Diversity of sugar alcohols produced by Aureobasidium spp. and the influence of selected nutrients on mannitol synthesis

Sugar alcohols are a group of diverse polyols derived from monosaccharides that have been used in many commercial products. Mannitol, a six-carbon sugar alcohol, is a natural sweetener produced by many plants such as pumpkin, celery and olive, and microorganisms including yeast, bacteria, fungi, lichen and algae. Mannitol has been widely used as a sugar substitute in food and pharmaceutical industries, especially the products for diabetic patients, since it does not affect blood sugar and insulin levels. Mannitol can be produced via fermentation by yeasts such as Candida spp. and Aureobasidium spp. In nature, most microorganisms produce mannitol from glucose, fructose, sucrose and glycerol with different efficacies. Thus, this study was aimed to (i) study the diversity of polyols produced by seven Aureobasidium strains including A. pullulans (NRRL Y-2311-1, CBS 135684, PBUAP55 and PBUAP102) A. thailandense (NRRL58543, PBUAP17 and PBUAP70) (ii) select the best strain for mannitol production and (iii) investigate the effect of carbon and nitrogen sources on the mannitol production. It was found that these Aureobasidium strains were able to produce a variety of sugar alcohols including mannitol, arabitol, xylitol and glycerol in the strain-dependent manner. Aureobasidium thailandense PBUAP70 produced the highest yield of mannitol at 14% conversion comparing with the rest by using glucose as substrate. Glucose and ammonium sulphate were found to be enhanced the mannitol production of this strain up to 16% conversion which was the highest of the carbon and nitrogen sources tested.

糖醇是一类由单糖衍生而来的多元醇，已被广泛应用于诸多商业产品中。甘露醇作为一种六碳糖醇，是由南瓜、芹菜、橄榄等多种植物，以及酵母菌、细菌、真菌、地衣和藻类等微生物所产生的天然甜味剂。由于其不会影响血糖与胰岛素水平，甘露醇被广泛用作食品及制药工业中的代糖，尤其适用于糖尿病患者相关产品。甘露醇可通过假丝酵母属（Candida spp.）和出芽短梗霉属（Aureobasidium spp.）等酵母菌发酵制备。在自然环境中，多数微生物可利用葡萄糖、果糖、蔗糖及甘油合成甘露醇，但合成效率各不相同。因此，本研究旨在：(i) 探究7株出芽短梗霉属菌株所产多元醇的多样性，其中包括4株短梗霉（A. pullulans：NRRL Y-2311-1、CBS 135684、PBUAP55及PBUAP102）和3株泰国出芽短梗霉（A. thailandense：NRRL58543、PBUAP17及PBUAP70）；(ii) 筛选出甘露醇产量最优的菌株；(iii) 探究碳源与氮源对该菌株甘露醇合成的影响。研究发现，这些出芽短梗霉属菌株可依菌株特异性合成多种糖醇，包括甘露醇、阿拉伯醇、木糖醇及甘油。以葡萄糖为底物时，泰国出芽短梗霉PBUAP70的甘露醇转化率最高，可达14%，优于其余供试菌株。实验结果表明，葡萄糖与硫酸铵可使该菌株的甘露醇转化率提升至16%，为本次测试所用碳源与氮源组合中的最优水平。