Food Biochemistry and Food Processing (2 edition)

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630 Part 5: Fruits, Vegetables, and Cereals

Table 33.2.Carbohydrate Composition of Worts

Origin Type of Wort

Danish Lager
11 ◦P

Canadian
Lager 13◦P

British Pale
Ale 10◦P

Canadian Corn
Adjunct Wort

All-Malt Wort
18 ◦P

Fructose (g/L) (%)a 2.1
2.7

1.5
1.6

3.3
4.8

1.3
1.3

3.0

Glucose (g/L) (%)a 9.1
11.6

10.3
10.9

10.0
14.5

14.7
15.7

13.0

Sucrose (g/L) (%)a 2.3
2.9

4.2
4.5

5.3
7.7

1.8
1.9
Maltose (g/L) (%)a 52.4
66.6

60.4
64.2

38.9
56.5

62.8
67.0

80.0

Maltotriose (g/L) (%)a 12.8
16.3

17.7
18.8

11.4
16.5

13.2
14.1

24.0

Total ferm. sugars (g/L) 78.7 94.1 68.9 93.8 121.0
Maltotetraose (g/L) 2.6 7.2 2.0
Higher sugars (g/L) 21.3 26.8 25.2
Total dextrins (g/L) 23.9 34.0 25.2
Total sugars (g/L) 102.6 128.1 94.1 117.5

Source: Patel and Ingledew (1973), Huuskonen et al. (2010), and Hough et al. (1982).
aPercent of the total fermentable sugars.

Brewing strains consume the wort sugars in a specific se-
quence: glucose is consumed first, followed by fructose, maltose,
and finally maltotriose. The uptake and consumption of maltose
and maltotriose is repressed or inactivated at elevated glucose
concentrations. Only when 60% of the wort glucose has been
taken up by the yeast, the uptake and consumption of maltose
will start. Maltotriose uptake is inhibited by high glucose and
maltose concentrations. When high amounts of carbohydrate
adjuncts (e.g., glucose) or high-gravity wort are employed, the
glucose repression is even more pronounced, resulting in fer-
mentation delays (Stewart and Russell 1993).
The efficiency of brewer’s yeast strains to effect alcoholic fer-
mentation is dependent upon their ability to utilize the sugars
present in wort. This ability very largely determines the fer-
mentation rate as well as the final quality of the beer produced.
In order to optimize the fermentation efficiency of the primary
fermentation, a detailed knowledge of the sugar consumption
kinetics, which is linked to the yeast growth kinetics, is required
(Willaert 2001).

Maltose and Maltotriose Metabolism

The yeastSaccharomyces cerevisiaetransports the monosaccha-
rides across the cell membrane by the hexose transporters. There
are 20 genes encoding hexose transporters (Dickinson 1999,
Rintala et al. 2008): 18 genes encoding transporters (HXT1to
HXT17,GAL2) and two genes encoding (SNF3,RGT2). The
disaccharide maltose and the trisaccharide maltotriose are trans-
ported by specific transporters into the cytoplasm, where these
molecules are hydrolyzed by the sameα-glucosidase yielding
two or three molecules of glucose, respectively (Panchal and
Stewart 1979, Zheng et al. 1994a).

Maltose utilization in yeast is conferred by any one of five
MALloci:MAL1toMAL4andMAL6(Bisson et al. 1993,
Dickinson 1999). Each locus consists of three genes (MALx1,
where x stands for one of the five loci): gene 1 encodes a
maltose transporter (permease), gene 2 encodes a maltase (α-
glucosidase), and gene 3 encodes a transcriptional activator of
the other two genes. Thus, for example, the maltose transporter
gene at theMAL1locus is designatedMAL61. The three genes
of aMALlocus are all required to allow fermentation. Alterna-
tively, some authors use gene designations such as for theMAL1
locus:MAL1T(transporter=permease),MAL1R(regulator),
andMAL1S(maltase). The genetic and biochemical analysis of
maltose fermentation by yeast cells revealed a series of five un-
linked telomere-associated multigeneMALloci:MAL1(chro-
mosome VII),MAL2(chromosome III),MAL4(chromosome
II), andMAL6(chromosome VIII). TheMALloci exhibit a very
high degree of homology and are telomere linked, suggesting
that they evolved by translocation from telomeric regions of
different chromosomes (Michels et al. 1992). Since a fully func-
tional or partial allele of theMAL1locus is found in all strains
ofS. cerevisiae, this locus is proposed as the progenitor of the
otherMALloci (Chow et al. 1983), as allS. cerevisiaestrains,
and even its closest related yeast speciesS. paradoxus, contain
MAL1sequences near the right telomere of chromosome VII.
The genes in theMALloci show a high degree of sequence
and functional similarity, but there can be extensive variability,
and several different alleles that determine distinct phenotypes
(i.e.,MAL-inducible andMAL-constitutive strains) have been
described (Novak et al. 2004). Gene dosage studies performed
with laboratory 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
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