Flavours

Alright, I haven't even read over this yet, so if you wish to point out pedantic grammatical errors you can go and get fucked. Otherwise, enjoy:

Yeast organoleptic compounds

Overview

The organoleptic (or flavour) compounds produced by yeast can be divided into five categories: alcohols, esters, aldehydes and ketones, sulphur-containing compounds and acids (Berry and Watson 1987; Verstrepen et al. 2003c). Yeast can also affect flavour by altering compounds already present in the substrate (e.g. wort or must)(Berry and Watson 1987), but it is those compounds produced as a by-product of yeast metabolism that are of most interest here.

Flavour thresholds

The absolute amount of a compound is not the most important determinant when evaluating its influence on flavour, as different compounds have different odour and flavour thresholds. A low flavour threshold means that the compound is detectable at low concentrations, while a compound with a high flavour threshold must be present in large amounts to be detectable (Berry and Watson 1987). However, even if a compound is present at levels below its flavour threshold, it can still affect the flavour profile through synergistic interactions with other compounds (the so-called “matrix effect”)(Meilgaard 1975). Further, some compounds are desirable at low concentrations, but undesirable in large amounts. Ethyl acetate, for example, imparts a “fruity” aroma at low levels, but is described as “solvent-like” at high concentrations (Verstrepen et al. 2003b; Swiegers et al. 2006).

Aldehydes and ketones

The majority of the flavour-active aldehydes and ketones result from chemical reactions in the pre- and post-fermentation stages rather than as a direct result of yeast metabolism. An exception is pyruvate, and to a lesser extent a-ketobutyric acid, which both influence the mouth-feel of beer and wine(Berry and Watson 1987). Acetaldehyde is another important flavour-active aldehyde synthesized by yeast. It has an apple-like and nutty aroma, and can affect the colour of red wines. It is rarely present in beer or wine at values above its flavour threshold (Berry and Watson 1987; Swiegers et al. 2006).

The vicinal diketones, 2,3-butanedione (also known as diacetyl) and 2,3-pentanedione are very important flavour compounds with very low flavour thresholds. However, they are formed by a non-enzymatic reaction that takes place outside of the yeast cell. This conversion of a-acetohydroxy acids released by yeast cells is an oxidative decarboxylation that is enhanced by the presence of oxygen (Swiegers et al. 2006). Yeast is involved in the enzymatic reduction of these compounds into the corresponding 2,3-diols, however (Berry and Watson 1987).

Sulphur-containing compounds

Sulphur-containing compounds are usually present in very small amounts, have low flavour thresholds, and are often associated with negative flavour descriptors such as cabbage, rotten-egg, sulphurous, garlic, onion and rubber. They fall into five major groups: sulfides, polysulfides, heterocyclic compounds, thioesters and thiols(Swiegers et al. 2006).

The major flavour-active sulphur containing compounds, sulphur dioxide and hydrogen sulphide, are formed as by-products of cysteine and methionine biosynthesis from inorganic sulphate, and can also be formed during the degradation of these amino acids. Many sulphur compounds are formed by the interaction of yeast with hop compounds in brewing, and pesticides and other sulphur containing compounds found in grape must in winemaking (Berry and Watson 1987; Swiegers et al. 2006). While associated with negative flavours, sulphur dioxide is often considered desirable in beer and wine due to its anti-oxidant and anti-microbial properties (Yoshida et al. 2008).

Organic acids

Organic acids produced by yeast are derived from three areas of yeast metabolism; those derived from the breakdown of pyruvate, those derived from the breakdown of certain amino acids via the Ehrlich Pathway (the fusel acids) and those produced from malonyl-CoA by the fatty acid synthetase pathway (Berry and Watson 1987; Hazelwood et al. 2006; Saerens, S. et al. et al. 2008). It is rare that the acids derived from the breakdown of pyruvate are present in concentrations even close to their flavour thresholds, with the exception of acetic acid. These acids can affect the mouthfeel of beverages however, and also affect the final pH. 2006; Hazelwood

Fusel acids

The breakdown of certain amino acids, including leucine, isoleucine, valine, methionine, tyrosine, tryptophan and pheylalanine, involves a sequential transamination and decarboxylation. The resulting aldehyde can then either be reduced to an alcohol or oxidised to form an acid in a process referred to as the Ehrlich Pathway (Vuralhan, Z et al. 2005; Hazelwood et al. 2006; Hazelwood et al. 2008). The fate of the aldehyde depends on the redox state (essentially the oxygen availability) of the cell (Vuralhan, Z. et al. 2003). Fusel acids (and alcohols) may also be synthesized from a-keto acids derived from the pathway used by the cell to produce amino acids (Berry and Watson 1987). Unlike higher alcohols which freely diffuse across the cell membrane, organic acids are actively transported out of the cell by Pdr12p (Hazelwood et al. 2006).

Short and medium chain fatty acids

This is the major class of acids produced by yeast. In terms of flavour they are important both in their own right and, in their activated form, as precursors in the synthesis of ethyl esters (Berry and Watson 1987; Saerens, SM et al. 2008a). In Saccharomyces, malonyl-CoA is synthesized by Acc1p, a biotin-dependent acetyl-CoA carboxylase. Malonyl-CoA is the substrate for the fatty acid synthase subunit (FAS), encoded by FAS1 and FAS2. This complex is responsible for the production of saturated fatty acids, including palmitic (16:0) and stearic acid (18:0). However, during alcoholic fermentation short and medium chain fatty acids, including hexanoic (caproic, C6) acid, octanoic (caprylic, C8) acid and decanoic (capric, C10) acid may be prematurely released from the FAS complex. (Berry and Watson 1987; Furukawa et al. et al. 2008a). It has been known for some time that these compounds are produced via this route rather than via b-oxidation of longer chain fatty acids(Taylor and Kirsop 1977). Further, Furukawa and co-workers showed that altering the expression of FAS1 and FAS2, by both overexpression and by inositol-induced downregualtion (mediated by the transcription factor OPI1) affected levels of both medium chain fatty acids and ethyl esters (Furukawa et al. 2003). 2003; Marchesini and Poirier 2003; Saerens, SM

Alcohols

While some 45 alcohols have been identified in beer, ethanol, n-propanol, isobutanol, 2-methyl-1-butanol (amyl alcohol), 3-methyl-1-butanol (isoamyl alcohol) and phenylethanol are the most important from a flavour perspective (Berry and Watson 1987; Verstrepen et al. et al. 2006). While other alcohols such as glycerol can affect the mouth-feel and apparent sweetness of a beverage, the so-called “fusel alcohols” or higher alcohols are the flavour compounds present in beer and wine at the highest concentrations (Verstrepen et al. et al. 2006; Hazelwood et al. 2008). At high concentrations, higher alcohols are associated with solvent-like off flavours but are desirable at lower concentrations. Phenylethanol in particular imparts a floral, rose-like flavour and is particularly desirable in white wines(Berry and Watson 1987; Swiegers et al. 2006). Ethanol is a precursor in the synthesis of both ethyl esters and ethyl acetate, and the fusel alcohols are precursors in the synthesis of acetate esters (Verstrepen et al. 2003b; Saerens, S. et al. 2006; Swiegers et al.. 2003c; Swiegers 2003b; Swiegers 2006)

The Ehrlich Pathway and higher alcohol synthesis

The majority of the alcohols produced by yeast are products of the breakdown of amino acids via the Ehrlich Pathway. German biochemist Felix Ehrlich first noticed the similarity between leucine and isoamyl alcohol in 1904, and proposed that amino acids were split by a “hydrating” enzyme to form the corresponding fusel alcohol in 1907(Hazelwood et al. 2008). The classical Ehrlich Pathway as it is viewed today was proposed by Neubauer and Fromherz in 1911, and involves a sequential transamination, decarboxylation and reduction. Recently the formation of fusel acids resulting from the oxidation of the fusel aldehyde has been included in the pathway (Vuralhan, Z. et al. 2003; Hazelwood et al. 2008). The amino acids broken down via this pathway include leucine, isoleucine, valine, phenylalanine, tryptophan, tyrosine and methionine. Table 1 shows the products of the breakdown of these amino acids.

Fusel alcohols may also be derived from the pathway responsible for amino acid biosynthesis. In this situation the a-keto acid made by the cell for amino acid biosynthesis is decarboxylated and reduced to the corresponding amino acid. Propanol is produced solely by this mechanism, and is not derived from the breakdown of threonine (Berry and Watson 1987; Saerens, S. et al. 2008b)

Esters

Volatile esters synthesized by yeast that are found in beverages can be divided into two groups. Ethyl esters are synthesized in a reaction catalysed by Eeb1p and Eht1p from ethanol and medium chain fatty acids (Saerens, S. et al. 2006). Acetate ester synthesis is catalysed by the alcohol acetyl transferases Atf1p and Atf2p (and LgAtf1p in lager yeasts) from acetyl-CoA and higher alcohols. Because acetate ester synthesis requires an activated substrate, a process which consumes ATP, it is considered to be an energy requiring process (Peddie 1989; Mason and Dufour 2000). The fact that it also consumes acetyl-CoA, an important precursor in lipid synthesis, amino acid synthesis and the TCA cycle (among others), has led some researchers to suggest that acetate ester synthesis is not a futile process (Verstrepen et al. 2003a). Over-expression of the ethyl ester synthesizing genes has no effect on the amount of ethyl esters produced, while over-expression of ATF1 and ATF2 results in a significant increase in the formation of these acetate esters. From this it was concluded that in ethyl ester synthesis substrate availability was the determining factor, but in acetate ester synthesis it is the activity of the enzymes responsible that determine the amount of esters produced (Verstrepen et al. et al. 2006). Increasing substrate availability will also lead to an increase in acetate ester synthesis, but this has a much less pronounced effect than the over-expression of the alcohol acetyl transferases (Verstrepen et al. 2003b). 2003b; Saerens, S.

Ester levels may also be affected by the action of esterases. Both Eeb1p and Eht1p have been found to have ethyl esterase activity (Saerens, S. et al. 2006), while Iah1p (Isoamyl acetate hydrolysing enzyme) has esterase activity against acetate esters (Fukuda et al. 1996). It has been suggested that final ester concentration is a result of the balance between ester synthesis and the action of esterases (Fukuda et al. 1998).

Something weird happened to some of the reference formatting when I copy pasted it - sorry about that.