GUWAHATI: Indian Institute of Technology (IIT) Guwahati researchers have developed an ultrasound-assisted fermentation method to produce a safe sugar substitute called ‘Xylitol’ from sugarcane biogases (the residue left after crushing of sugar cane).
This method overcomes the operational limitations of chemical methods of synthesis and the time delays associated with conventional fermentation.
With increasing awareness of the adverse effects of white sugar (sucrose), not only for patients with diabetes but also for general health, there has been a rise in the consumption of safe alternative sweeteners.
Xylitol, a sugar alcohol derived from natural products, has potential ant diabetic and anti-obesogenic effects, is a mild prebiotic and protects teeth against caries.
The research team was led by Prof. V.S. Moholkar, Department of Chemical Engineering, IIT Guwahati, and included Dr. Belachew Zegale Tizazu and Dr. Kuldeep Roy who co-authored the research papers.
This Research was published in two peer-reviewed journals Bioresource Technology (https://doi.org/10.1016/j.biortech.2018.07.141) and Ultrasonics Sonochemistry (https://doi.org/10.1016/j.ultsonch.2018.06.014)
“The use of ultrasound during the fermentation process not only reduced the time of fermentation to 15 hours (against almost 48 hours in conventional processes) but also increased the yield of the product by almost 20%,” Prof. V.S. Moholkar, Department of Chemical Engineering, IIT Guwahati, said.
“The researchers used only 1.5 hours of ultrasonication during the fermentation, which means that not much ultrasound power was consumed in the process. Thus, xylitol production from sugarcane bagasse using ultrasonic fermentation is a potential opportunity for forwarding integration of sugarcane industries in India,” he said.
Xylitol is industrially produced by a chemical reaction in which wood-derived D-xylose, a costly chemical, is treated with nickel catalyst at very high temperatures and pressures that makes the process highly energy consuming.
Only 8-15% of the Xylose is converted to Xylitol and the method requires extensive separation and purification steps, all of which translate to high price for the consumer.
Fermentation is a biochemical process that is attractive to tackle these issues.
Fermentation is not a new process – the conversion of milk into curd in many households in India is fermentation.
In fermentation, one substance is converted to another using various types of micro-organisms like bacteria and yeast.
However, fermentation processes are slow – the conversion of milk into curd, for example, takes many hours, and that poses a major hurdle in using these processes on commercial scales.
The IIT Guwahati researchers used two approaches to overcome the two problems:
First, they used sugarcane bagasse, the waste fibrous material produced after extracting juice from sugarcane, as the raw material. This overcomes the cost limitations of current xylitol synthesis methods and provides a method to upcycle a waste product.
Secondly, they used a new type of fermentation process wherein, the microbe-induced synthesis of xylitol is hastened by the application of ultrasound waves.
The researchers first hydrolyzed the hemicellulose in bagasse into five-carbon (pentose) sugars such as xylose and arabinose.
For this, they chopped 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 this to bring about fermentation.
Under normal conditions, the fermentation of the xylose to xylitol would take 48 hours, but the team hastened the process by subjecting the mixture to ultrasound waves.
Ultrasound is sound that has a frequency higher than the human ear can hear. When a solution containing microbial cells is subjected to low-intensity ultrasonic waves, microbial cells eat, digest and excrete faster.
Without ultrasound, only 0.53g xylitol was produced per gram of xylose, but on subjecting the process to ultrasound, the yield was 0.61g/gram of xylose.
This number translates to 170 g of xylitol per kilogram of bagasse. The yield could be further increased to 0.66g/gram of xylose and the fermentation time reduced to 15 hours by immobilizing the yeast in polyurethane foam.
While the IIT Guwahati team’s findings are exciting, a challenge must be overcome to scale up the process.
“The present research has been carried out on a laboratory scale. Commercial implementation of sonic fermentation requires the design of high power sources of ultrasound for large-scale fermenters, which in turn requires large-scale transducers and RF amplifiers, which remains a major technical challenge,” said Moholkar.