Physiology, ecology and industrial applications of aroma formation in yeast

Abstract

Yeast cells are often employed in industrial fermentation processes for their ability to efficiently convert relatively high concentrations of sugars into ethanol and carbon dioxide. Additionally, fermenting yeast cells produce a wide range of other compounds, including various higher alcohols, carbonyl compounds, phenolic compounds, fatty acid derivatives and sulfur compounds. Interestingly, many of these secondary metabolites are volatile and have pungent aromas that are often vital for product quality. In this review, we summarize the different biochemical pathways underlying aroma production in yeast as well as the relevance of these compounds for industrial applications and the factors that influence their production during fermentation. Additionally, we discuss the different physiological and ecological roles of aroma-active metabolites, including recent findings that point at their role as signaling molecules and attractants for insect vectors.

Figure 1.

Overview of aroma compound production. This review covers a large array of aroma compounds produced during yeast fermentation. The basic fermentation of pyruvate (green/red) leads to several carbon-based compounds, including ethanol and carbon dioxide. Pyruvate also feeds into the anabolism of amino acids, leading to production of vicinal diketones (pink). Metabolism of amino acids is responsible for numerous aroma compounds including higher alcohols and esters (purple) as well as sulfur-containing compounds (blue). Additionally, the phenolic compounds are derived from molecules found in the media (orange). Compounds shown in darker shades are considered intermediates while lighter shades are aroma compounds discussed in this review. Dotted lines indicate import/export of compounds, solid lines represent biochemical reactions (not indicative of number of reactions).

Figure 2.

Production of ethanol, acetaldehyde, acetic acid, and CO₂. Fermentable carbons are assimilated from the medium and converted to glycerol or pyruvate via glycolysis. Pyruvate can be shuttled towards the TCA cycle and respiration (left) or towards alcoholic fermentation (right). For some conversions, multiple enzymes can perform the reaction and are indicated on the figure. Note: Ald4, Ald5 and Adh3 are mitochondrial enzymes but perform the same reactions as the other cytosolic ALD and ADH enzymes.

Figure 3.

Production of vicinal diketones. The vicinal diketones are produced as by-products during the isoleucine-leucine-valine (ILV) biosynthetic pathways. Gene names correlate with nomenclature from S. cerevisiae (Saccharomyces Genome Database). OYE = ‘Old Yellow Enzyme’. Dotted lines indicate import/export, solid lines indicate biochemical reactions. Note: dotted line from sugar to pyruvate also encompasses glycolysis.

Figure 4.

The Ehrlich pathway. There are several routes that can direct carbon compounds into the production of amino acids and subsequently the higher alcohols. This scheme depicts the most direct connections between the amino acids and the respective higher alcohols through the three-step Ehrlich Pathway (general reactions depicted at top). Dotted lines indicate multiple steps. Note: the reduction step can be carried out by over 10 different enzymes which vary in localization, regulation and substrate specificity; AdhX = alcohol dehydrogenase (Adh1, Adh2, Adh3, Adh4, Adh6, Adh7); AadX = aryl alcohol dehydrogenase (Aad3, Aad4, Aad6, Aad10, Aad14, Aad15, Aad16).

Figure 5.

Ester synthesis in yeast. Left: general scheme of both types of ester production. Esters are the result of condensation reactions between an alcohol and an acetyl/acyl-CoA. (A) Acetate esters are produced through the actions of Atf1 and Atf2. (B) Fatty acid esters are produced by Eeb1 and Eht1. Right: examples of some of the most common esters discussed in this review. General aroma descriptors are listed in italics.

Figure 6.

Sulfate reduction pathway leading to the production of sulfur-containing amino acids and compounds. (1) Extracellular sulfate is taken up through two transporters, Sul1 and Sul2, and sequentially reduced to sulfite and sulfide. (2) Excess sulfide can be converted to hydrogen sulfide which diffuses out of the cell or (3) assimilated into amino acid synthesis pathways. (4) Production of α-ketobutyrate links this pathway to threonine and the branched amino acid synthesis pathways (Fig. 2). (5) Methionine can be acted on by a lyase to form methanethiol, which is a major precursor for numerous sulfur-containing aroma compounds. (6) Methanethiol can also be produced via transamination of methionine, which is also the first step of the Ehrlich pathway (Fig. 3). Adapted from Landaud (2008), Pereira et al. (2008), and Saccharomyces Genome Database (Cherry et al.2012).

Figure 7.

Production of phenolic compounds. Hydroxycinnamic acids are released during pre-processing of biomass. Yeast cells can decarboxylate these toxic compounds to less harmful forms through the actions of Fdc1. Fdc1 requires a cofactor FMN which is produced by Pad1. The compounds are then secreted and can be further reduced by a vinylphenol reductase, typically by contaminating yeast or bacterial species.

Figure 8.

Summary of the ecological roles of aroma compounds. This review has summarized a variety of physiological and ecological roles of yeast aroma compounds. This figure depicts some of the major organisms described to illustrate the vast number of compounds that they interact with. Positive (+) indicates a generally positive interaction such as attraction, increased growth or behavior. Negative (–) indicates a negative interaction such as inhibited growth or repulsion.