Abstract
Short- and medium-chain volatile esters with flavors and fruity fragrances, such as ethyl acetate, butyl acetate, and butyl butyrate, are usually value-added in brewing, food, and pharmacy. The esters can be naturally produced by some microorganisms. As ester-forming reactions are increasingly deeply understood, it is possible to produce esters in non-natural but more potential hosts. Clostridia are a group of important industrial microorganisms since they can produce a variety of volatile organic acids and alcohols with high titers, especially butanol and butyric acid through the CoA-dependent carbon chain elongation pathway. This implies sufficient supplies of acyl-CoA, organic acids, and alcohols in cells, which are precursors for ester production. Besides, some Clostridia could utilize lignocellulosic biomass, industrial off-gas, or crude glycerol to produce other branched or straight-chain alcohols and acids. Therefore, Clostridia offer great potential to be engineered to produce short- and medium-chain volatile esters. In the review, the efforts to produce esters from Clostridia via in vitro lipase-mediated catalysis and in vivo alcohol acyltransferase (AAT)-mediated reaction are comprehensively revisited. Besides, the advantageous characteristics of several Clostridia and clostridial consortia for bio-ester production and the driving force of synthetic biology to clostridial chassis development are also discussed. It is believed that synthetic biotechnology should enable the future development of more effective Clostridia for ester production.
Highlights
Short- and medium-chain volatile esters (C2–C12) with flavors and fruity fragrances are usually value-added in brewing, food, and pharmacy (Rodriguez et al, 2014; Aleksander et al, 2019)
Ethyl acetate, ethyl lactate, butyl acetate, and ethyl hexanoate are the main components of the flavor substances in Baijiu (Chinese liquor) (Yi et al, 2019)
We summarized the advances of ester production by Clostridia including in vitro lipase catalysis and in vivo acyltransferase reaction
Summary
Short- and medium-chain volatile esters (C2–C12) with flavors and fruity fragrances are usually value-added in brewing, food, and pharmacy (Rodriguez et al, 2014; Aleksander et al, 2019). Short- and medium-chain fatty acid esters are mainly produced by concentrated sulfuric acid-mediated esterification of acids and alcohols (Cull et al, 2003). This method has certain risks in terms of safety, health, and environment, because it usually causes serious equipment corrosion, as well as a large amount of wastewater and residues (Jermy and Pandurangan, 2005). A lot of efforts have been paid to develop nonnatural but more potential strains as microbial cell factories for short- and medium-chain volatile ester production (Rodriguez et al, 2014; Kruis et al, 2017). We suggested some promising clostridial chassis for bio-ester production and discussed the driving force of synthetic biology in this field
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