# The Small of Synthetic Biology: Engineering Strategies for Aroma Compound Production in Yeast
###### Abstract  
Yeast--especially _Saccharomyces cerevisiae_--have long been a preferred workhorse for the production of numerous recombinant proteins and other metabolites. _S. cerevisiae_ is a noteworthy aroma compound producer and has also been exploited to produce foreign bioflavour compounds. In the past few years, important strides have been made in unlocking the key elements in the biochemical pathways involved in the production of many aroma compounds. The expression of these biochemical pathways in yeast often involves the manipulation of the host strain to direct the flux towards certain precursors needed for the production of the given aroma compound. This review highlights recent advances in the bioengineering of yeast--including _S. cerevisiae_--to produce aroma compounds and bioflavours. To capitalise on recent advances in synthetic yeast genomics, this review presents yeast as a significant producer of bioflavours in a fresh context and proposes new directions for combining engineering and biology principles to improve the yield of targeted aroma compounds. a 2018 27 June 2018 27  
Aroma compounds are structurally remarkably heterogeneous. They can have cyclic or non-cyclic, saturated or unsaturated, straight-chain or branched-chain structures bearing all kinds of functional groups (e.g., alcohols, aldehydes, ketones, esters and ethers) and, in some cases, have nitrogen and sulphur within the structure. Certain aroma compounds are even inorganic in nature. If made enzymatically, aroma compounds are derived from the pool of precursor molecules from the core metabolism of the cell (i.e., the carbohydrates, fatty acid, nucleotides and amino acids). Odour thresholds (i.e., the concentration ranges at which a given aroma compound is detected or sensed) are key parameters in aroma compound studies. Most aroma compounds on the market are produced by isolating natural compounds from plant or animal or by chemical synthesis. However, there is a clear swing away from chemically-produced aroma compounds and aroma compounds that require extensive extraction from plants or animals towards the production and use of aroma compounds of (micro) biological origin--also called biofilavours. This is despite the fact that the chemically produced compounds are identical to their natural counterparts. Reasons for such a change in market preferences include the fact that chemical synthesis can often result in environmentally detrimental production processes and in undesired racemic mixtures. Also, extraction of aroma compounds from plants or animal sources can be resource-intensive and cost-inefficient because of low yields. In addition, multiple purification steps often lead to product loss and degradation. Consumer aversion toward chemical compounds relates especially to food and home-care products. Despite changing preferences in consumer markets, the financial implication of aroma compound generation remains a strong consideration as those derived from chemical synthesis are, in general, markedly less expensive than those derived from natural sources. In this context, researchers are directing their research efforts toward producing aroma compounds from microbial sources. This usually involves _Escherichia coli_ or _S. cerevisiae_ as cell factories by incorporating genes that code for enzymes that are relevant to the production of the given compound in a recombinant host [8]. Despite the campaigns against genetically-modified organisms (GMOs) in some sections of global consumer markets, there are numerous food ingredients derived from GMOs that are commercially-available the world over. However, in the case of such GM food-ingredients that comply with regulatory safeguards, high yields using cost-effective substrates have not yet been achieved in many instances. This review primarily focusses on recent advances in research aimed at the production of aroma compounds in yeast.