by Yuda Benjamin

Sugar cane energy

New scientific method helps to identify most cost-effective sugar cane varieties

Sugar cane has been gazetted to contribute to bio-ethanol production

The mandatory blending of biofuels has been gazetted at minimum concentrations of 5% for biodiesel and 2% to 10% for bio-ethanol, and the country’s biofuels strategy allows the use of grain sorghum, sugar cane and sugar beet for bio-ethanol production; and sunflower, canola and soyabeans for biodiesel production.

This has exciting possibilities for the local biofuels market, says Professor Emile van Zyl, Stellenbosch University senior chair of energy research in biofuels. He points out that the mandatory blending of biofuels with petrol and diesel could lead to the creation of a vibrant biofuels market which, in turn, could kick-start a bioenergy industry in South Africa.

Traditionally produced from high-cost sugars such as sucrose found in sugar cane, or starch found in maize and wheat, ethanol is widely regarded as one of the promising alternative fuels that could one day replace petrol.

Sugar cane is one of the crops that the South African Biofuels Strategy has identified for ethanol production because of its high productivity per unit area of land compared to other crops. It also contains high amounts of sugar (the sweet juice extracted from sugar cane), which can be fermented directly by yeast to produce ethanol.

However, sugar cane is not fully utilised for the production of ethanol. Carbohydrates in sugar cane fibre (the residue left after sweet juice extraction) can be used to increase ethanol production because of its availability and the fact that it does not have an impact on food production. This could also help avoid restrictions on the use of food crops for ethanol production.

Sugar cane fibre, like other agricultural residues, is mainly composed of carbohydrates protected by a gluey material called 'lignin', which makes it difficult to access the carbohydrates. To access the carbohydrates and convert them to simple sugars suitable for fermentation, the fibre must first be put through an expensive pre-treatment process to disrupt the structure. Once this is done, it has to go through another costly process called 'enzymatic hydrolysis', which uses enzymes as a biological catalyst.

Not surprisingly, the biggest challenge in processing sugar cane fibre for ethanol is to reduce the costs.

A new scientific method developed as part of my doctoral research in process engineering helps to identify the most cost-effective sugar cane varieties for the production of ethanol.

I evaluated 115 fibres from different sugar cane varieties to determine their chemical components, and passed them through pre-treatment and enzymatic hydrolysis processes for the initial screening. From the 115 fibres, 34 varieties were selected and evaluated further at various pre-treatment magnitudes. After the evaluation, six potential varieties were identified, with each variety being investigated further to determine the maximum carbohydrates that could be extracted.

Finally, three varieties were selected as the best for ethanol production because they yielded high sugar cane productivity, high sugar content in sweet juice, high levels of carbohydrates in the fibre and a low lignin content.

Almost 90% of the carbohydrates could be extracted as simple sugars from the selected varieties compared to 72% obtained from sugar cane already existing in the market. The selected varieties required less pre-treatment and less enzymes during enzymatic hydrolysis.

The sugars released from the fibres after pre-treatment and enzymatic hydrolysis differed substantially because of the chemical composition of the fibres.

The study shows that sugar cane fibre has the potential to boost the production of ethanol and, in so doing, address the challenge of diminishing fossil fuels reserves and reduced petroleum production. The new method can be applied by the agricultural and biofuels sector to select the best varieties and to use the entire crop for ethanol production.

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