Industrial Biotechnology, Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg SE-412 96, Sweden.
Biotechnol Biofuels. 2014 Apr 8;7(1):54. doi: 10.1186/1754-6834-7-54.
Economically feasible cellulosic ethanol production requires that the process can be operated at high solid loadings, which currently imparts technical challenges including inefficient mixing leading to heat and mass transfer limitations and high concentrations of inhibitory compounds hindering microbial activity during simultaneous saccharification and fermentation (SSF) process. Consequently, there is a need to develop cost effective processes overcoming the challenges when working at high solid loadings.
In this study we have modified the yeast cultivation procedure and designed a SSF process to address some of the challenges at high water insoluble solids (WIS) content. The slurry of non-detoxified pretreated spruce when used in a batch SSF at 19% (w/w) WIS was found to be inhibitory to Saccharomyces cerevisiae Thermosacc that produced 2 g l-1 of ethanol. In order to reduce the inhibitory effect, the non-washed solid fraction containing reduced amount of inhibitors compared to the slurry was used in the SSF. Further, the cells were cultivated in the liquid fraction of pretreated spruce in a continuous culture wherein the outflow of cell suspension was used as cell feed to the SSF reactor in order to maintain the metabolic state of the cell. Enhanced cell viability was observed with cell, enzyme and substrate feed in a SSF producing 40 g l-1 ethanol after 96 h corresponding to 53% of theoretical yield based on available hexose sugars compared to 28 g l-1 ethanol in SSF with enzyme and substrate feed but no cell feed resulting in 37% of theoretical yield at a high solids loading of 20% (w/w) WIS content. The fed-batch SSF also significantly eased the mixing, which is usually challenging in batch SSF at high solids loading.
A simple modification of the cell cultivation procedure together with a combination of yeast, enzyme and substrate feed in a fed-batch SSF process, made it possible to operate at high solids loadings in a conventional bioreactor. The proposed process strategy significantly increased the yeast cell viability and overall ethanol yield. It was also possible to obtain 4% (w/v) ethanol concentration, which is a minimum requirement for an economical distillation process.
经济可行的纤维素乙醇生产要求该过程能够在高固体负荷下运行，这目前带来了技术挑战，包括低效混合导致的传热和传质限制，以及在同步糖化和发酵（SSF）过程中高浓度抑制化合物抑制微生物活性。因此，需要开发具有成本效益的工艺，克服高固体负荷下的挑战。
在这项研究中，我们修改了酵母培养程序并设计了 SSF 工艺，以解决高水不溶性固体（WIS）含量下的一些挑战。在 19%（w/w）WIS 的分批 SSF 中使用未经解毒预处理的云杉浆时，发现对产热酿酒酵母 Thermosacc 具有抑制作用，仅产生 2 g/L 的乙醇。为了降低抑制作用，与浆体相比，含有减少抑制剂的未洗涤固体部分被用于 SSF。此外，细胞在预处理云杉液体部分的连续培养中进行培养，其中细胞悬浮液的流出物用作 SSF 反应器中的细胞进料，以维持细胞的代谢状态。在 SSF 中观察到增强的细胞活力，其中含有细胞、酶和底物进料，在 96 小时后生产 40 g/L 的乙醇，对应于基于可用己糖糖的 53%理论产率，而在 SSF 中仅使用酶和底物进料而不使用细胞进料则导致在高固体负荷 20%（w/w）WIS 含量下仅 37%的理论产率。分批 SSF 还显著缓解了混合问题，这在高固体负荷下的分批 SSF 中通常具有挑战性。
通过简单修改细胞培养程序并在分批 SSF 中结合酵母、酶和底物进料，可以在常规生物反应器中在高固体负荷下运行。所提出的工艺策略显著提高了酵母细胞活力和总体乙醇产率。还可以获得 4%（w/v）的乙醇浓度，这是经济蒸馏过程的最低要求。
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