Compared with currently available strains, a robust yeast strain ( Saccharomyces cerevisiae NGY10) that can produce up to 15.5% more ethanol when glucose or lignocellulose biomass — rice and wheat straw — is fermented has been isolated by researchers from the International Centre for Genetic Engineering and Biotechnology, DBT-ICGEB Centre for Advanced Bioenergy Research, Delhi.
In India, ethanol production is mostly by fermenting molasses to meet the annual target of 5% blending of petrol with ethanol. But with India setting a target of blending petrol with 10% of biofuel by 2022, other sources such as rice and wheat straw have to be considered. Fermenting lignocellulose efficiently to generate more ethanol than what is currently possible is therefore necessary. To that end, the strain isolated by ICGEB becomes important.
The team led by Dr. Naseem A. Gaur from the Yeast Biofuel Group at ICGEB isolated 500 yeast-like colonies from different natural habitats — distillery waste, dairy waste, hot springs, sewage and algal bloom. After screening, 25 yeast-like colonies were chosen and an additional nine yeast strains from the National Culture collection of Industrial Microorganisms (NCIM), Pune, were included for evaluation. Of these, one strain was found to suitable for fermenting rice and wheat straw. The results were published in the journal Biotechnology for Biofuels.
Lignocellulose is comprised of lignin, cellulose and hemicellulose. While cellulose is rich in hexose or C6 (glucose) sugar, hemicellulose, which accounts for about 30% of the composition, is made mostly (more than 90%) of pentose or C5 (xylose and arabinose).
Three challenges
Ethanol production by fermenting lignocellulose biomass faces three challenges. During the fermentation process, the temperature increases from about 30 degree C to 40 degree C. Since the commercially available yeast strains are good at fermenting at 30 degree C, the fermentor has to be cooled down when the temperature increases. This increases the cost of ethanol production. Second, lignocellulose biomass (rice and wheat straw) contains a mixture of hexose and pentose sugars.
Though yeast can ferment glucose (hexose sugar), it cannot ferment pentose sugar (xylose and arabinose) that make up 30% of the composition. Finally, the pretreatment of lignocellulose (to breakdown the recalcitrant structure of the biomass) results in the production of three main inhibitors (furfural, 5-HMF and acetic acid). These inhibitors reduce the fermentation performance of yeast, leading to reduced ethanol production.
Functional superiority
Unlike currently available, commercially used yeast strains, the strain (NGY10) isolated by the ICGEB team has been found to be thermotolerant and can continue to ferment the biomass even when the temperature increases to 40 degree C. “The strain (NGY10) displayed a negligible reduction in the growth even in the presence of all three fermentation inhibitors at 40 degree C. And it produced more ethanol than currently available industrial yeast strains,” says Dr. Gaur. “But the NGY10 strain was not able to ferment the pentose sugar (xylose and arabinose).”
“Our strain showed better efficiency than the industrial strains now available in producing ethanol from lignocellulose. Also, the NGY10 isolate produced 11.1 % and 15.49 % more ethanol compared with the industrial yeast (Angel yeast) when glucose and pretreated lignocellulose were fermented, respectively,” says Ajay Kumar Pandey from ICGEB and first author of the paper.
Engineering the strain
The NGY10 strain can be metabolically engineered so it can ferment both hexose and pentose sugars leading to increased production of ethanol using lignocellulose.
This will increase the quantity of ethanol produced from lignocellulose biomass but also reduce the cost of ethanol production.
“We have almost engineered the strain to make it capable of fermenting both pentose and hexose sugars,” Dr. Gaur says. “The productivity and ethanol yield from pentose sugars after metabolic engineering are encouraging and comparable with yield obtained with glucose (hexose sugar) fermentation.”
