Formation of flavour compounds

A multitude of compounds contribute to beer flavour. Many of these are derived directly from the raw materials used to produce the wort. In this respect the blend of malt and hops are influential. However, fermentation has the most significant impact on flavour development. Both ethanol and carbon dioxide contribute to beer flavour, imparting 'warming' and 'mouth tingle' characters, respectively. The essential character of any beer is determined by the plethora of other yeast metabolites, that arise during fermentation. Many of these are flavour-active at the concentrations found in beer. The balance of flavour metabolites formed is largely a consequence of the combination of yeast strain and wort composition which are used. In addition, a prime aim of fermentation management is to control conditions so as to ensure that these metabolites are produced in desired quantities.

Yeast metabolites, which contribute to beer flavour, are diverse chemically and include organic acids, medium chain-length aliphatic alcohols ('fusel alcohols'), aromatic alcohols, esters, carbonyls and various sulphur-containing compounds. Many hundreds of components may be detected in beers, although many remain to be positively identified. Some of these make a positive contribution; others impart undesirable notes. Meilgaard (1974) reported on the flavour thresholds and flavours of more than 200 beer components. Flavour thresholds varied to an enormous extent. Thus, the threshold detection concentration for ethanol was 13 g 1 whereas, for tertiary amyl mercaptan a value of 0.07 ngl 1 was recorded! Flavour impact was shown to be influenced by the chain-length of the molecule. Thus, for aliphatic components, the greatest intensity was generally shown by compounds with 8-10 carbon atoms. Molecules of longer or shorter chain-lengths were less flavour-active.

Meilgaard (1975) ranked, in order of importance, the positive contributions to beer flavour as: ethanol, hop bitterness, carbon dioxide, 'banana esters' (isoamyl acetate), 'apple esters' (ethyl acetate) and 'fusel alcohols'. In terms of flavour defects the corresponding ranking was: 'sulphury' (dimethyl sulphide + hydrogen sulphide), 'toffee/butterscotch' (diacetyl) and 'stale' (2-trans-nonenal).

Predictably with such a wide range of components deriving from the results of yeast metabolism it is not possible to ascribe a single underlying rationale that explains their appearance in beer. Potential metabolic mechanisms are responses to reduced water activity, cellular redox balancing reactions, maintenance of intracellular pH, shock excretion, stress responses and counter ion nutrient uptake. Some of these are discussed in more detail in the following sections of this chapter. In addition, the significance of overflow metabolism should not be overlooked. With regard to carbon flow from assimilation of wort nutrients to formation of extracellular metabolites, it is noteworthy to consider the crucial importance of pyruvate and acetyl-CoA as major branch points (Fig. 3.11).

The significance of carbon flow from sugars and through pyruvate with respect to phenomena such as the Crabtree effect has been discussed previously (see Section 3.4.1). It is suggested that carbon flow from pyruvate to ethanol and acetyl-CoA is regulated only by the relative affinities of pyruvate dehydrogenase and pyruvate decarboxylase and that ethanol formation represents overflow metabolism. Furthermore, acetyl-CoA can arise from acetaldehyde, via the intermediary of acetate, thereby bypassing pyruvate dehydrogenase.

The evidence suggests that this bypass represents the major route for acetyl-CoA formation in brewery fermentations. Thus, the lipoamide dehydrogenase, which is an essential component of the multi-enzyme pyruvate dehydrogenase complex, is subject to glucose repression (Roy & Dawes, 1987). The absence of this enzyme, which is also the glucose repressible component of the 2-oxoglutarate dehydrogenase complex, is

Formation Sulphur Flavour Compounds

Diacetyl Pentanedione dihydroxymethylvalerate-► 2-methylbutanol

Fig. 3.11 Central roles of pyruvate and acetyl-CoA in pathways leading from the utilisation of wort nutrients and formation of flavour-active metabolites.

Diacetyl Pentanedione dihydroxymethylvalerate-► 2-methylbutanol

Fig. 3.11 Central roles of pyruvate and acetyl-CoA in pathways leading from the utilisation of wort nutrients and formation of flavour-active metabolites.

responsible for the lack of a complete citric acid cycle in anaerobically grown S. cerevisiae. From another viewpoint, Flikweert et al. (1996) demonstrated that mutant strains of S. cerevisiae lacking pyruvate decarboxylase could not grow on glucose, even under non-fermentative conditions. This suggests that cytosolic formation of acetyl-CoA via the intermediary of acetaldehyde is essential to yeast. On the other hand, Wenzel et al. (1993) detected activity of pyruvate dehydrogenase during growth of yeast under anaerobic conditions and aerobic growth on ethanol, where it has no apparent catabolic role. These authors suggested that this may indicate an anabolic role in supplying acetyl-CoA for mitochondrial amino acid synthesis.

It seems reasonable to assume, therefore, that pyruvate dehydrogenase does not play a role in sugar catabolism in brewery fermentations. It follows that the very large quantities of sugars present in brewery worts must be channelled largely though pyruvate decarboxylase and thence, via either alcohol dehydrogenase or aldehyde dehydrogenase to generate ethanol or acetate, respectively. Presumably the inability of these enzymes to contain the carbon flux accounts for the appearance of pyruvate in beer (Fig. 3.10). Similar overflow mechanisms may account for the appearance of other metabolites in beer. Many important groups of flavour-active yeast metabolites arise in beer during early to mid-fermentation when the yeast is actively assimilating both sugars and other nutrients (Fig. 3.12). In the data given it may be seen that ester levels attained peak values coincident with the time at which uptake of wort free amino acids ceased. Both VDK and higher alcohols achieved maximum concentrations before this, when the wort was approximately two-thirds attenuated.

The period of maximum production of extracellular by-products of yeast metabolites occurs when there is a high rate of sugar uptake and dissimilation, coupled

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  • otho proudfoot
    When are beer flavour components detected?
    5 months ago

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