Food Biochemistry and Food Processing

29 Biochemistry and Fermentation of Beer 661

transporter (permease), gene 2 encodes a maltase
(-glucosidase), and gene 3 encodes a transcription-
al activator of the other two genes. Thus, for exam-
ple, the maltose transporter gene at the MAL1locus
is designated MAL61. The three genes of a MAL
locus are all required to allow fermentation. Some
authors persist in using gene designations such as
for the MAL1locus: MAL1T(transporter
perme-
ase), MAL1R(regulator) and MAL1S(maltase). The
five MALloci each map to a different chromosome.
The MALloci exhibit a very high degree of homolo-
gy and are telomere linked, suggesting that they
evolved by translocation from telomeric regions of
different chromosomes (Michels et al. 1992). Since
a fully functional or partial allele of the MAL1locus

is found in all strains of S. cerevisiae, this locus is pro-

posed as the progenitor of the other MALloci (Chow

et al. 1983). Gene dosage studies performed with lab-

oratory strains of yeast have shown that the transport

of maltose in the cell may be the rate-limiting step in

the utilization of this sugar (Goldenthal et al. 1987).

Constitutive expression of the maltose transporter

gene (MALT) with high-copy-number plasmids in a

lager strain of yeast has been found to accelerate the

fermentation of maltose during high-gravity (24°P)

brewing (Kodama et al. 1995). The constitutive

expression of MALSand MALRhad no effect on

maltose fermentability.

The control over MALgene expression is exerted

at three levels. The presence of maltose induces,

Figure 29.1.Schematic overview of the brewing process (input flows are indicated on the left side and output flows
on the right side).