660 Part VI: Fermented Foods
wort. For the production of a high quality beer, a
well-controlled fermentation needs to be performed.
During fermentation, major flavor-active compounds
are produced (and some of them are again degraded)
by the yeast cells. The metabolism of the most im-
portant fermentation by-products during main and
secondary fermentation is discussed in detail. The
latest trend in beer fermentation technology is pro-
cess intensification using immobilized cell technol-
ogy. This new technology is explained, and some
illustrative applications—small and large scale—are
discussed.
THE BEER BREWING PROCESS
The principal raw materials used to brew beer are
water, malted barley, hops, and yeast. The brewing
process involves extracting and breaking down the
carbohydrate from the malted barley to make a sugar
solution (called “wort”), which also contains essen-
tial nutrients for yeast growth, and using this as a
source of nutrients for “anaerobic” yeast growth.
During yeast fermentation, simple sugars are con-
sumed, releasing energy and producing ethanol and
other metabolic flavoring by-products. The major
biological changes that occur in the brewing process
are produced by naturally produced enzymes from
barley (during malting) and yeast. The rest of the
brewing process largely involves heat exchange, sep-
aration, and clarification, which only produce minor
changes in chemical composition when compared
with the enzyme-catalyzed reactions. Barley is able
to produce all the enzymes that are needed to de-
grade starch, -glucan, pentosans, lipids, and pro-
teins, which are the major compounds of interest to
the brewer. An overview of the brewing process is
shown in Figure 29.1, where the input and output
flows also are indicated. Table 29.1 gives a more
detailed explanation of each step in the process.
CARBOHYDRATE
METABOLISM—ETHANOL
PRODUCTION
CARBOHYDRATEUPTAKE
Carbohydrates in wort make up 90–92% of wort
solids. Wort from barley malt contains the fermen-
table sugars sucrose, fructose, glucose, maltose, and
maltotriose, and some dextrin material (Table 29.2).
The fermentable sugars typically make up 70–80%
of the total carbohydrate (MacWilliam 1968). The
three major fermentable sugars are glucose and the
-glucosides maltose and maltotriose. Maltose is by
far the most abundant of these sugars, typically
accounting for 50–70% of the total fermentable sug-
ars in an all-malt wort. Sucrose and fructose are pre-
sent in low concentrations. The unfermentable dex-
trins play little part in brewing. Wort fermentability
may be reduced or increased by using solid or liquid
adjuncts.
Brewing strains consume the wort sugars in a spe-
cific sequence: 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 con-
centrations. Only when 60% of the wort glucose has
been taken up by the yeast, will the uptake and con-
sumption of maltose start. Maltotriose uptake is in-
hibited by high glucose and maltose concentrations.
When high amounts of carbohydrate adjuncts (e.g.,
glucose) or high-gravity wort are employed, the glu-
cose repression is even more pronounced, resulting
in fermentation delays (Stewart and Russell 1993).
The efficiency of brewer’s yeast strains to effect
alcoholic fermentation is dependent upon their abil-
ity to utilize the sugars present in wort. This ability
very largely determines the fermentation 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 con-
sumption kinetics, which is linked to the yeast
growth kinetics, is required (Willaert 2001).Maltose and Maltotriose MetabolismThe yeast Saccharomyces cerevisiaetransports the
monosaccharides across the cell membrane by the
hexose transporters. There are 19, or possibly 20,
genes encoding hexose transporters (Dickinson
1999). The disaccharide maltose and the trisaccha-
ride maltotriose are transported by specific trans-
porters into the cytoplasm, where these molecules
are hydrolyzed by the same -glucosidase, yielding
two or three molecules of glucose, respectively
(Panchal and Stewart 1979, Zheng et al. 1994).
Maltose utilization in yeast is conferred by any
one of five MALloci: MAL1to MAL4and MAL6
(Bisson et al. 1993, Dickinson 1999). Each locus
consists of three genes: gene 1 encodes a maltose