Study On The Xylitol Fermentation From Corn Cob Hemicellulosic Hydrolysate

Xylitol, a five-carbon sugar alcohol, has many interesting applications in the food, pharmaceutical, and odontological industries, owing to its high sweetening power, its anticariogenic properties, and its insulin-independent metabolism. The bioconversion of detoxified hemiceilulosic hydrolysate to xylitol by microorganisms could be a cheaper alternative to the current chemical process, since it is a simple process, with great specificity and low energy requirements. However, the success of fermentations for xylitol production depends on the productivity of the strain and its tolerance to different toxic or inhibitory compounds existing in the hydrolysates. In addition, a number of culture process parameters proved to have significant effects on xylitol production in hemiceilulosic hydrolysate media. One of the most important control variables in this bioconversion is the aeration level, which affects the biochemical pathways in the xylose metabolism. The production of biomass is favored by aerobic conditions, while under anaerobic conditions xylose can not be assimilated by yeast, whereas xylitol is formed in oxygen-limited incubation conditions.An adapted Candida sp strain with enhanced resistance to the inhibitors in the hydrolysate can directly ferment the simply detoxified corn cob hemiceilulosic hydrolysate to xylitol. In the present study, the combined effects of shaking speed, C/N ratio, initial pH, and inoculum level on the fermentation of corn cob hemiceilulosic hydrolysate to xylitol were investigated using an orthogonal experimental design methodology in shaking flasks. As a result, the optimum fermentation conditions were as follows: 180 r/min, a C/N ratio of 50, initial pH5.5, and an inoculum level of 5% (volume ratio). The optimum concentration factor of hydrolysate was between 3.0 and 3.72. In order to evaluate the effect of aeration rate on the xylitol fermentation on corn cob hemiceilulosic hydrolysate in fermentator, batch fermentations were carried out in a 3.7 L stirred fermentor using four different aeration strategies, including three lands of two-stage aeration strategies, which provided relative higher aeration rate in the early stage but reduced it in the later stage, and including a one-stage aeration strategy that it provided a constant aeration rate. With respect to xylitol yield, the results indicated that two-stage aeration strategy was significantly superior toone-stage aeration strategy. The highest xylitol yield (0.75 g/g) was obtained with oxygen supply strategy C (3.75 L/min for first 24 h, then lowered it to 1.25 L/min, 2.5 L fermentation medium was employed). In this process, without extensive detoxification of hydrolysate, an adapted Candida sp can efficiently ferment the simply treated corn cob hemicellulosic hydrolysate to xylitol under the optimized fermentation conditions. This process can effectively reduce the pretreatment costs, showing broad prospects of industrial applications.Hemicellulosic sugars and inhibitors in corn cob hydrolysate had significant effects on xylitol production. The optimum initial xylose concentration in the culture media was about 100g/L. The inhibition effects of acetic acid and furfural in hydrolysate increased obviously when their concentration exceeded Ig/L. Under the conditions of the initial pH 6.0 instead of pH 4.0, the inhibition effect of acetic acid was reduced. K+, Mg2+, Na+, andH2PO4- could promote xylitol fermentation, especially in the presence of Mg . The optimum salt concentration ranged from 0 to 4g/L. At high ion concentration, cell growth and fermentation was suppressed. In ratios characteristics of hydrolysate, the synthetic media was composed of hemicelluosic sugars (xylose, glucose, arabinose, fructose). The time course of xylitol fermentation on synthetic media was in good agreement with that on the hydrolysate. This result showed flexibility to further study the kinetics of xylitol fermentation on hemicellulosic hydrolysate.The cells of ' Candida sp. were immobilized in Ca-alginate beads and used for...