Food Biochemistry and Food Processing (2 edition)

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33 Biochemistry of Beer Fermentation 635

acid from leucine). There are significant differences in the way

eachα-ketoacid is subsequently decarboxylated. The catabolism

of phenylalanine to 2-phenylethanol and of tryptophan were also

studied (Dickinson et al. 2003). Phenylalanine and tryptophan

are first deaminated to 3-phenylpyruvate and 3-indolepyruvate,

respectively, and then decarboxylated. These studies revealed

that all amino acid catabolic pathways studied to date use a sub-

tle different spectrum of decarboxylases from the five-membered

family that comprises Pdc1p, Pdc5p, Pdc6p, Ydl080cp, and

Ydr380wp. Using strains containing all possible combinations of

mutations affecting the sevenAADgenes (putative aryl alcohol

dehydrogenases), fiveADHandSFA1(other alcohol dehydroge-

nase genes), showed that the final step of amino acid catabolism

can be accomplished by any one of the ethanol dehydrogenases

(Ahd1p, Ahd2p, Ahd3p, Ahd4p, Ahd5p) or Sfa1p (formalde-

hyde dehydrogenase).

In the anabolic pathway, the higher alcohols are synthesized

fromα-keto acids during the synthesis of amino acids from the

carbohydrate source. The pathway choice depends on the indi-

vidual higher alcohol and on the level of available amino acids

available. The importance of the anabolic pathway decreases

as the number of carbon atoms in the alcohol increases (Chen

1978) and increases in the later stage of fermentation as wort

amino acids are depleted (MacDonald et al. 1984). Yeast strain,

fermentation conditions, and wort composition all have signif-

icant effects on the combination and levels of higher alcohols

that are formed (MacDonald et al. 1984).

Conditions that promote yeast cell growth such as high lev-

els of nutrients (amino acids, oxygen, lipids, zinc,...)andin-

creased temperature and agitation stimulate the production of

higher alcohols (Engan 1969, Engan and Aubert 1977, Landaud

et al. 2001, Boswell et al. 2002). The synthesis of aromatic al-

cohols is especially sensitive to temperature changes. On the

other hand, conditions that restrict yeast growth, such as lower

temperature and higher pressure, reduce the extent of higher

alcohol production. Higher pressures can reduce the extent of

cell growth and, therefore, the production of higher alcohols

(Landaud et al. 2001). The yeast strain, fermentation conditions,

and wort composition have all significant effects on the pattern

and concentrations of synthesized higher alcohols. Supplemen-

tation of wort with valine, isoleucine, and leucine induces the

formation of isobutanol, amyl alcohol, and isoamyl alcohol, re-

spectively (Ayr ̈ ̈ap ̈aa 1971, Kodama et al. 2001). The overexpres- ̈

sion of the branched chain amino acid transferases genesBAT1

andBAT2result in an increased production of isoamyl alcohol

and isobutanol (Lilly et al. 2006).

Biosynthesis of Esters

Esters are very important flavor compounds in beer. They have

an effect on the fruity/flowery aromas. Table 33.4 shows the

most important esters with their threshold values, which are

considerably lower than those for higher alcohols. The major

esters can be subdivided into acetate esters and C 6 –C 10 medium-

chain fatty acid ethyl esters. They are desirable components of

beer when present in appropriate quantities and proportions but

can become unpleasant when in excess.

Esters are produced by yeast both during the growth phase

(60%) and also during the stationary phase (40%) (NN 2000).

They are formed by the intracellular reaction between a fatty

acyl-coenzyme A and an alcohol:

R′OH+RCO−ScoA→RCOOR′+CoASH (1)

This reaction is catalyzed by alcohol acyltransferases

(AATases; or ester synthethases) of the yeast. Since acetyl CoA

is also a central molecule in the synthesis of lipids and sterols,

ester synthesis is linked to the fatty acid metabolism (see also

Fig. 33.2). The majority of acetyl-CoA is formed by oxidative

decarboxylation of pyruvate, while most of the other acyl-CoAs

are derived from the acylation of free CoA, catalyzed by acyl-

CoA synthase.

Several enzymes are involved in the synthesis, the best char-

acterized are AATase I and II that are encoded byATF1and

ATF2. Alcohol acetyltransferase (AAT) has been localized in

the plasma membrane (Malcorps and Dufour 1987) and found

to be strongly inhibited by unsaturated fatty acids, ergosterol,

heavy metal ions, and sulfydryl reagents (Minetoki et al. 1993).

Subcellular fractionation studies conducted during the batch

fermentation cycle demonstrated the existence of both cy-

tosolic and membrane-bound AAT (Ramos-Jeunehomme et al.

1989, Ramos-Jeunehomme et al. 1991). In terms of controlling

Table 33.4.Major Esters in Beer (Adapted From Dufour and Malcorps 1994.)

Compound Flavor Threshold (mg/L) Aroma

Concentration Range (mg/L) in

48 Lagers

Ethyl actetate 20–30, 30(a) Fruity, solvent-like 8–32 (18.4)(a),5–40(b)

Isoamyl acetate 0.6–1.2, 1.2(a) Banana, peardrop 0.3–3.8 (1.72),<0.01–2.8(b)

Ethyl caproate (ethyl

hexanoate)

0.17–0.21, O.21(a) Apple-like with note

of aniseed

0.05–0.3 (0.14), 0.01–0.54(b)

Ethyl caprylate

(ethyl octanoate)

0.3–0.9, 0.9(a) Apple-like 0.04–0.53 (0.17), 0.01–1.2(b)

2-Phenylethyl

acetate

3.8(a) Roses, honey, apple,

sweetish

0.10–0.73 (0.54)

Source: (a) Meilgaard (1975b) and (b) immobilised cells (Willaert and Nedovic 2006).

aMean value.