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Method developed by IIT Guwahati researchers to produce ‘xylitol’, which has potential anti-diabetic effects, overcomes limitations of synthetic chemical methods and delays associated with conventional fermentation

Researchers at India’s Guwahati Institute of Technology have developed an ultrasonically assisted fermentation method to produce a safe sugar substitute called ‘xylitol’ from sugar cane bagasse (the residue left after grinding sugar cane). This method overcomes the operational limitations of synthetic chemical methods and the delays associated with conventional fermentation.

With growing awareness of the harmful effects of white sugar (sucrose), not only for patients with diabetes but also for health in general, the consumption of safe alternative sweeteners has increased. Xylitol, a sugar alcohol derived from natural products, has potential anti-diabetic and anti-obesogenic effects, is a mild prebiotic and protects teeth against cavities.

The research team was led by Prof. VS Moholkar, Department of Chemical Engineering, IIT Guwahati, and included Dr. Belachew Zegale Tizazu and Dr. Kuldeep Roy, co-authors of the research papers. This research has been published in two peer-reviewed journals Bioresource Technology and Ultrasonics Sonochemistry.

Xylitol is produced industrially by a chemical reaction in which wood-derived D-xylose, an expensive chemical, is treated with a nickel catalyst at very high temperatures and pressures, making the process very energy intensive. Only 8-15% of xylose is converted to xylitol and the method requires numerous separation and purification steps, all of which result in a high price for the consumer.

Fermentation is an interesting biochemical process to solve these problems. Fermentation is not a new process – the conversion of milk into curds in many households in India is fermentation. During fermentation, one substance is converted into another with the help of various types of microorganisms such as bacteria and yeasts. However, fermentation processes are slow – converting milk to curd, for example, takes many hours, which poses a major obstacle to using these processes on a commercial scale.

Researchers at IIT Guwahati used two approaches to overcome the above two problems:

– First, they used sugarcane bagasse, the fibrous waste produced after extracting juice from sugarcane, as a raw material. This overcomes the cost limitations of current xylitol synthesis methods and provides a method for recycling a waste product.

– Second, they used a new type of fermentation process in which microbe-induced xylitol synthesis is accelerated by the application of ultrasonic waves.

The researchers first hydrolyzed hemicellulose from bagasse into five-carbon sugars (pentoses) such as xylose and arabinose. For this, they cut the bagasse into small pieces and treated them with dilute acid. The sugar solution was then concentrated and a form of yeast called Candida tropicalis was added to it to cause fermentation.

Under normal conditions, fermentation of xylose to xylitol would take 48 hours, but the team sped up the process by subjecting the mixture to ultrasound waves. Ultrasound is a sound whose frequency is higher than what the human ear can hear. When a solution containing microbial cells is subjected to low intensity ultrasound waves, the microbial cells eat, digest and excrete faster.

Without ultrasound, only 0.53 g of xylitol was produced per gram of xylose, but subjecting the process to ultrasound the yield was 0.61 g/gram of xylose. This number translates to 170g of xylitol per kilogram of bagasse. The yield could be further increased to 0.66 g/gram xylose and the fermentation time reduced to 15 hours by immobilizing the yeast in polyurethane foam.

Although the findings of the IIT Guwahati team are exciting, there is a challenge to be met to scale up the process.