Xylitol production from xylose and co-fermenting sugars by a hyperacidophilic Candida tropicalis using fed-batch fermentation strategy

On industrial scale, xylitol is currently manufactured by the catalytic hydrogenation of xylose, but, as it is well-known, the process still includes expensive separation and purification steps. Alternatively, it can also be produced by microbiological methods with xylose as carbon source. This route could be of economical interest since xylose-to-xylitol conversion is a selective process and high yields can be achieved. Among yeast, the best xylitol producers were considered those from genus Candida, because, unlike the most common Saccharomyces spp., they contain the enzyme xylose reductase, which catalyses the first step of xylose metabolism. Several Candida strains convert D-xylose to xylitol in high yield, including C.boidinii, C.guillermondii, C. parapsilosis and C. tropicalis, due to their high D-xylose uptake rate and xylitol production capacity. Despite the large information available on the effect of operating parameters on xylitol production by Candida yeasts, only few studies have been performed on C. tropicalis DSM 7524, which has the peculiarity to be hyperacidophilic. The hyperacidophilic behavior of C. tropicalis makes this strain particularly promising for an industrial application, due tothe possibility to work in non-sterile conditions. In particular Candida tropicalis is an excellent model organism in xylitol production studies, because it has well-developed pentose phosphate pathway (PPP) and can grow on xylose as a sole carbon and energy source. In this work several preliminary batch fermentation, both in flask and on lab fermenter scale were carried out to clarify the effects of pH, temperature, aeration rate and substrate concentration on xylitol production. Because a high concentration of a polyhydroxyl compound, as xylitol, increases the osmotic pressure, which could affect cell metabolism and interfere with the membrane transport system, the use of a fed-batch strategy has been also described. The maximum xylitol yield was 83,66% (w/w) on consumed D-xylose in microaerophilic conditions (kla 2 h-1). Scaling up on 3 L fermenter, using a fed-batch strategy, the best xylitol yield was 86,84% (w/w), against a 90% of theoretical yield.