BLBS102-c33 BLBS102-Simpson March 21, 2012 14:5 Trim: 276mm X 219mm Printer Name: Yet to Come
632 Part 5: Fruits, Vegetables, and CerealsData indicated that all knownα-glucoside transporters present in
S. cerevisiae, including the maltose permeasesMAL31,MAL61,
MPH2,andMHP3, allowed growth of the yeast cells on both
maltose and maltotriose (Day et al. 2002a, 2002b). Their kinetic
analysis of maltose and maltotriose uptake indicated that all
these transporters, includingAGT1permease, could transport
both sugars with practically the same affinities and capacity.
Recently, to better understand maltotriose utilization by yeast
strains, an analysis of maltotriose and maltotriose utilization by
52 laboratory and industrialSaccharomycesyeast strains was
performed (Duval et al. 2009). Microarray comparative genome
hybridization (aCGH) was used to correlate the observed phe-
notypes with copy number variations in genes known to be in-
volved in maltose and maltotriose utilization by yeasts. The
results showed thatS. pastorianusstrains utilized maltotriose
more efficiently thanS. cerevisiaestrains and highlighted the
importance of theAGT1gene for efficient maltotriose utiliza-
tion byS. cerevisiaeyeasts. The fermentation performance of
a lager (S. pastorianus) strain was improved when itsAGT1
gene was replaced with theAGT1gene of an ale (S. cerevisiae)
strain, since theATG1gene of the lager strains studied con-
tained a premature stop codon and did not encode functional
transporters (Vidgren et al. 2005, 2009). The transformants
with repairedAGT1had higher maltose transport activity, es-
pecially after growth on glucose, which represses endogeneous
α-glucoside transporter genes. The sequences of twoAGT1-
encodedα-glucosidase transporters with different efficiencies of
maltotriose transport in twoSaccharomycesstrains were com-
pared (Smit et al. 2008). The amino acids Thr^505 and Ser^557 ,
which are respectively located in the transmembrane (TM) seg-
ment TM^11 and on the intracellular segment after TM^12 of the
AGT1-encodedα-glucosidase transporters, are critical for effi-
cient transport of maltotriose inS. cerevisiae. It was also shown
that maltotriose utilization could be improved by attaching a
maltase encoded byMAL32to the yeast cell surface (Dietvorst
et al. 2007).Glycogen and Trehalose MetabolismGlycogen and trehalose are the main storage carbohydrates in
yeast cells (Panek 1991). The synthesis of both compounds
commences with the formation of uridine diphosphate (UDP)-
glucose catalyzed by UDP-glucose pyrophosphorylase.
Glycogen is a reserve carbohydrate that is metabolized during
periods of starvation. It is a branched polysaccharide composed
of linearα-(1,4)-glycosyl chains withα-(1,6)-linkages (similar
to starch but with a higher degree of branching). It is synthesized
starting from glucose via glucose-6-phosphate and glucose-1-
phosphate. Glycogen is synthesized from glucose, via glucose-6-
phosphate and glucose-1-phosphate (Franc ̧ois and Parrou 2001).
UDP serves as a carrier of glucose units and is formed by a
two-step reaction, catalyzed by phosphoglucomutase and UDP-
glucose phosphorylase. Glycogen synthesis is initiated by glyco-
genin that produces a shortα-(1,4)-glucosyl chain, which is
elongated by glycogen synthase. Theα-(1,6)-glucosidic bonds
are formed by the branching enzyme Glc3. The dissimilation
of glycogen occurs through the action of glycogen phosphory-lase, which releases glucose-1-phosphate from the nonreducing
ends of the glycogen chains, and the debranching enzyme Gdb1,
which transfers a maltosyl unit to the end of an adjacent linear
α-(1,4) chain and releases glucose by cleaving the remaining
α-(1,6)-linkage.
When yeast cells are pitched in aerated wort, an immediate
glycogen mobilization is observed (Quain 1988, Boulton 2000).
Glycogen accumulates during the exponential growth phase, af-
ter oxygen has been consumed. When the yeast growth is ceased
toward the end of the primary fermentation, are maximum glyco-
gen levels obtained. In the stationary phase, the glycogen levels
decline slowly. Glycogen provides energy for the synthesis of
sterols and unsaturated fatty acids during the aerobic phase of the
beer fermentation, and energy for the cellular maintanance func-
tions during the stationary phase in the storage phase between
cropping and pitching (Quain and Tubb 1982, Boulton et al.
1991). The glycogen content is directly related to subsequent
fermentation performance. Therefore, yeast storage occurs best
at a low temperature, without agitation and under an atmosphere
of nitrogen or carbon dioxide to minimize glycogen breakdown
(Murray et al. 1984).
The regulation of the glycogen content is complex and occurs
in part by the cAMP/PKA pathway (Smith et al. 1998). Glycogen
accumulation is repressed by a high PKA-activity (Franc ̧ois and
Parrou 2001). The glycogen content increases and the PKA-
activity is reduced, when an essential nutrient is progressively
consumed from the growth medium.
Trehalose is a disaccharide (α-d-glucopyranosyl-1,1-α-d-
glucopyranoside) which contains two molecules ofd-glucose
(Boulton and Quain 2006). Trehalose biosynthesis is catalyzed
by the trehalose synthase complex, which forms trehalose-6-
phosphate from UDP-glucose and glucose-6-phosphate, and
next dephosphorylates it to trehalose. Trehalose is degradaded by
the neutral (Nth1) or the acid (Ath1) trehalase (Panek and Panek
1990, Franc ̧ois and Parrou 2001). Like glycogen, trehalose also
accumulates in yeast under conditions of nutrient limitation.
It has been observed that trehalose rapidly accumulates in re-
sponse to environmental stress, such as dehydration, heating
and osmotic stress (during high gravity brewing) (Majara et al.
1996, Hounsa et al. 1998). Under stress conditions, the higher
levels of trehalose protect the cells by binding to membranes and
proteins (Franc ̧ois and Parrou 2001). Trehalose accumulation in
response to stress is, however, a transient phenomenon (Parrou
et al. 1997).
In the beginning of the primary fermentation, high glucose
levels activate the camp/PKA pathway (Zahringer et al. 2000). ̈
This results in posttranslational activation of neutral treha-
lase and in induction of itsNTH1gene, and in repression of
the trehalose synthase genes, which results in reduced levels
of trehalose.Wort FermentationBefore the fermentation process starts, wort is aerated. 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 membrane (Rogers and Stewart 1973). It has been