29 Biochemistry and Fermentation of Beer 665ily of sugar transporters (Nelissen et al. 1995). Like
Mal61p, Agt1p is a high-affinity, maltose/proton
symporter, but Mal61p is capable of transporting
only maltose and turanose, while Agt1p transports
these two -glucosides as well as several others,
including isomaltose, -methylglucoside, maltotri-
ose, palatinose, trehalose, and melezitose. AGT1ex-
pression is maltose inducible, and induction is medi-
ated by the Mal activator.
Brewing strains of yeast are polyploid, aneuploid,
or, in the case of lager strains, alloploid. Recently,
Jespersen et al. (1999) examined 30 brewing strains
of yeast (five ale strains and 25 lager strains) with
the aim of examining the alleles of maltose and mal-
totriose transporter genes contained by them. All the
strains of brewer’s yeast examined, except two, were
found to contain MAL11and MAL31sequences, and
only one of these strains lacked MAL41. MAL21was
not present in the five ale strains and 12 of the lager
strains. MAL61was not found in any of the yeast
chromosomes other than those known to carry MAL
loci. Sequences corresponding to the AGT1gene
(transport of maltose and maltotriose) were detected
in all but one of the yeast strains.
Wort maltotriose has the lowest priority for up-
take by brewer’s yeast cells and incomplete mal-
totriose uptake results in yeast-fermentable extract
in beer, material loss, greater potential for microbio-
logical stability, and sometimes atypical beer flavor
profiles (Stewart and Russell 1993). Maltotriose
uptake from wort is always slower with ale strains
than with lager strains under similar fermentation
conditions. However, the initial transport rates are
similar to those of maltose in a number of ale and
lager strains. Elevated osmotic pressure inhibits the
transport and uptake of glucose, maltose, and mal-
totriose, with maltose and maltotriose being more
sensitive to osmotic pressure than glucose in both
lager and ale strains. Ethanol (5% w/v) stimulated
the transport of maltose and maltotriose, due in all
probability to an ethanol-induced change in the plas-
ma membrane configuration, but had no effect on
glucose transport. Higher ethanol concentrations in-
hibited the transport of all three sugars.
WORTFERMENTATION
Before the fermentation process starts, wort is aerat-
ed. This is a necessary step since oxygen is required
for the synthesis of sterols and unsaturated fatty
acids, which are incorporated in the yeast cell mem-
brane (Rogers and Stewart 1973). It has been shown
that both ergosterol and unsaturated fatty acids
increase in concentration as long as oxygen is pres-
ent in the wort (e.g., Haukeli and Lie 1979). A max-
imum concentration is obtained in 5–6 hours after
pitching, but the formation rate is dependent upon
pitching rate and temperature. Unsaturated fatty
acids can also be taken up from the wort, but not all
malt wort contains sufficient unsaturated lipids to
support a normal yeast growth rate. Adding lipids to
wort, especially unsaturated fatty acids, might be an
interesting alternative (Moonjai et al. 2000, 2002).
The oxygen required for lipid biosynthesis can also
be introduced by oxygenation of the separated yeast
cells.
Different devices are used to aerate the cold wort:
ceramic or sintered metal candles, aeration plants
employing Venturi pipes, two-component jets, static
mixers, or centrifugal mixers (Kunze 1999). The
principle of these devices is that very small air (oxy-
gen) bubbles are produced and quickly dissolve dur-
ing turbulent mixing.
As a result of this aeration step, carbohydrates are
degraded aerobically during the first few hours of
the “fermentation” process. The aerobic carbohy-
drate catabolism for a lager fermentation typically
takes 12 hours.
During the first hours of the fermentation pro-
cess, oxidative degradation of carbohydrates occurs
through glycolysis and the Krebs (tricarboxylic
acid, TCA) cycle. The energy efficiency of glucose
oxidation is derived from the large number of
NADH 2 produced for each mole of glucose oxi-
dized to CO 2. The actual wort fermentation gives
alcohol and carbon dioxide via the Embden-
Meyerhof-Parnas (glycolytic) pathway. The reduc-
tive pathway from pyruvate to ethanol is important
since it regenerates NAD. Energy is obtained sole-
ly from the ATP-producing steps of the Embden-
Meyerhof-Parnas pathway. During fermentation, the
activity of the TCA cycle is greatly reduced, al-
though it still serves as a source of intermediates for
biosynthesis (Lievense and Lim 1982).
Lagunas (1979) observed that during aerobic
growth of S. cerevisiae, respiration accounts for less
than 10% of glucose catabolism, the remainder
being fermented. Increasing sugar concentrations
resulting in a decreased oxidative metabolism is
known as the Crabtree effect. This was traditionally