ENEA Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Laboratory of Technology and Engineering for Biomass, S.S. 106 Jonica, 75026 Rotondella (MT), Italy.
N Biotechnol. 2013 Sep 25;30(6):591-7. doi: 10.1016/j.nbt.2013.02.003. Epub 2013 Feb 27.
Bioethanol can be produced from several biomasses including lignocellulosic materials. Besides 6-carbon sugars that represent the prevalent carbohydrates, some of these feedstocks contain significant amounts of 5-carbon sugars. One common limit of the major part of the xylose-fermenting yeasts is the diauxic shift between the uptake of glucose and xylose during the fermentation of mixed syrups. Thus, optimized fermentation strategies are required. In this paper the ability of Scheffersomyces stipitis strain NRRLY-11544 to ferment mixed syrups with a total sugar concentration in the range 40-80 g/L was investigated by using mono cultures, co-cultures with Saccharomyces cerevisiae strain Bakers Yeast Type II and single cultures immobilized in silica-hydrogel films. The experimental design for the fermentations with immobilized cells included the process analysis in function of two parameters: the fraction of the gel in the broth and the concentration of the cells loaded in the gel. Furthermore, for each total sugars level, the fermentative course of S. stipitis was analyzed at several glucose-to xylose ratios. The results indicated that the use of S. stipitis and S. cerevisiae in free co-cultures ensured faster processes than single cultures of S. stipitis either free or immobilized. However, the rapid production of ethanol by S. cerevisiae inhibited S. stipitis and caused a stuck of the process. Immobilization of S. stipitis in silica-hydrogel increased the relative consumption rate of xylose-to-glucose by 2-6 times depending on the composition of the fermentation medium. Furthermore the films performances appeared stable over three weeks of continuous operations. However, on the whole, the final process yields obtained with the immobilized cells were not meaningfully different from that of the free cells. This was probably due to concurrent fermentations operated by the cells released in the broth. Optimization of the carrier characteristics could improve the performances of the process with immobilized cells.
生物乙醇可以由多种生物质生产，包括木质纤维素材料。除了代表主要碳水化合物的六碳糖外，其中一些原料还含有大量的五碳糖。大多数木糖发酵酵母的一个常见限制是在混合糖浆发酵过程中葡萄糖和木糖吸收之间的双相转化。因此，需要优化发酵策略。在本文中，使用单培养物、与酿酒酵母菌株 Baker’s Yeast Type II 的共培养物以及固定在硅石-水凝胶膜中的单培养物，研究了酿酒酵母 NRRLY-11544 菌株在总糖浓度为 40-80 g/L 的范围内发酵混合糖浆的能力。用于固定化细胞的发酵实验设计包括根据两个参数进行过程分析：凝胶在培养基中的分数和凝胶中加载的细胞浓度。此外，对于每个总糖水平，在不同的葡萄糖与木糖比下分析了 S. stipitis 的发酵过程。结果表明，与游离的 S. stipitis 单培养物或固定化的 S. stipitis 单培养物相比，游离的 S. stipitis 和 S. cerevisiae 的共培养物可确保更快的过程。然而，S. cerevisiae 的快速乙醇生产抑制了 S. stipitis 并导致过程停滞。S. stipitis 的硅石-水凝胶固定化增加了木糖对葡萄糖的相对消耗速率，取决于发酵培养基的组成，可增加 2-6 倍。此外，在连续三周的运行中，膜的性能表现稳定。然而，总的来说，固定化细胞的最终过程收率与游离细胞的收率没有明显差异。这可能是由于在培养基中释放的细胞同时进行发酵。载体特性的优化可以提高固定化细胞过程的性能。
