2009_Book_FoodChemistry

20.1 Beer 897
active aroma ingredients more extractable. Prior
to grinding, part of the inert material is separated
and thus lupulin-enriched concentrates are
obtained.
Hops are extracted with a mixture of water and an
organic solvent (e. g., alcohol, diethylether), giving
extracts of varying compositions. Recently,
a hops extraction process using supercritical carbon
dioxide has become important.
Isomerized extracts, in which humulon has been
converted into isohumulon by heat treatment, are
suitable for a cold hopping procedure. In traditional
beer hopping this conversion is achieved
by boiling the wort for a long time. Isomerized
extracts are used in the main fermentation or at
a later step in brewing.
Boiling of hops results in the loss of a large portion
of oil constituents with the steam. The addition
of hops shortly before the end of the boiling
process or the use of hop resins or concentrates
may greatly enhance the hop aroma of the product.
Phenolic constituents in hops contribute to
protein coagulation during wort boiling. A part of
protein-tannin complexes formed may precipitate
at low temperatures after long storage, resulting
in turbidity in the beer.
20.1.2.4 Brewing Water
The water used for wort preparation in a brewery
has a great influence on beer quality and character.
The salt constituents of water can change the
pH of the mash and wort. Bicarbonate ions cause
a pH increase, while Ca 2+ and Mg 2+ ions cause
a pH decrease. Heating of water which contains
bicarbonates increases the alkalinity according to
the equation:
HCO ⊖ 3 + H⊕ ⇋ CO 2 + H 2 O (20.3)
in which the equilibrium is shifted to the left
since, during heating, the CO 2 component escapes
as a gas. Ca and Mg ions react with secondary
phosphates in wort to form insoluble tertiary
phosphates, releasing protons which add to
the acidity of the water:
3Ca 2⊕ + 2HPO 2⊖
4 ⇋ Ca 3 (PO 4 ) 2 + 2H ⊕ (20.4)
Magnesium sulfate in high concentrations imparts
an unpleasant bitter taste to beer. Manganese
and iron salts induce turbidity, discoloration and
taste deterioration. Concentrations of NaCl or nitrate
(>300 mg/l) which are too high interfere
with fermentation. During fermentation, nitrate is
reduced to nitrite, which is toxic for yeast.
The unique character of different kinds of beer
(Pilsen, Dortmund, Munich, Burton-on-Trent),
without doubt, can historically be ascribed to the
brewing water used in those places, with residual
alkalinity playing the major role. Water, low in
soluble bicarbonates of calcium, magnesium,
sodium or potassium, and soluble carbonates
and hydroxides, is suitable for strongly-hopped
light beers, such as Pilsener, while alkaline water
is suitable for dark beers, such as those from
Munich.
Preparation of brewing water is mainly directed
to the removal of carbonates. Precipitation by
heating with lime is customary. Furthermore,
when lime water is used without heating, water
softening occurs. Removal of excess salt by
ion-exchange resins is also advantageous. Today
any water can be treated to match the requirement
of a desired type of beer.
20.1.2.5 Brewing Yeasts
Brewing yeasts are exclusively strains of Saccharomyces.
Two types are recognized: top
fermenting yeasts for temperatures >10 ◦ C,
and bottom fermenting yeasts used down to
0 ◦ C. The top fermenting yeasts, e. g., Saccharomyces
cerevisiae Hansen, rise to the surface
during fermentation in the form of large budding
(“sprouting”) associations. They ferment
raffinose only partially since they lack the enzyme
melibiase. The bottom fermenting yeasts,
e. g., Saccharomyces carlsbergensis Hansen,
settle to the bottom during fermentation and
completely ferment all sugars including raffinose.
There are yeasts with high fermentation
ability which remain suspended for a long time,
giving a high fermentation rate. Yeasts with low
fermentation ability flocculate early and settle to
the bottom (super-flocculent yeasts) and hence
are unable to continue active fermentation. Pure
cultures of many yeast strains currently in use
are derived from a single yeast cell and are
used as “starter yeast” in plant operations. After
the main fermentation, a part of the yeast is