Fig. 1

Possible strategies to produce BB via a single strain aided by lipase or alcohol acyltransferases (AATs). a The microorganism can produce butanol then external butyrate and lipase are added in the fermentation to undergo the esterification process. b Some clostridia strains, such as C. tyrobutyricum, can produce high butyrate. Thus, it requires an external addition of butanol and lipase to the fermentation for butyl butyrate synthesis. c Engineered microorganisms with the strong C4 pathway expressed with AAT can accumulate butyryl-CoA and butanol for BB biosynthesis. In case they have low levels of aldehyde and alcohol dehydrogenase activity, butanol needs to be externally supplemented to the fermentation medium (indicated by a purple dashed line)