假丝酵母发酵玉米芯半纤维素水解液生产木糖醇

采用一株驯化过的假丝酵母(Candida sp.)直接发酵经过简单脱毒处理的玉米芯半纤维素水解液生产木糖醇。确定了水解液的最适浓缩倍数在3.0～3.72的范围内。利用正交实验，确定了摇瓶分批发酵工艺条件的最适组合为：摇床转速180r/min，起始C/N为50，起始pH 5.5，接种量5% (体积比)。在此基础上，重点研究了在发酵罐中通气量对酵母发酵玉米芯水解液生产木糖醇的影响。结果表明采用先高后低的分段通气发酵在木糖醇得率方面明显优于恒定通气发酵；其中，在0～24h，3.75 L/min；24～108h，1.25 L/min的分段通气条件下(装液量为2.5L)，木糖醇得率(木糖醇/木糖，g/g) 达到0.75 g/g。该结果将有助于建立一种高效的、大规模的利用玉米芯半纤维素水解液发酵生产木糖醇的工艺。

Xylitol Production from Corn Cob Hemicellulosic Hydrolysate by Candida sp.

Xylitol, a five\|carbon sugar alcohol, has many interesting application s in the food, pharmaceutical, and odontological industries, owing to its high s weetening power, its anticariogenic properties, and its insulin\|independent met a bolism. The bioconversion of detoxified hemicellulosic 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 pr oductivity of the strain and its tolerance to different toxic or inhibitory comp ounds existing in the hydrolysates. In addition, a number of culture process par ameters proved to have significant effects on xylitol production in hemicellulos ic hydrolysate media. One of the most important control variables in this biocon version is the aeration level, which affects the biochemical pathways in the xyl ose metabolism. The production of biomass is favored by aerobic conditions, whil e under anaerobic conditions xylose cannot be assimilated by yeast, whereas xyl itol is formed in oxygen_limited incubation conditions. An adapted Candida s p. with enhanced resistance to the inhibitors in the hydrolysate can directly fe rment the simply detoxified corn cob hemicellulosic hydrolysate to xylitol. In the pr esent study, the combined effects of shaking speed, C/N ratio, initial pH, and i noculum level on the fermentation of corn cob hemicellulosic hydrolysate to xyli tol by an adapted \%Candida\% sp. were investigated using an orthogonal experime ntal design in flask. As a result, the optimum fermentation conditions we re as follows: 1 80 r/min, a C/N ratio of 50, initial pH 5 5, and an inoculum level of 5% (volum e ratio). Moreover, the optimum concentration factor of hydrolysate varied betwee n 3 0 and 3 72 was obtained. Based on these results, in order to evaluate the effect of aeration rate on the fermentation of corn cob hemicellulosic hydrolysa te to xylitol in fermentor, batch fermentations were carried out in a 3 7 L stirre d fermentor using four different aeration strategies, including three kind of two -stage aeration strategies, which provided relatively high aeration rate in t he early stage but reduced it in the later stage, and including a one-stage aerati on strategy provided a constant aeration rate. With respect to xylitol y i eld, the results indicated that two-stage aeration strategy was significantly s uperior to one-stage aeration strategy. The highest xylitol yield (0 75 g/g) w as 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, w ithout extensive detoxification of hydrolysate, an adapted Candida sp. can effici ently ferment the simply treated corn cob hemicellulosic hydrolysate to xylitol under the optimized fermentation conditions. Thi s work should help the development of an efficient process for producing xylitol from corn cob hemicellulosic hydrolysate on a larger scale by bioconversion.