11.05.2021

-  Chijian Zhang, Chengwei Ma, Shubhang Sharma, Wei Wang, An-Ping Zeng - Institute of Bioprocess and Biosystems Engineering  -

1, 3-propanediol (1,3-PDO) is an important chemical material used mainly in the production of polyester, e.g., polytrimethylene terephthalate (PTT), but also for the synthesis of pharmaceutical intermediates and as cosmetic ingredient. Realizing highly efficient microbial production of this versatile chemical from raw glycerol, e.g. generated as an abundant byproduct of industrial biodiesel production, is of economic and environmental significance and have drawn vast attentions. 

To realize industrial scale microbial production of 1,3-PDO, based on our previous success in producing 1,3-PDO from raw glycerol by a Clostridium pasteurianum wild-type strain under unsterile conditions, adaptive laboratory evolution (ALE) of this strain was applied to obtain a highly efficient and raw glycerol tolerant 1,3-PDO producer strain. This was realized by using a continuous adaptive evolution system that automatically monitors cell growth in real time to determine the cycles of adaptation to the increase in raw glycerol concentration. The C. pasteurianum strain obtained was able to produce up to 74.2 g/L of 1,3-PDO within 14 hours  in a fed-batch fermentation of raw glycerol, which was about 48% higher than that of the wild-type strain, and the volumetric productivity was even increased for over 150%, reaching 5.3 g/l.h and representing the most efficient raw glycerol fermentation to 1,3-PDO under unsterile conditions in an optimized minimal medium without the requirement of yeast extract (Fig. 1). 

Subsequently, a novel downstream process was developed to recover and purify 1,3-PDO from the fermentation broth (Fig. 2). The novelty of this downstream process lies in that (a) glycerol is supplied as a supporting agent in the direct salt removal step using a thin film evaporator to prevent salt precipitation and decline of heat transfer, and then recovered by alcohol precipitation for reuse. (b) esters formed by reactions of 1,3-PDO with organic acids (mainly acetic and butyric acids) during the first vacuum distillation are eliminated by alkaline hydrolysis, resulting in successful separation of highly pure 1,3-PDO (> 99 wt%) in the second vacuum distillation and an overall 1,3-PDO yield of  nearly 80%. 

Finally, the integrated fermentation and downstream process was successfully transferred to 1 m³ pilot scale fermentation, an important step to the anticipated industrial production of 1,3-PDO from raw glycerol by C. pasteurianum (Fig. 3).

• Chijian Zhang