Chemistry Reference
In-Depth Information
in yeast proteins. The formation of ethanol occurs via the Embden-Meyerhof-Parnas pathway
(also called the glycolytic pathway) where, theoretically, 1 g of glucose will yield 0.51 g of
ethanol and 0.49 g of CO
2
.
Brewers yeast types are of the genus
Saccharomyces
. In wort, brewers yeast absorbs
dissolved sugars, simple nitrogenous matter (amino acids and very simple peptides), vita-
mins, ions, etc., through their outside cell membrane (the plasma membrane). Then they
employ a structured series of reactions known as metabolic pathways to use these sub-
stances for growth and fermentation. The two main types of beer, lager and ale are fermented
with
Saccharomyces uvarum
(
carlsbergensis
) and
Saccharomyces cerevisiae
, respectively.
S. cerevisiae
has the ability to take up a wide range of sugars, for example glucose, fruc-
tose, mannose, galactose, sucrose, maltose, maltotriose and raffinose. Unlike
S. cerevisiae
,
strains of
S. uvarum
(
carlsbergensis
) possess the MEL gene, which means that through
an extracellular enzyme (α-galactosidase or melibiase), they are able to utilize melibiose
(glucose-galactose). Wort components like dextrins, β-glucan and soluble proteins are not
metabolized by strains of brewer's yeast.
Wort contains the sugars sucrose, fructose, glucose, maltose and maltotriose together with
dextrins. The initial step in the utilization of any sugar by yeast is usually either its intact
passage across the cell membrane or its hydrolysis outside the cell membrane followed by
entry into the cell by some or all of the hydrolysis products. Maltose and maltotriose are
examples of sugars that pass intact across the cell membrane whereas sucrose is hydrolyzed
by an extracellular enzyme and the hydrolysis products are taken up into the cell. Maltose
(50-60% of wort fermentable sugar) and maltotriose (20% of wort fermentable sugar) are
the major sugars in brewers wort and consequently, a brewers yeast's ability to use these two
sugars is vital and depends upon the correct genetic complement. It is probable that brewer's
yeast possesses independent uptake mechanisms (maltose and maltotriose permease) to
transport the two sugars across the cell membrane into the cell. Once inside the cell, both
sugars are hydrolyzed to glucose units by the α-glucosidase (maltase) system. Once the
sugars are inside the cell, they are converted via the glycolytic pathway into pyruvate. There
are many more enzymatic-induced processes occurring during yeast metabolism. This is not
the subject of this chapter and thus the reader is referred to more specific texts
10
covering
the broad area of brewing fermentation.
8.5.2 Exogenous enzymes applied during fermentation
Exogenous enzymes (Table 8.10), apart from the application areas described below, are
applied in fermentation as tools to help possible difficulties occurring later in the process. In
the case of downstream beer filtration problems, β-glucanase can be added to the fermenter
(or during maturation) to assist in degradation of residual glucans, which would otherwise
cause the filters to block. This is a last option technique, as fermentation temperatures are
low, hence enzyme activities are lower. It is more economical to apply the enzymes during the
brewhouse stages of the process. Another area for enzyme application during fermentation
is to reduce haze problems in the final beer. This will be covered in the next section.
If filtration problems are due to protein, starch or non-starch polysaccharides or combi-
nations of these materials, then this can be detected via laboratory tests applying enzyme,
filtration and flow rate tests to the wort. A visual representation of this is shown in Fig. 8.4. In
this case, β-glucanase was very effective in restoring filtration to normal rates. The control
beer had a β-glucan level of 300 mg L
−
1
β-glucan. With the addition of β-glucanase at
