Biotechnology and Bioprocess Engineering 2021; 26(1): 125-136  
Development of a Cell-recycled Continuous Fermentation Process for Enhanced Production of Succinic Acid by High-yielding Mutants of Actinobacillus succinogenes
Soo Yeon Kim, Sun Ok Park, Jeong Yeon Yeon, and Gie-Taek Chun
Soo Yeon Kim†, Sun Ok Park†, Jeong Yeon Yeon, Gie-Taek Chun*
College of Biomedical Science, Kangwon National University, Chuncheon 24341, Korea
Tel: +82-33-250-8547
E-mail: gtchun@kangwon.ac.kr
Sun Ok Park
STR Biotech Co., Ltd., Chuncheon 24232, Korea
†Equal contribution.
Received: September 10, 2020; Revised: October 8, 2020; Accepted: October 9, 2020; Published online: February 28, 2021.
© The Korean Society for Biotechnology and Bioengineering. All rights reserved.

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Abstract
Succinic acid (SA), a four-carbon dicarboxylic acid utilized as a platform chemical for valuable industrial products, is a major fermentation product of Actinobacillus succinogenes cells, synthetized during anaerobic metabolism. In this study, cell-recycled continuous fermentation (CRCF) was carried out in order to maximize the appreciably stable biocatalytic activity of SA-producing cells and the growthassociated mode of SA biosynthesis. Stable and long operations of CRCF could be carried out through an efficient decanter system developed in our laboratory, which could effectively separate highly dense microbial cells from the outlet stream. This allowed to overcome the wash-out phenomenon encountered at relatively low dilution rates in conventional continuous fermentation systems without cell recycling. Through careful assessment of the effects of dilution rate, composition of feeding medium, and cell recycling ratio on SA yield via the CRCF process, volumetric SA productivity could be enhanced to 3.86 g/(L·h), an approximately 5.1-fold increase compared to parallel continuous fermentation without cell recycling. A higher dilution rate and a 24% increment in SA production via increased density of active cells inside the bioreactor of the CRCF system were the probable factors inducing such a considerable enhancement in volumetric SA productivity during CRCF. Since volumetric productivity is the most important parameter determining the cost-effectiveness of a given fermentation bioprocess, it is quite evident that CRCF is a promising alternative to conventional batch or continuous fermentation without cell recycling for mass production of SA.
Keywords: succinic acid, Actinobacillus succinogenes, cell-recycled continuous fermentation, dilution rate, recycling ratio, feeding medium composition

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