Optimization of xylitol production through Candida tropicalis in xylose hydrolysate from rice husk

Owing to the extensive cultivation and production of rice, which consequently generates a high amount of rice husk, rice husk is an easily available lignocellulosic waste; however, the applications it currently has do not entirely exploit its potential. This study aims the production of xylitol as an opportunity to value this residue through fermentation of rice husk hydrolysate. For it, the best particle size of rice husk among 0.25-0.6 mm, 0.6-1.2 mm, and unmilled rice husk (URH) for dilute acid hydrolysis was determined by using a randomized complete block design (RCBD) to select the particle size that produced the highest amount of xylose release; subsequently, adaptation of Candida tropicalis was carried out as an alternative to common detoxification methods, by increasing the concentration of hydrolysate in the medium in 20% every 2 days, and then, xylitol fermentation from the best particle size hydrolysate with C. tropicalis was optimized performing a central composite design (CCD) 2(2). The best particle size for dilute acid hydrolysis was URH and produced a xylose concentration of 12.85 g/L, and the optimal conditions for xylitol fermentation were 4.41 g/L of inoculum and 68.28 g/L of xylose and accomplished a concentration of 36.74 g/L of xylitol, a yield of 0.58 g/g xylose, and a volumetric productivity of 0.34 g/L/h, providing evidence that rice husk has the potential to be valorized as a source of fermentable sugars for the production of xylitol as a high-value metabolite and proving adaptation method as a viable alternative for the strain's growth as well as xylitol production.