Fermentation strategies explored for xylitol production

This chapter reviews different fermentation strategies for xylitol production using synthetic media or hemicellulosic hydrolyzates as carbon source. Most of the published works were carried out with free cells in batch operation because of its versatility and easy use in preliminary tests, where the age of inoculum, cell recycling, initial cell concentration, pH, temperature, type and concentration of nutrients in the culture medium, initial xylose concentration, presence of carbon sources other than xylose, and dissolved oxygen level were selected as the main variables. Conversely, continuous fermentation systems were shown to offer additional advantages such as high productivity for long periods of time, elimination of idle time for cleaning and sterilization and simplicity to perform an automated control. The most attractive equipments employed for this purpose were the continuous flow stirred tank, crossflow membrane and submerged membrane bioreactors. The use of cells immobilized by adsorption, entrapment, or covalent binding showed several advantages compared to the free ones, including higher cell density and possible biomass recycling for continuous operation. Repeated-batch fermentations were also investigated to evaluate the durability of immobilized cells with the aim of implementing the technology into a continuous process or scaling up the conversion of xylose to xylitol. Seeking the long-term stability, the packed bed and fluidized bed bioreactors proved to be the most effective equipments; however, their hydrodynamic characteristics and the influence of aeration rate on fermentation performance still deserve further efforts. Finally, the fed-batch process, mainly with free cells, was also reported as an effective tool to keep the substrate at a suitable level throughout the whole fermentation process.