Study On The Xylitol Fermentation From Hemicellulosic Hydrolysate Of Corncob

Corncob is a kind of cheap renewable lignocellulosic resource. The bioconversion of hemicellulosic hydrolysate to xylitol by microorganisms could be a cheaper alternative to the traditional chemical process, since it is a simple process, with great specificity and low energy requirements. Key factors in xylitol production, such as detoxification of hemicellulosic hydrolysate, strain adaptation, fermentation parameters optimization and xylitol recovery were investigated in this work. The main results were as follows:The optimal kinetic parameters for hydrolysis were concentration of H₂SO₄ of 1.00%, hydrolysis reaction temperature of 108 ℃, hydrolysis reaction time of 3 h and the ratio of liquid/solid of 8.It was found that acetic acid and furfural were the primary fermentation inhibitor in corncob hemicellulosic hydrolysate. When the concentrations of acetic acid and furfural exceeded 1.20 g/L and 0.50 g/L respectively, xylitol fermentation process ceased. Detoxification methods, such as lime neutralization, evaporation, absorption with activated charcoal and treatment with anion-exchange resins could remove a given amount of inhibitor. The results indicated that lime neutralization followed by evaporation could remove 55.20% of acetic acid and 73.30% of furfural in the hemicellulosic hydrolysate.Strain adaptation was carried out by increasing the inhibitor concentration in hemicellulosic hydrolysate medium gradually. A well-adapted strain of Candida sp. ZU04 was isolated in the 19th batch by this continuous adaptation process. Candida sp. ZU04 adaptability of acetic acid was increased from 1.20 g/L to 5.41 g/L compared with initial strain. With the strain, xylitol was produced from hemicellulosic hydrolysate untreated with activated carbon or ion-exchange resins, and the xylitol yield increased 4.18 folds compared with initial strain. This process can effectively reduce the detoxification treatment costs, showing broad prospects of industrial applications.The optimum fermentation conditions by Candida sp.ZU04 were as follows: xylose concentration, 100 g/L;corn steep liquor concentration, 2.00%;temperature,32 ℃;initial pH value, 6.00;initial cell concentration, 0.60 g/L. Time course of xylitol production from corn cob hemicellulosic hydrolysate by Candida sp.ZU04 was investigated. Time course showed that biomass increased at the expense of oxygen uptake and xylose consumption at the initial stage of fermentation, then xylitol beganto accumulate when the cells grew well. If this fermentation stage was prolonged, xylitol yield would be improved to some extent.According to the xylitol fermentation mechanism, high aeration rate was applied in the initial stage of fermentation process, and partial inhibitors were decomposed and glucose was consumed by Candida sp.ZU-04 for biomass growth. In the latter fermentation phase aeration rate was reduced, the high ratio of NADH/NAD+ reduce activities of xylitol dehydrogenase(XDH), so xylitol yield could be improved. The maximum xylitol yield (76.00%) and volumetric productivity (0.76 g-L^-h"1) were obtained with the two-phase aeration (0²4 h, 3.75 L/min, 24⁹6 h, 0.75 L/min).Xylitol production from hemicellulosic hydrolysate by recycling cells of Candida sp.ZU04 was proved to be an effective method. In 10 repeated fermentation batches using recycling cells, the average xylitol yield, xylitol concentration and fermentation time were 77.72%, 98.20 g/L and 55.10 h respectively.Xylitol recovery from fermentation broth is a difficult step due to the low xylitol concentration and complex components in the fermentation broth. In order to optimize the decoloration of xylitol broth, macroreticular resins and activated carbons were applied in xylitol fermentation broth decoloration. Results showed that activated carbon AC3 performed best in the absorption of coloring matters. The absorption ratio of coloring matters reached 96.00% and the loss of xylitol was 3.90% when treated with 3% activated carbon AC3 at pH 6.0, 80°C for 40 min. The clarified xylitol fermentation broth was treated with a strong cation-exchange resin D001 and a weak anion-exchange resin D301 to desalt the salt and absorb the impurity after which xylitol crystallization was attempted. Results showed that most of salt was removed from the xylitol broth by the combined D001 and D301 resins. After treating with activated carbon and combined resins, xylitol purity degree was increased from 61.19% to 91.00%.Xylitol crystallization kinetics was investigated in the xylitol broth and xylitol solution. As far as xylitol solution is concerned, xylitol concentration does not drop in the initial crystallization phase, so xylitol solution has longer crystallization time than xylitol broth. Temperature and initial xylitol concentration are two key factors in xylitol crystallization process. Xylitol crystallization rate increased with the increased xylitol concentration. However, xylitol crystallization rate decreased when the crystallization temperature was enhanced. By adding l%0 xylitol crystal seeds, crystallization time was shortened, and improved crystallization rate was obtained. The best result in term of xylitol crystal yield (58.00%) was obtained withconcentrated xylitol broth (750 g/L of xylitol) at temperature (—5°C) by adding l%o xylitol crystal seeds.The xylitol crystallization kinetic model was as follows:(C-C*) 05- (Co-C*) °-5=-0.5KtThe mathematic model was set up for this complex xylitol crystallization process. Comparing with the experiment data, this model could excellently simulate to the experiment data.The xylitol crystal obtained from the xylitol broth has regular tetrahedral shape, and its purity reached 99.9% (analysed by HPLC). After identifying by the IR . MS and tfMF, the crystal is ascertained as xylitol, and xylitol crystal matches the trade grade level.The research work developed an environmental-friendly and economical xylitol production process by fermentation on corncob hemicellulosic hydrolysate using the adapted strain of Candida sp.Z\J04, showing broad prospects of industrial applications.