3.3 The whole-cell catalysis for ultra-high titer of 5-HVA production in SOS-BR
The background of the previous section mentioned that the maximum yield of 5-HVA prepared by biological method is 55 g/L during 28 h, and with 10 g/L byproduct (GTA) produced. However, we found that pH regulation can selectively and directionally prepare 5-HVA by G. oxydans . Therefore, based on the fact, in-depth development of SOS technology for ultra-high titer production of 5-HVA is the key procedure to establishing the feasibility of industrial by biological method.
Consequently, SOS-BR which was designed for whole-cell catalysis by aerobic microorganisms with incomplete oxidation was employed as intensified technology to conduct 5-HVA bio-production, with the results shown in Figure 3. From the perspective of production, 102.3 g/L of 5-HVA accumulated during 48 h whole-cell catalysis with an average productivity of 2.1 g/L/h. Simultaneously, the production of the first 10 h was 58.9 g/L with the productivity of 5.9 g/L/h which has exceeded the supreme level of 5-HVA (55 g/L during 28 h). It was importantly that the broth contained only 0.2 g/L substrate and without any diacid production at 48 h, that is, the yield of 5-HVA was as high as 99.8%. Moreover, concerning the dissolved oxygen (DO) level, SOS-BR could always maintain a high DO level during whole-cell catalysis process, which met the demand of G. oxydans for oxygen. At the same time, due to the strict sealing environment of the entire system, the use cost of oxygen was greatly saved, and the economy of the entire bioprocess was improved. In conclusion, combined with pH control and SOS-BR technology, we successfully achieved the preparation of 5-HVA with superior quality and ultra-high titer, which provides a promising avenue for the industrialization of 5-HVA and even hydroxyl acids.
Finally, the entire process is illustrated in Figure 4. The study focused on the whole-cell catalysis for the selective and directionality preparation if hydroxyl acids. At present, the treatment of primary diols has been mature which making diols a cheap chemical(He et al., 2018; Kim et al., 2020). Furthermore, in the downstream treatment of hydroxyl acid, electrodialysis system can be applied to simultaneously acidify and concentrate the fermentation broth, and the obtained high titer of hydroxyl acids can be easily crystallized(Du et al., 2021). In summary, the entire process can realize highly selective oxidation and directional regulation of hydroxyl acids, which greatly promotes the industrial production of hydroxyl acids with C2-C6.