668 Part VI: Fermented Foods
depleted (MacDonald et al. 1984). Yeast strain, fer-
mentation conditions, and wort composition all have
significant 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 levels of nutrients (amino acids, oxygen, lip-
ids, zinc, etc.), increased temperature, and agita-
tion—stimulate the production of higher alcohols.
The synthesis of aromatic alcohols is especially sen-
sitive 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.
BIOSYNTHESIS OFESTERS
Esters are very important flavor compounds in beer.
They have an effect on the fruity/flowery aromas.
Table 29.4 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 com-
ponents of beer when present in appropriate quanti-
ties and proportions but can become unpleasant
when in excess. Ester formation is highly dependent
on the yeast strain used (Nykänen and Nykänen
1977, Peddie 1990) and on certain fermentation
parameters such as temperature (Engan and Aubert
1977, Gee and Ramirez 1994, Sablayrolles and Ball
1995), specific growth rate (Gee and Ramirez 1994),
pitching rate (Maule 1967, D’Amore et al. 1991,
Gee and Ramirez 1994), and top pressure. Addi-
tionally, the concentrations of assimilable nitrogen
compounds (Hammond 1993, Calderbank and Ham-
mond 1994, Sablayrolles and Ball 1995), carbon
sources (Pfisterer and Stewart 1975; White and
Portno 1979; Younis and Stewart 1998, 2000), dis-
solved oxygen (Anderson and Kirsop 1975a,b;
Avhenainen and Mäkinen 1989; Sablayrolles and
Ball 1995), and fatty acids (Thurston et al. 1981,
1982) can influence the ester production rate.
Esters are produced by yeast both during the
growth phase (60%) and during the stationary phase
(40%). They are formed by the intracellular reaction
between a fatty acyl-coenzyme A (acyl-CoA) and an
alcohol:ROH RCO-SCoA →RCOORCoASH (1)This reaction is catalyzed by an alcohol acyltrans-
ferase (or ester synthetase). Since acetyl-CoA is also
a central molecule in the synthesis of lipids and
sterols, ester synthesis is linked to fatty acid metab-
olism (see also Fig. 29.2).
Alcohol acetyltransferase (AAT) has been local-
ized in the plasma membrane (Malcorps and Dufour
1987) and found to be strongly inhibited by unsatu-
rated fatty acids, ergosterol, heavy metal ions, and
sulphydryl reagents (Minetoki et al. 1993). Subcell-
ular fractionation studies conducted during the batch
fermentation cycle demonstrated the existence of
both cytosolic and membrane-bound AAT (Ramos-
Jeunehomme et al. 1989, Ramos-Jeunehomme et al.
1991). In terms of controlling ester formation on a
metabolic basis, it has further been shown that the
ester-synthesizing activity of AAT is dependent on its
positioning within the yeast cell. An interesting fea-Table 29.4.Major Esters in Beer
Concentration
Flavor Range (mg/L)
Compound Threshold (mg/L) Aroma in 48 Lagers
Ethyl actetate 20–30, 30a Fruity, solventlike 8–32 (18.4)*
Isoamyl acetate 0.6–1.2, 1.2a Banana, peardrop 0.3-3.8 (1.72)
Ethyl caproate (ethyl 0.17–0.21, 0.21a Applelike with note 0.05–0.3 (0.14)
hexanoate) of aniseed
Ethyl caprylate (ethyl 0.3–0.9, 0.9a Applelike 0.04–0.53 (0.17)
octanoate)
2-Phenylethyl acetate 3.8a Roses, honey, 0.10–0.73 (0.54)
apple, sweetish
Source:Dufour and Malcorps 1994.
Reference:aMeilgaard 1975b.
*Mean value.