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CH H3C CH

CO 3C C

COOH CO2 C=O

NADH NAD

2-Ketovalerate

Transamination ^

Valine

H3C CH3 Isobutyraldehyde CH

2-Methylpropan-1-ol ' H

Fig. 2.28 The anabolic route to higher alcohols in yeast. Note: Fig. 2.29 shows how acetolactate is derived from pyruvate.

Even more important than FAN levels, though, is the yeast strain, with ale strains producing more of these compounds than lager strains. Fermentations at higher temperatures increase higher alcohol production. Conditions favouring increased yeast growth (e.g. excessive aeration or oxygenation) promote higher alcohol formation, but this can be countered by application of a top pressure on the fermenter. The reasons why increased pressure has this effect are unclear, but it has been suggested that it may for some reason be due to an accumulation of carbon dioxide. Whatever the reason, it is pertinent to mention that beer produced in different sizes and shapes of vessel, displaying different hydrostatic pressures, do produce higher alcohols (and thereof esters) to different extents. This can be a problem for product matching between breweries (e.g. in franchise brewing operations).

Various esters may make a contribution to the flavour of beer (Table 2.6). The esters are produced from their equivalent alcohols (ROH), through catalysis by the enzyme alcohol acetyl transferase (AAT), with acetyl-coenzyme A being the donor of the acetate group:

ROH + CH3COSCoA ^ CH3COOR + CoASH

Clearly the amount of ester produced will depend inter alia on the levels of acetyl-CoA, of alcohol and of AAT. Esters are formed under conditions when the acetyl-CoA is not required as the prime building block for the synthesis of key cell components. In particular, acetyl-CoA is the starting point for the synthesis of lipids, which the cell requires for its membranes. Thus, factors promoting yeast production (e.g. high levels of aeration/oxygenation) lower ester production, and vice versa.

However, perhaps the most significant factor influencing the extent of ester production is yeast strain, some strains being more predisposed to generating esters than others. This may relate to the amount of AAT that they contain. The factors that dictate the level of this enzyme present in a given yeast strain are not fully elucidated, but it does seem to be present in raised quantities when the yeast encounters high-gravity wort, and this may explain the disproportionate extent of ester synthesis under these conditions.

Whereas the esters and higher alcohols can make positive contributions to the flavour of beer, few beers (with the possible exception of some ales) are helped by the presence of VDKs, diacetyl and (less importantly) pentanedione (Table 2.7). Elimination of VDKs from beer depends on the fermentation process being well-run. These substances are offshoots of the pathways by which yeast produces the amino acids valine and isoleucine (and therefore there is a relationship to the anabolic pathway of higher alcohol production).

The pathway for diacetyl production is shown in Fig. 2.29 because it is more significant (with respect to diacetyl being present at higher levels and with a lower flavour threshold). The precursor molecules leak out of the yeast and break down spontaneously to form VDKs. Happily, the yeast can mop up the VDK, provided it remains in contact with the beer and is in good condition.

Reductases in the yeast reduce diacetyl successively to acetoin and 2,3-butanediol, both of which have much higher flavour thresholds than diacetyl.

Table 2.7 VDKs in beer.

Flavour threshold (mg L 1) Perceived character

Diacetyl Pentanedione

Butterscotch Honey

Diacetyl

Diacetyl

Acetoin

HO OH

2,3-Butanediol

Fig. 2.29 The production and elimination of diacetyl by yeast.

Table 2.8 Some sulphur-containing substances in beer.

S-containing compound Flavour threshold (mg L Perceived character

Table 2.8 Some sulphur-containing substances in beer.

S-containing compound Flavour threshold (mg L Perceived character

Hydrogen sulphide

0.005

Rotten eggs

Sulphur dioxide

25

Burnt matches

Methanethiol

0.002

Drains

Ethanethiol

0.002

Putrefaction

Propanethiol

0.0015

Onion

Dimethyl sulphide

0.03

Sweetcorn

Dimethyl disulphide

0.0075

Rotting vegetables

Dimethyl trisulphide

0.0001

Rotting vegetables, onion

Methyl thioacetate

0.05

Cooked cabbage

Diethyl sulphide

0.0012

Cooked vegetables, garlic

Methional

0.25

Cooked potato

3-Methyl-2-butene-1-thiol

0.000004-0.001

Lightstruck, skunky

2-Furfurylmercaptan

Rubber

Many brewers allow a temperature rise at the end of fermentation to facilitate more rapid removal of VDKs. Others introduce a small amount of freshly fermenting wort later on as an inoculum of healthy yeast (a process known as Krausening). Persistent high diacetyl levels in a brewery's production may be indicative of an infection by Pediococcus or Lactobacillus bacteria. If the ratio of diacetyl to pentanedione is disproportionately high, then this indicates that there is an infection problem.

In many ways the most complex flavour characters in beer are due to the sulphur-containing compounds. There are many of these in beer (Table 2.8) and they make various contributions. Thus, many ales have a deliberate hydrogen sulphide character, not too much, but just enough to give a nice 'eggy' nose. Lagers tend to have a more complex sulphury character. Some lagers are relatively devoid of any sulphury nose. Others, though, have a distinct DMS character, while some have characters ranging from cabbagy to burnt rubber. This range of characteristics renders substantial complexity to the control of sulphury flavours.

All of the DMS in a lager ultimately originates from a precursor, S-methylmethionine (SMM), produced during the germination of barley (Fig. 2.30). SMM is heat sensitive and is broken down rapidly whenever the temperature gets above about 80°C in the process. Thus, SMM levels are lower in the more intensely kilned ale malts and, as a result, DMS is a character more associated with lagers. SMM leaches into wort during mashing and is further degraded during boiling and in the whirlpool. If the boil is vigorous, most of the SMM is converted to DMS and this is driven off. In the whirlpool, though, conditions are gentler and any SMM surviving the boil will be broken down to DMS but the latter tends to stay in the wort. Brewers seeking to retain some DMS in their beer will specify a finite level of SMM in their malt and will manipulate the boil and whirlpool stages in order to deliver a certain level of DMS into the pitching wort. During fermentation, much DMS will be lost with the gases, so the level of DMS required in the wort will be somewhat

COOH

Homoserine

H2NCH u

S-methylmethionine Dimethyl sulphide Dimethyl sulphoxide

Synthesised in barley embryo during germination

* E.g. malt kilning, wort boiling 1 At curing temperatures in a a a kilning

2. By yeast/bacterial metabolism

Fig. 2.30 The origin of DMS in beer.

Sulphate —► Activated sulphate

Sulphite

Serine

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