binod et al., 2010; Fierro et al., 2013).

The first-generation bioethanol is produced mainly from different crops such as corn, wheat, sugarcane, rice and barley.

The use of these crops for ethanol production can lead to shortage of food (Ajanovic, 2011; Månsson et al., 2014). Thus, non-food biomass such as lignocellulosic materials have become an attractive low-cost material for ethanol production.

Lignocellulosic biomass from agricultural residues, municipal paper wastes, dedicated energy crops and other sources are projected to be the major renewable feedstocks for sustainable production of biofuels (Banerjee et al., 2010).


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CSN Team.


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