With around two million tonnes of cotton stalk waste, India is one of the largest countries in the world where utilization or recycling of waste is next to negligible. However, scientists from CSIR’s National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram, Kerala have been able to turn cotton stalk waste into wealth. They have produced ethanol from discarded cotton-stalks by using a combination of chemical and biological techniques.
The stalks were first treated with an acid, alkali and different enzymes to breakdown the complex organic polymers of the stalk. The acid helps to remove hemicellulose, a polymer of the cell wall and the alkali extracts lignin, a binding matrix in the cell wall, made of complex phenolics. These treatments expose cellulose, the major component made of glucose to the action of enzymes. The cellulose was further treated using enzymes to convert it into glucose.
To convert the glucose into ethanol, fermentation was carried out using a novel yeast strain. Here, the scientists isolated the yeast-Saccharomycescerevisiae-RRP-03N, from a rotting wild fruit they have found in the Silent Valley National Park in Palakkad, Kerala. In spite of several inhibitors of microbial growth produced during chemical treatment, the yeast performed better than distiller’s yeast strains in fermenting the cotton stalk hydrolysate. The yeast showed a glucose conversion efficiency of 76 per cent and the entire glucose was utilised by the yeast in just 24 hours and converted into alcohol. This performance was superior to any other organism reported for fermentation of cotton stalk. The final alcohol obtained can be made to fuel grade bio-ethanol (>99 per cent purity), after distillation and dehydration using molecular sieves, which is an existing technology practiced in the distilleries.
Bio-ethanol has a number of advantages over conventional fuels as it comes from a renewable resource. It is mandatory to blend 10 per cent ethanol with petrol. Bio-ethanol presently in use is obtained by fermentation of sugar cane molasses which is a byproduct of sugar production, and has food value. Most of this first generation ethanol finds its way into consumer applications, primarily as liquor. Converting the agro-residues to ethanol reduces the food vs fuel competition.