674 Part VI: Fermented Foods
“quality” of these excreted materials depend on the
yeast concentration, yeast strain, yeast metabolic
state, and the temperature (N.N. 2000). Rapid excre-
tion of material is best achieved at a temperature of
5–7°C during 10 days (Van de Meersche et al.
1977).
When the conditioning period is too long or when
the temperature is too high, yeast cell autolysis will
occur. Some enzymes are liberated (e.g., -
glucosidase) that will produce glucose from traces of
residual maltose (N.N. 2000). At the bottom of a fer-
mentation tank, the amount of -amino-nitrogen can
rise to 40–10,000 mg/L, which accounts for an
increase of 30 mg/L for the total beer volume. The
increase in amino acid concentration in the beer has a
positive effect on the flavor fullness of the beer.
Undesirable medium-chain fatty acids can also be
produced in significant amounts if the maturation
temperature is too high (Masschelein 1981).
Measurement of these compounds indicates the level
of autolysis and permits the determination of the most
appropriate conditioning period and temperature.Figure 29.4.The remethylation, transulfuration, and sulfur assimilation pathways. Genes and enzymes catalyzing
individual reactions are 1,sulphate permease; 2,ATP sulphurylase; 3, MET1; 4,adenylylsulfate kinase (EC 2.7.1.25);
5, MET10: sulfite reductase (EC 1.8.1.2); 6,sulphite permease; 7, MET17: O-acetylhomoserine (thiol)-lyase (EC
2.5.1.49); 8, CYS4: cystathionine -synthase (CBS, EC 4.2.1.22); 9, CYS3: cystathionine -lyase (EC 4.4.1.1); 10,
MET6: methionine synthase (EC 2.1.1.14); 11, SAM1and SAM2: S-adenosylmethionine synthetase (EC 2.5.1.6); 12,
SAH1: S-adenosylhomocysteine hydrolase (EC 3.3.1.1); 13, SHM1and SHM2: serine hydroxymethyltransferase
(SHMT, EC 2.1.2.1); 14, MET12and MET13: methylenetetrahydrofolate reductase (MTHFR, EC 1.5.1.20). “X” repre-
sents any methyl group acceptor; THF, tetrahydrofolate; CH 2 -THF, 5,10-methylenetetrahydrofolate; CH 3 -THF,
5-methyltetrahydrofolate. (Partly adapted from Chan and Appling 2003.)