Acetoin, also known as 3-hydroxybutanone or acetyl methyl carbinol, is formed in bacteria from pyruvate by action of the two enzymes, namely α-acetolactate synthase, that catalyzes the condensation of two pyruvate molecules with a single decarboxylation to afford α-acetolactate, and α-acetolactate decarboxylase that decarboxylates this last one to acetoin (Juni, 1952).
Acetoin is a flavor commonly used in the food industry where it replaces diacetyl because it is regarded as safer (Huang, 2011). Acetoin is used as a flavor ingredient in formulations for raspberry, strawberry, vanilla, walnut, rum, butter, butterscotch, caramel, coconut, coffee and fruit flavors. Acetoin may be added to alcoholic and nonalcoholic beverages such as cream soda. Its buttery flavor is well suited for ice cream, ices, candy, baked goods, margarine, gelatin desserts, cottage cheese, margarine and shortenings. Acetoin is also used by the cosmetic industry in perfumes, and fragrances as an aroma carrier. Finally, acetoin is one of the top chemical additives in cigarettes and as a flavor agent in electronic cigarettes. Acetoin is furthermore a precursor of Methyl Vinyl Ketone (MVK) which is a useful intermediate for chemistry.
The traditional chemical synthesis of acetoin is faced the drawback of the petroleum deficiency and environmental pollution, whereas the market for acetoin as a food flavor is currently still growing by an annual rate of 5 to 5.5%, and expected to reach $14 billion in 2018. The fragrance market is expected exceed $16 billion in 2018.
Many chemicals that could only be produced by traditional chemical processes in the past can now have the potential to be generated biologically, using renewable resources (Danner & Braun, 1999; Hatti-Kaul et al., 2007). Microbial production of acetoin is one such example. Interest in this bioprocess has increased remarkably because acetoin has a large number of industrial applications, as above-mentioned, and microbial production will alleviate the dependence on oil supply for the production of platform chemicals. Saccharomyces cerevisiae is an especially well suited platform for such bioprocesses (Nielsen 2013) Regarding the microbial production of acetoin, most studies used microorganisms, such as Candida glabrata, Bacillus subtilis, to produce acetoin (Shubo Li et al., Microbial Cell Factories 2014, 13:55; Silbersack J et al., Appl Microbiol Biotechnol. 2006 December; 73(4):895-903;
Acetoin production by a GRAS (i.e. generally recognized as safe) microorganism would thus be desirable. Yeast, and more particularly Saccharomyces cerevisiae, is an appropriate microorganism in this context. S. cerevisiae is known to produce acetoin naturally, but the yield and productivity of acetoin production are poor. Ethanol production is indeed the most obvious barrier for the efficient acetoin production in S. cerevisiae because pyruvate, a key intermediate, is preferentially used for producing ethanol rather than acetoin.
Therefore, for obvious reasons, to improve the production of acetoin through microbial processes, and more particularly of the conversion of pyruvate to acetoin, remains a constant aim. More particularly, there is still a need in a stable recombinant microorganism having an enhanced production yield of acetoin, in particular compatible with industrialization requirements.