Center for Microbial Ecology and Technology (CMET), Faculty of Bioscience Engineering Ghent University, Coupure Links 653, 9000 Ghent, Belgium; Center for Advanced Process Technology for Urban Resource recovery (CAPTURE), Frieda Saeysstraat, 9052 Ghent, Belgium.
Center for Microbial Ecology and Technology (CMET), Faculty of Bioscience Engineering Ghent University, Coupure Links 653, 9000 Ghent, Belgium; Center for Advanced Process Technology for Urban Resource recovery (CAPTURE), Frieda Saeysstraat, 9052 Ghent, Belgium; Cross-disciplinary Research in Environmental Technologies (CRETUS), Department of Chemical Engineering, Universidade de Santiago de Compostela, Spain.
Sci Total Environ. 2023 Feb 20;860:160501. doi: 10.1016/j.scitotenv.2022.160501. Epub 2022 Nov 24.
Medium chain carboxylic acids (MCCA) such as caproic acid have a plethora of applications, ranging from food additives to bioplastics. MCCA can be produced via microbial chain elongation using waste and side-streams as substrates, a process that can be more sustainable than conventional production routes. Most chain elongation studies have focused on mesophilic conditions, with only two recent studies hinting at the possibility of thermophilic chain elongation, but a systematic study of its mechanisms is lacking. Here, we investigated thermophilic chain elongation from grass juice, to understand the effect of key operational parameters (pH, temperature, substrate) on the process performance and to establish the key microbial genera and their role in the system. The genus Caproiciproducens was identified as responsible for thermophilic chain elongation, and caproic acid production was most favorable at pH 6.0 and 50 °C among the conditions tested, reaching an average concentration of 3.4 g/L. Batch experiments showed that the substrate for caproic acid production were glucose and xylose, while lactic acid led to the production of only butyric acid. Fed-batch experiments showed that substrate availability and the presence of caproic acid in the system play a major role in shaping the profile of thermophilic chain elongation. The increase of the total sugar concentration by glucose addition (without changing the organic load) during continuous operation led to a microbial community dominated (75 %) by Caproiciproducens and increased by 76 % the final average caproic acid concentration to 6.0 g/L (13 g/L) which represented 32 % (g/g) of the total carboxylic acids. The highest concentration achieved was 7.2 g/L (day 197) which is the highest concentration reported under thermophilic conditions thus far. The results of this work pave the way to the potential development of thermophilic systems for upgrading various underexplored abundant and cheap sugar-rich side-streams to caproic acid.
中链羧酸（MCCA），如己酸，具有广泛的应用，从食品添加剂到生物塑料。MCCA 可以通过微生物链伸长反应利用废物和副产物作为底物生产，这一过程比传统的生产途径更具可持续性。大多数链伸长研究都集中在中温条件下，只有最近的两项研究暗示了嗜热链伸长的可能性，但对其机制的系统研究仍缺乏。在这里，我们研究了来自草汁的嗜热链伸长，以了解关键操作参数（pH 值、温度、底物）对过程性能的影响，并确定关键微生物属及其在系统中的作用。确定 Caproiciproducens 属负责嗜热链伸长，在测试的条件下，pH 值为 6.0 和 50°C 时最有利于己酸的生产，平均浓度达到 3.4 g/L。批处理实验表明，葡萄糖和木糖是生产己酸的底物，而乳酸仅导致丁酸的生成。补料分批实验表明，底物的可用性和系统中己酸的存在在塑造嗜热链伸长的特征方面起着主要作用。在连续运行过程中通过添加葡萄糖增加总糖浓度（不改变有机负荷）会导致 Caproiciproducens 占主导地位的微生物群落（75%），并将最终平均己酸浓度提高 76%，达到 6.0 g/L（13 g/L），占总羧酸的 32%（g/g）。达到的最高浓度为 7.2 g/L（第 197 天），这是迄今为止在嗜热条件下报道的最高浓度。这项工作的结果为开发用于将各种未充分利用的丰富廉价富含糖的副产物升级为己酸的嗜热系统铺平了道路。
