Abstract:
Thirty strains of Aureobasidium pullulans were screened in the production medium containing 1% (w/v) corn cob as carbon source for the production of xylanase functioning well at pH 7 and room temperature (28±2°C). It was found that A. pullulans AP 46 produced the highest xylanase activity at 2.70±0.11 unit/ml and this enzyme was highly stable at this condition with approximately 60% of its original activity remained after 4-day storage. When the xylanase production of A. pullulans AP 46 was optimized, this strain yielded the highest xylanase activity at 5.19±0.08 unit/ml when cultured in the production medium containing 3.89% (w/v) rice straw as carbon source and 0.75% (w/v) sodium nitrate as nitrogen source at pH 5.5 for 72 hours. The crude xylanase was used to hydrolyze xylans extracted from biomasses of fast-growing plants and weeds, including cogon grass, napier grass, vetiver grass and water hyacinth, in order to produce xylooligosaccharides. After the extraction, the highest yield of xylan at 28.33±3.33% (w/w) was obtained from napier grass and the extracted xylans from all plants comprised the majority of high-molecular-weight polymers. When these xylans was hydrolyzed at pH 7 and room temperature, the amounts of reducing sugars in cogon grass and vetiver grass xylan hydrolysates were the highest and not significantly different from each other. Moreover, the hydrolysis of vetiver grass xylan provided the highest amount of xylobiose and xylotriose combined. Therefore, the optimization of xylooligosaccharide production was carried out using the vetiver grass xylan as substrate. When the xylanase was used for xylan hydrolysis at the concentration of 27.94 unit/g xylan for 92 hour 19 minutes, the highest amount of reducing sugars was achieved at 23.65±1.34 mg/100 mg xylan. The produced xylooligosaccharides were partially purified and tested for their prebiotic property. It was found that they can significantly stimulate the growth of Lactobacillus brevis and Lactobacillus casei when compared with glucose.