BLBS102-c33 BLBS102-Simpson March 21, 2012 14:5 Trim: 276mm X 219mm Printer Name: Yet to Come
33 Biochemistry of Beer Fermentation 641concentrations in beer (Hansen and Kielbrandt 1996b). Beers
produced with increased levels of sulfite showed an improved
flavor stability.AcetaldehydeAldehydes—in particular, acetaldehyde (green apple-like
flavor)—have an impact on the flavor of green beer. The accu-
mulation of acetaldehyde is dependent on the kinetic properties
of enzymes responsible for its formation (pyruvate decarboxy-
lase and acetaldehyde dehydrogenase) and dissimilation (alco-
hol dehydrogenase) (Boulton and Quain 2006). Acetaldehyde
synthesis is linked to yeast growth (Geiger and Piendl 1976).
Its concentration is maximal at the end of the growth phase and
is reduced at the end of the primary fermentation and during
maturation by the yeast cells. As with diacetyl, levels may be
enhanced if yeast metabolism is stimulated during transfer, es-
pecially by oxygen ingress. Removal also requires the presence
of enough active yeast. Fermentations with early flocculating
yeast cells can result in too high acetaldehyde concentrations at
the end.Development of Flavor FullnessDuring maturation, the residual yeast will excrete compounds
(i.e., amino acids, phosphates, peptides, nucleic acids,...) into
the beer. The amount and “quality” of these excreted materials
depend on the yeast concentration, yeast strain, its metabolic
state and the temperature (NN 2000). Rapid excretion 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 temper-
ature is too high, yeast cell autolysis will occur. Some enzymes
are liberated (e.g.,α-glucosidase), which will produce glucose
from traces of residual maltose (NN 2000). At the bottom of
a fermentation tank, the amount ofα-amino-nitrogen can rise
to 40–10,000 mg/L, which account for an increase of 30 mg/L
for the total beer volume. The increase in amino acid concen-
tration in the beer has a positive effect on the flavorfullness of
the beer. Undesirable medium-chain fatty acids can also be pro-
duced 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.BEER FERMENTATION USING
IMMOBILIZED CELL TECHNOLOGYThe advantages of continuous fermentation—such as greater
efficiency in utilization of carbohydrates and better use of
equipment—led also to the development of continuous beer
fermentation processes. Since the beginning of the twentieth
century, many different systems using suspended yeast cells
have been developed. The excitement for continuous beer fer-
mentation led—especially during the 1950 and 1960s—to the
development of various interesting systems. These systems can
be classified as (i) stirredversusunstirred tank reactors, (ii)single-vessel systems versus a number of vessels connected in
series, (iii) vessels that allow yeast to overflow freely with the
beer (“open system”) versus vessels that have abnormally high
yeast concentrations (“closed” or “semiclosed system”) (Well-
hoener 1954, Coutts 1957, Bishop 1970, Hough et al. 1982,).
However, these continuous beer fermentation processes were
not commercially successful due to many practical problems,
such as the increased danger of contamination (not only during
fermentation but also during storage of wort in supplementary
holdings tanks that are required since the upstream and down-
stream brewing processes are usually not continuous), changes
in beer flavor (Thorne 1968) and a poor understanding of the
beer fermentation kinetics under continuous conditions. One of
the well-known exceptions is the successful implementation of a
continuous beer production process in New Zealand by Morton
Coutts (Dominion Breweries), which is still in use today (Coutts
1957, Hough et al. 1982).
In the 1970, there was a revival in developing continuous
beer fermentation systems due to the progress in research on
immobilization bioprocesses using living cells. Immobilization
gives fermentation processes with high cell densities, resulting
in a drastic increase in fermentation productivities compared to
the traditional time-consuming batch fermentation processes.
The last 30 years, ICT has been extensively examined and
some designs have reached already commercial exploitation.
Immobilized cell systems are heterogeneous systems in which
considerable mass transfer limitations can occur, resulting in a
changed cell yeast metabolism. Therefore, successful exploita-
tion of ICT needs a thorough understanding of mass transfer and
intrinsic yeast kinetic behavior of these systems.Carrier MaterialsCell immobilization can be classified into four categories based
on the mechanism of cell localization and the nature of sup-
port material: (i) attachment to the support surface, which can
be spontaneous or induced by linking agents; (ii) entrapment
within a porous matrix; (iii) containment behind or within a bar-
rier; and (iv) self-aggregation, naturally or artificially induced
(Karel et al. 1985, Willaert and Baron 1996). Various cell im-
mobilization carrier materials have been tested and used for
beer production/bioflavoring. Selection criteria are summarized
in Table 33.5. Depending on the particular application, reac-
tor type and operational conditions, some selection criteria will
be more appropriate. Examples of selected carrier materials for
particular applications are tabulated in Table 33.6.
Cell immobilization by self-aggregation is based on the for-
mation of cell clumps or flocs. Actually, flocs are formed at the
end of the primary fermentation when flocculent strains are used.
Flocculent strains can also be used in continuous fermentation
systems (see “Coutts continuous beer fermentation system” de-
scribed in the preceding text). Very high yeast concentrations
may be achieved by the use of inclined tubes, still zones around
outlet pipes and by holding the yeast in a filter (Hough et al.
1982). Growth of the sedimentary yeast may be controlled by
the amount of air injected, while carbon dioxide is used to cause
some mixing. A flocculent strain has also been used in the tower