PROCESS DEVELOPMENT AND OPTIMIZATION FOR PRODUCTION OF XYLITOL FROM LIGNOCELLULOSIC BIOMASS HYDROLYSATE

Abstract

Lignocellulosic biomass (LCB) stands out as an abundant, inexpensive, and promising renewable source of energy that can be used to produce fuels and value-added products. LCB comprises of major 3 components: cellulose, hemicellulose and lignin. These three components can be further transformed into commercially viable and sustainable products like ethanol, xylitol, acetic acid, glutamic acid, glucuronic acid, succinic acid, vanillin, etc. Thus, they can contribute significantly towards developing cost-effective integrated biorefineries. Among these, xylitol has been a tremendously increasing area of interest. With no petrochemical alternative, xylitol turns out to be one of the highest-valued products, which may be produced by utilizing lignocellulosic biomass. Its large-scale production is still carried out through the chemical route by dehydrogenation of xylose under high pressure and temperature. The biochemical route is the potential substitute for the chemical route as it involves milder process conditions and can utilise both industrial and agricultural wastes, thereby reducing the overall production cost. However, a biochemical scheme has not been adopted yet at the industrial scale. Microbial xylitol production links elements of sustainability, health, biotechnology, and economics, making it a relevant and engaging research topic. The present work embarks upon systematically developing the optimisation process for xylitol synthesis using fermentation. The significant medium components were identified using the Plackett Burman design followed by central composite designs to obtain the optimal concentration for the critical medium components in shaker flasks. Subsequently, the effect of operational parameters was examined using the one factor-ata- time method in a 1 L bioreactor. Co-generation of high-value-added products like xylitol, which has various applications in the pharmaceutical and food industries, can support the economics of bioethanol in an integrated biorefinery. Therefore, a sequential fermentation approach using Z.mobilis and C.tropicalis to produce bioethanol and xylitol from mixed sugars (glucose and xylose) in a single bioreactor was proposed. The sequential fermentation without ethanol separation resulted in lower xylose uptake due to the repressive effect of ethanol on C.tropicalis. Thus, an in-situ distillation technique was employed for bioethanol recovery prior to xylose fermentation by C.tropicalis. Fed-batch configuration was further incorporated into this strategy to enhance the xylitol production. For further improvement in the xylitol yield and productivity in batch fermentation, co-substrate supplementation was adopted to resolve the issue of cell maintenance, since the synchronized metabolism of a co-substrate and xylose could maintain constant NADPH regeneration and meet the requirement of carbon intermediates and energy for cell maintenance and growth, which was inadequately fulfilled by xylose consumption under restricted oxygen, enhancing xylitol biosynthesis. The impact of co-substrate supplementation (glucose, fructose, maltose, sucrose, and glycerol) on xylitol biosynthesis by C.tropicalis ATCC 13803 using synthetic xylose in a 1L bioreactor was examined. Glycerol supplementation improved xylitol yield by 1.12 and 1.21 folds, respectively, higher than the without co-substrate supplementation. Fed-batch configuration with glycerol supplementation further enhanced the xylitol productivity. The ability of C.tropicalis for bioethanol and xylitol synthesis under anaerobic and aerobic conditions, respectively was also studied. A novel strategy of integrated sequential anaerobic bioethanol production and aerobic xylitol production via combined glycerol supplementation and xylose fed-batch from the mixed sugars was proposed. This strategy resulted in reduced fermentation time, higher yield and productivity compared to the sequential fermentation for ethanol and xylitol production by Z.mobilis and C.tropicalis respectively. The potential of xylitol synthesis using wheat straw by C.tropicalis in bioreactors was explored. The effect of varying kLa and xylose concentration from hemicellulosic fraction on xylitol synthesis was studied. Subsequently, scale-up for xylitol synthesis was also performed using kLa as a static parameter in 0.5 L, 1 L, and 5 L bioreactor. The control experiments were done using pure xylose for a comparative analysis The approach developed in the present work would offer valuable assistance for the growth of larger commercial-scale fermentation processes.