Department of Spirit and Yeast Technology, Institute of Fermentation Technology and Microbiology, Faculty of Biotechnology and Food Sciences, Lodz University of Technology, Wolczanska 171/173, 90-924 Lodz, Poland.
Institute of Organic Chemistry, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland.
Int J Mol Sci. 2019 Apr 3;20(7):1659. doi: 10.3390/ijms20071659.
The qualitative and quantitative composition of volatile compounds in fermented distillery mash determines the quality of the obtained distillate of agricultural origin (i.e., raw spirit) and the effectiveness of further purification steps. Propan-2-ol (syn. isopropyl alcohol), due to its low boiling point, is difficult to remove by rectification. Therefore, its synthesis needs to be limited during fermentation by yeast, while at the same time controlling the levels of acetaldehyde and acetic acid, which are likewise known to determine the quality of raw spirit. Lactic acid bacteria (LAB) are a common but undesirable contaminant in distillery mashes. They are responsible for the production of undesirable compounds, which can affect synthesis of propan-2-ol. Some bacteria strains are able to synthesize isopropyl alcohol. This study therefore set out to investigate whether LAB with yeast are responsible for conversion of acetone to propan-2-ol, as well as the effects of the amount of LAB inoculum and fermentation parameters (pH and temperature) on the content of isopropyl alcohol, acetaldehyde, lactic acid and acetic acid in fermented mashes. The results of NMR and comprehensive two-dimensional gas chromatography coupled with time of flight mass spectrometry (GC × GC-TOF MS) analysis confirmed the ability of the yeast and LAB strains to metabolize acetone via its reduction to isopropyl alcohol. Efficient fermentation of distillery mashes was observed in all tested mashes with an initial LAB count of 3.34-6.34 log cfu/mL, which had no significant effect on the ethanol content. However, changes were observed in the contents of by-products. Lowering the initial pH of the mashes to 4.5, without and with LAB (3.34-4.34 log cfu/mL), resulted in a decrease in propan-2-ol and a concomitant increase in acetaldehyde content, while a higher pH (5.0 and 5.5) increased the content of propan-2-ol and decreased acetaldehyde content. Higher temperature (35 °C) promoted propan-2-ol synthesis and also resulted in increased acetic acid content in the fermented mashes compared to the controls. Moreover, the acetic acid content rose with increases in the initial pH and the initial LAB count.
发酵蒸馏醪中挥发性化合物的定性和定量组成决定了所获得的农业起源馏分（即原酒）的质量和进一步纯化步骤的效果。由于其沸点低，异丙醇（又称异丙醇）难以通过精馏去除。因此，需要通过酵母在发酵过程中限制其合成，同时控制乙醛和乙酸的水平，这同样会影响原酒的质量。乳酸菌（LAB）是发酵醪中的常见但不受欢迎的污染物。它们负责产生不良化合物，这会影响异丙醇的合成。一些细菌菌株能够合成异丙醇。因此，本研究旨在调查酵母和 LAB 是否负责将丙酮转化为异丙醇，以及 LAB 接种量和发酵参数（pH 和温度）对发酵醪中异丙醇、乙醛、乳酸和乙酸含量的影响。NMR 和全面二维气相色谱与飞行时间质谱联用（GC×GC-TOF MS）分析的结果证实了酵母和 LAB 菌株通过将丙酮还原为异丙醇来代谢丙酮的能力。在所有测试的醪液中，初始 LAB 计数为 3.34-6.34 log cfu/mL 时，醪液的发酵效率很高，这对乙醇含量没有显著影响。然而，副产物的含量发生了变化。将醪液的初始 pH 降低至 4.5，不添加和添加 LAB（3.34-4.34 log cfu/mL），导致异丙醇含量降低，乙醛含量增加，而较高的 pH（5.0 和 5.5）增加了异丙醇的含量并降低了乙醛的含量。较高的温度（35°C）促进了异丙醇的合成，并且与对照相比，发酵醪中的乙酸含量也增加了。此外，随着初始 pH 和初始 LAB 计数的增加，乙酸含量增加。
