Study On The Key Technique Of Xylitol Fermentation From Corn Cob Hemicellulosic Hydrolysate

Xylitol, a five-carbon sugar alcohol, has many applications in the food, pharmaceutical, and odontological industries, owing to its sweetening power equal to sucrose, anticariogenic properties, and insulin-independent metabolism. Xylitol is currently manufactured by catalytic hydrogenation of xylose, an expensive process mainly because it involves a large number of purification steps. In addition, the nickel catalyst used can pollute the environment. This research investigated the bioconversion of xylose to xylitol by Candida sp., a biological xylitol production process, which holds great economic and social value compared to the chemical process. This microbial production of xylitol requires neither xylose purification nor high temperatures and pressures. It bears many advantages, including high specificity and low energy requirements, and more importantly, no environmental pollution.Generally, synthetic xylose solution was used as the fermentation medium for producing xylitol. However, the high overall processing costs prevent it from a large scale application. Corn cob, consisting of mainly lignocellulosic residue, has many advantages: renewable, widespread, inexpensive, and xylose-abundant. Firstly, the degradation of corn cob hemicellulose into fermentable xylose, which can be performed by dilute sulphuric acid hydrolysis. Subsequently, the corn cob hemicellulosic hydrolysates could be efficiently converted to xylitol by the adapted Candida sp. cells. Totally,39.4 g/L reducing sugar exists in the corn cob hemicellulosic hydrolysates, which were prepared under the optimal process parameters (Reuse of wash water and unconcentrated hemicellulosic hydrolysates). Xylose constitutes 81.9% of the total reducing sugar, and concentrations of acetic acid, furfuraldehyde and other fermentation inhibitors were lower than the maximum allowable concentration of each inhibitor. In this thesis, the corn cob hemicellulosic hydrolysates could be directly and efficiently converted to xylitol without decolorization or ion-exchanged by the adapted Candida sp. cells. This process can effectively reduce the pretreatment costs, and its fermentation results are desirable, laying favourable foundation for industrial applications.In this study, we optimized the major processing parameters for xylitol fermentation by an adapted Candida sp. Moreover, a series of scale-up xylitol fermentation experiments were performed in a 3.7-L fermentator. Meanwhile, batch xylitol fermentations were carried out under the optimum culture conditions by Candida sp. Fermentation on synthetic xylose solutions produces xylitol at an average rate of 2.46 g/(L·h) and yield of 0.83 g xylitol/g xylose, whereas fermentation on hemicellulosic hydrolysates generates the average xylitol productivity (0.66 g/(L·h)) and xylitol yield (0.75 g xylitol/g xylose). In addition, fed-batch cultures of hemicellulosic hydrolysates were employed, which could alleviate the inhibition of xylose, xylitol, acetic acid, furfuraldehyde, and other inhibitors, and therefore might increase the fermentation efficiency. As for the continuous xylitol fermentation on hemicellulosic hydrolysates, the best results were obtained with a dilution rate of 0.015 h-1, compared to batch fermentation, the xylitol productivity and xylitol yield increased by 63.23% and 5.33%, respectively.In the early stage of the xylitol fermentation by Candida sp., relatively high oxygen supply favoured higher cell growth, but in the later stage, relatively low oxygen supply, led to substantial accumulation of xylitol. Therefore, in this research, immobilized Candida sp. cells were used for xylitol fermentations on corn cob hemicellulosic hydrolysates, which can omit the cell growth phase, direct toward xylitol biosynthesis immediately, shorten the fermentation period, readily employ continuous production, and easily automate the process. In a 2.0 L bubble column bioreactor, xylitol fermentations on corn cob hemicellulosic hydrolysates were carried out under the optimized conditions by Candida sp. cells immobilized in calcium alginate, the fermentation results were good and continuously stable, the immobilized cells could be successively reused, rendering the process efficient and economical, and other beneficial characteristics.The process parameters of decolorization, purification and crystallization of the xylitol fermented broths had been investigated primarily, and the results confirmed the feasibility of xylitol crystallization from purified fermentation broths, showing broad prospects of industrial applications. This work developed an environmental-friendly, simple, efficient, and economical xylitol production process by fermentation on corn cob hemicellulosic hydrolysates using the adapted Candida sp. The results should not only favor higher biconversion of corn cob, an important renewable resource, but also help the persistent development of xylitol industrial production.