Food Biochemistry and Food Processing

29 Biochemistry and Fermentation of Beer 667

isoamyl alcohol)] and aromatic (2-phenylethanol,
tyrosol, tryptophol) higher alcohols. Aliphatic high-
er alcohols contribute to the “alcoholic” or “solvent”
aroma of beer, and produce a warm mouthfeel. The
aromatic alcohol 2-phenylethanol has a sweet aroma
and makes a positive contribution to the beer aroma,
whereas the aroma of tyrosol and tryptophol are
undesirable.
Higher alcohols are synthesized by yeast during
fermentation via the catabolic (Ehrlich 1904) and
anabolic pathways (amino acid metabolism)
(Ehrlich 1904, Chen 1978, Oshita et al. 1995). In the
catabolic pathway, the yeast uses the amino acids of
the wort to produce the corresponding -keto acid
via a transamination reaction. The excess oxoacids
are subsequently decarboxylated into aldehydes and
further reduced (alcohol dehydrogenase) to higher
alcohols. This last reduction step also regenerates
NAD.
Dickinson et al. looked at the genes and enzymes
that are used by S. cerevisiaein the catabolism of
leucine to isoamyl alcohol (Dickinson et al. 1997),
valine to isobutanol (Dickinson et al. 1998), and
isoleucine to active amyl alcohol (Dickinson et al.
2000). In all cases, the general sequence of bio-
chemical reactions is similar, but the details for the
formation of the individual alcohols are surprisingly
different. The branched-chain amino acids are first
deaminated to the corresponding -keto acids (-
ketoisocapric acid from leucine, -ketoisovaleric

acid from valine, and -keto-
-methylvaleric acid

from leucine). There are significant differences in

the way each -keto acid is subsequently decar-

boxylated. Recently, the catabolism of phenylala-

nine 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 decar-

boxylated. These studies revealed that all amino acid

catabolic pathways studied to date use a subtly dif-

ferent spectrum of decarboxylases from the five-

member family that comprises Pdc1p, Pdc5p,

Pdc6p, Ydl080cp, and Ydr380wp. Using strains con-

taining all possible combinations of mutations

affecting the seven AADgenes (putative aryl alcohol

dehydrogenases), five ADH,and SFA1(other alco-

hol dehydrogenase 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 from -keto acids during the synthesis

of amino acids from the carbohydrate source. The

pathway choice depends on the individual 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 alco-

hol increases (Chen 1978), and increases in the

later stage of fermentation as wort amino acids are

Table 29.3.Major Higher Alcohols in Beer (N.N. 2000)

Concentration Concentration

Flavor Aroma Range (mg/L), Range (mg/L),

Compound Threshold (mg/l) or Tasteb Bottom Fermentation Top Fermentation

n-Propanol 600 c, 800b Alcohol 7–19 (12)*,f 20–45i

Isobutanol 100 c,80–100g,200b Alcohol 4–20 (12)f 10–24i

2-Methylbutanol 50 c, 50–60g, 70b Alcohol 9–25 (15)a 80–140i

3-Methylbutanol 50 c, 50–60g, 65b Fusely, pungent 25–75 (46)a 80–140i

2-Phenylethanol 5 a, 40c, 45–50g, Roses, sweetish 11–51 (28)f, 4–22g, 35–50g, 8–25a,

75 d,125b 16–42h 18–45i

Tyrosol 10 a, 10–20e, 20c, Bitter chemical 6–9a, 6–15a 8–12g, 7–22g

100 d,g, 200b

Tryptophol 10 a, 10–20e, 200d Almonds, solvent 0.5–14a 2–12g

Source:N.N. 2000.

References:aSzlavko 1973; bMeilgaard 1975a; cEngan 1972; dRosculet 1971;eCharalambous et al.1972; fValues in 48

European lagers, Dufour (unpublished data); gReed and Nogodawithana 1991; hIverson 1994; iDerdelinckx (unpublished

data).

*Mean value.