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