by Vuyisile L. Dube

ACROSS Southern Africa, common thatch grass, Hyparrhenia spp., covers vast stretches of landscape, used for roofing, fencing, and little else once it matures and loses its nutritional value. 

Research published in the NUST journal, the Zimbabwe Journal of Science and Technology (ZJST), suggests this overlooked plant could serve a far more valuable purpose: as a locally available feedstock for the production of bioethanol.

The study, led by Professor Thembekile Ncube and Mr Webster Ndlovu of NUST's Department of Applied Biology and Biochemistry, together with Professor Elbert Jansen van Rensburg of the University of Limpopo's Department of Microbiology, Biochemistry and Biotechnology, investigated whether pre-treated thatch grass could be broken down into fermentable sugars and subsequently converted into ethanol by two yeast species.

The process begins with size reduction and pre-treatment of the grass, using either acid or alkali to strip away lignin, the tough structural compound that acts as a barrier to sugar extraction. 

The pre-treated grass was then enzymatically saccharified, a process by which enzymes break down cellulose and hemicellulose into simpler fermentable sugars, using a mixture of Celluclast™ and an Aspergillus niger cellulase blend. 

The resulting sugar-rich liquid, known as a hydrolysate, was then fermented by two yeast strains: Saccharomyces cerevisiae WBSA 1386, the species most commonly associated with conventional fermentation, and Candida shehatae CSIR Y-0492, a species known for its ability to ferment xylose, a sugar found in hemicellulose that S. cerevisiae cannot typically utilise. 

Both organisms performed well, with ethanol production exceeding the theoretical expected yield. 

“Both S. cerevisiae WBSA 1386 and C. shehatae CSIR-Y 0142 were able to produce ethanol above the theoretical expected value of 0.5 g/L, indicating the use of sugars other than glucose in the production of ethanol. Ethanol production was at an average of 8.3 g/L for both organisms,” the paper notes.

This suggests that the yeasts drew on additional sugars present in the hydrolysate, including xylose and sucrose, to produce ethanol beyond what glucose alone could account for.

Equally significant was what did not make a difference. Neither the method of pre-treatment nor the addition of a nutrient supplement to the fermentation broth had a measurable effect on output. 

This finding potentially carries practical implications for anyone considering scaling the process up.

"The pre-treatment method and addition of nutrients had no significant effect on the amount of bioethanol produced from the grass hydrolysate,” noted the papers.

This suggests that industrial-scale bioethanol production from thatch grass would not necessarily require costly nutritional additives or strict adherence to a particular pre-treatment method, potentially lowering the barrier to commercial viability. 

The paper also highlighted the possibility that the yeast strains used showed some tolerance to inhibitory compounds that acid pre-treatment can introduce, compounds such as furfural and phenolic substances known to hamper fermentation in other studies.

The conclusion drawn by the research team is clear: pre-treated common thatch grass, once broken down into simple fermentable sugars, can serve as a viable substrate for bioethanol production. 

With fossil fuel depletion and climate pressures driving global interest in renewable energy, this research positions an abundant, low-value African grass as a credible contributor to a cleaner energy future. 

Below is the abstract and full paper.

Abstract

Fossil fuels, such as oil, coal and natural gases, still remain the prime sources of energy worldwide. These resources are likely to be depleted within the next few decades. Current environmental issues like global warming, acid rain and urban smog have led to a shift of focus to utilising renewable energy sources, such as solar, wind, and biofuels, which are less environmentally harmful and are sustainable. Ethanol is one of the most promising alternative biofuels in this respect. A number of biomass feedstocks have been considered for bioethanol production, among which grasses have been suggested. In this study, bioethanol was produced by the fermentation of a hydrolysate obtained from the saccharification of pre-treated common thatch grass (Hyparrhenia spp.). The grass is abundant in Southern Africa and can be used as feed during its early stages of development. Older grasses have low nutritional value due to lignification and are thus used for thatching, fences or are burnt to clear the veld. Size reduction was done to the grass, after which it was pre-treated with either alkali or acid to remove lignin, which is a barrier to saccharification. The grass was enzymatically saccharified using a mixture of cellulases, namely Celluclast™ and Aspergillus niger cellulase mixture, to produce glucose and other fermentable sugars. Candida shehatae CSIR Y-0492 and Saccharomyces cerevisiae WBSA 1386 were utilised to convert the glucose in the hydrolysate to ethanol. The effect of adding a nutrient supplement on the production of ethanol was also investigated. The pre-treatment method and addition of nutrients had no significant effect on the amount of bioethanol produced from the grass hydrolysate. Both S. cerevisiae WBSA 1386 and C. shehatae CSIR-Y 0142 were able to produce ethanol above the theoretical expected value of 0.5 g/L, indicating the use of sugars other than glucose in the production of ethanol. Ethanol production was at an average of 8.3 g/L for both organisms. The results obtained indicate that pre-treated thatch grass, when saccharified by cellulases and xylanases, can potentially be utilised for the production of bioethanol.

Key words: ethanolic fermentation, bioethanol, saccharification, thatch-grass, hydrolysate

The paper is accessible at: https://journals.nust.ac.zw/index.php/zjst/article/view/317