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 Bia Bia Bia Bia

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Bài gửiTiêu đề: Bia Bia Bia Bia   Thu Jun 02, 2011 12:10 pm

9.10 BEER
The popularity of products resulting from the conversion of sugars into
ethanol by yeasts is almost universal and there is hardly a culture without
its own indigenous alcoholic beverage. All that is required is a material
that will furnish sufficient fermentable carbohydrate; a condition fulfilled
by honey, cereals, root crops, palm saps and many fruits, pre-eminently
grapes, but also, apples, pears, plums and others. The ethanol concen-
tration achieved by fermentation is limited by the sugar content of the
raw material and also by the ethanol tolerance of the yeast which is
normally around 14%v/v. Sake, Section 9.12.2 below, is something of an
exception. Potency can be increased by distillation of a fermented wash
to produce spirits such as whisky, vodka, brandy, calvados and arrack,
and ethanol partially purified by distillation can also be added back to a
fermented product to give fortified wines such as port, sherry and
madeira. Here, we will concentrate on a single product which has spread
throughout the world and is now produced more widely than any other
alcoholic drink: European-style beer.
Brewing is thought to have originated inMesopotamia where it is said
that as much as 40% of total cereal production was used for this
purpose. Because of the relative complexity of the process, it is likely
that beer was a later discovery than wine. The Romans were disinterested

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Bài gửiTiêu đề: 22222222222222222222222222   Thu Jun 02, 2011 12:12 pm

and after tasting British ale in the 4th century the Emperor Julian was
compelled to pen a little poem:
Who made you and from what
By the true Bacchus I know you not
He smells of nectar
But you smell of goat.
Clearly the unhopped ale of the time was not to his taste, but even
today beer enjoys an inferior reputation to that of wine.
Barley is the principal cereal used in the production of beer, although
other cereals are occasionally used and wheat beers such as Berliner
Weisse and the Gueuze–Lambic beers of Belgium are notable exceptions.
Africa has a number of traditional beers produced from local cereals
such as sorghum or millet and some of these are produced on a
substantial industrial scale. These however are the result of a mixed
lactic/ethanolic fermentation and bear little resemblance to European-
style beers.
One reason for barleys pre-eminence is that the grain retains the
husk which affords protection during storage and transport and also
acts as an aid to filtration during wort separation. The gelatinization
temperature of malted barley starch is also low relative to that of other
cereals (52–59 1C) and this enables the starch to be gelatinized (solubili-
zed), prior to enzymic digestion, at temperatures which will not inac-
tivate the starch degrading enzyme a-amylase. A further advantage is
the presence in barley of substantial quantities of a second enzyme,
b-amylase, which is essential for the rapid conversion of starch and
dextrins to maltose.
Since the brewing yeast, Saccharomyces cerevisiae, is unable to fer-
ment starch, the first stage in the production of any alcoholic beverage
from starchy materials is conversion of the starch into fermentable
sugars. Human ingenuity has come up with a number of ways of doing
this. In the Oriental Technique, exemplified by products such as sake,
mould enzyme preparations like koji are used, whereas the prevalent
Western technique uses endogenous starch-degrading enzymes produced
in the grain through the process of malting. A third technique used in
some native cultures in South America is to use salivary amylase by
chewing the substrate so that it becomes coated with saliva and then
spitting it out to saccharify and ferment. This approach is not amenable
to industrialization and is not, as far as we are aware, the basis of any
large-scale commercial production of alcoholic beverages.
In malting, the grain is moistened by steeping in water and is then
spread on to a malting floor and allowed to germinate. During germi-
nation, hydrolytic enzymes, produced in the aleurone layer surrounding
the grain endosperm, attack the endosperm, mobilizing the nutrient and

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Bài gửiTiêu đề: 3333333333333333333333333   Thu Jun 02, 2011 12:13 pm

energy reserves it contains for the growing barley plant. To encouraged
this, maltsters sometimes add gibberellins, plant growth hormones which
are the natural regulators of this process.
The development from a seed to a plant is arrested by kilning which
reduces the moisture content of the malt to 3–5%. During kilning, some
non-enzymic browning reactions occur between amino acids and sugars
in the malt and these contribute to the final beer colour. Darker beers
tend to include malts that have been kilned at higher temperatures to
promote browning reactions.
Nowadays malts are usually bought in by brewers as one of their raw
materials and the brewing process proper starts with its conversion into a
liquid medium (wort) capable of supporting yeast growth: a step known
as mashing (Figure 9.11).
The malt is ground to reduce the particle size and increase the rate of
enzymic digestion and is then mixed with hot water. Water, known in
brewers parlance as liquor, is an important ingredient in brewing and
the quality of the local water was one of the reasons for the development
of traditional UK brewing centres such as Burton on Trent, London and
Edinburgh. In particular, calcium content has a significant impact on the
brewing process because calcium ions precipitate out as calcium phos-
phate during mashing. This decreases the wort pH from 6.0 to 5.4, nearer
the optimum for a number of malt enzymes, and thus increases the yield
of fermentable extract. Starchy adjuncts may be added during mashing
to boost the fermentable sugar content of the wort.
There are two traditional systems of mashing: the British technique of
infusion mashing where the mash is held in a single vessel at a constant
temperature of around 65 1C, and the continental decoction system
where the mash is heated through a range of temperatures by removing
a portion, heating it, then adding it back. Nowadays a number of
variations on these techniques are used so that the differences are less
distinct.
In mashing, a number of enzymic activities contribute to the produc-
tion of the clear liquid medium known as sweet wort. For instance, it
requires two enzymes operating in concert to break down starch into
maltose, a disaccharide of glucose fermentable by the brewing yeast.
Barley starch is composed of two fractions: amylose (20–25%), a linear
polymer of a-1,4-linked glucose units, and amylopectin (75–80%), a
branched polymer containing linear chains of a-1,4-linked glucose units
with branches introduced by occasional a-1,6-linkages. Alpha amylase
hydrolyses a-1,4-linkages to produce a mixture of lower molecular
weight dextrins while the exoenzyme, b-amylase, attacks dextrins at their
non-reducing end, snipping off maltose units. Limit dextrins containing
the a-1,6-linkages are left in the wort largely untouched unless the non-
malt enzyme amyloglucosidase is added to the mash.

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Bài gửiTiêu đề: 444444444444444444   Thu Jun 02, 2011 12:13 pm

Proteinases solubilize malt proteins and supply yeast nutrients so that
about 35–40% of malt protein is solubilized during mashing compared
with 90–95% of the malt starch. Phosphatases release inorganic phos-
phate, which is important for yeast nutrition and for contributing to the
buffering capacity of the wort. The activity of b-glucanases can also be
useful in breaking down b-glucans which can cause subsequent handling
problems with the beer.
After mashing, sweet wort is boiled. This stops the degradative proc-
esses by inactivating the malt enzymes. It also pasteurizes the wort,
completes ionic interactions such as calcium phosphate precipitation,
denatures and precipitates proteins and tannins which separate as a
material known as hot break or trub and helps dissolve any sugars which
may be added at this stage as an adjunct.
Hops are also added during boiling. These are the cones or strobili of
the plant Humulus lupulus whose principal purpose is the bittering of the
wort. The hop resin contains a-acids such as humulone and cohumulone
which are only partially soluble in wort. During boiling they isomerize to
isohumulones which are more soluble and more bitter than a-acids
(Figure 9.12). Although hop resins have some antibacterial action, they
play little part in assuring the bacteriological stability of beer as spoilage
bacteria such as lactobacilli rapidly acquire a tolerance to them.
Wort boiling lasts for 1–2 h during which 5–15% of the volume is
evaporated. Hop residues are then strained off, hot trub is removed in a
whirlpool separator, and the hopped wort cooled to the fermentation
temperature.
The yeasts used to brew ales and lagers are strains of Saccharomyces
cerevisiae, known as S. cerevisiae var. cerevisiae and S. cerevisiae var.
carlsbergensis (uvarum) respectively. The distinctions between the yeasts
used in ale and lager brewing are slight. Traditionally, ale yeasts were
regarded as top fermenters which formed a frothy yeast head on the
surface of brewing beer and was skimmed off to provide yeasts for
pitching (inoculating) subsequent batches, while lager yeasts were bot-
tom fermenters which formed little surface head and were recovered
from the bottom of the fermenter. Nowadays this is a less useful
distinction as many ales are brewed by bottom fermentation.
The cardinal temperatures of the two organisms differ and this is
reflected in the different temperatures used for lager fermentations (8–
12 1C) and for ale fermentations (12–18 1C). They can also be distin-
guished by the ability of S. cerevisiae var. carlsbergensis to ferment the
disaccharide melibiose, although this is of no practical import since the
sugar does not occur in wort.
During fermentation the yeast converts fermentable carbohydrate to
ethanol via the EMP pathway. Although this is an anaerobic process, a
vigorous fermentation is often helped by aeration of the wort before
pitching with yeast. This supplies oxygen, necessary for the synthesis of
unsaturated fatty acid and sterol components of the yeast cell membrane,
and may sometimes be repeated later in the fermentation.
A time course of a typical ale fermentation is shown in Figure 9.13. After
an initial vigorous phase during which there is active yeast growth, ethanol
production and a drop in pH as nitrogen is removed from the wort, there is
a second phase of slower ethanol production in the absence of further yeast

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Bài gửiTiêu đề: 55555555555555555   Thu Jun 02, 2011 12:15 pm

growth. Overall the yeast population increases about six-fold during fer-
mentation.Thisyeastcanberecycled,usuallyafteranacidwashtocontrol
bacterial contamination, but eventually its performance drops as viability
declines and it is used in animal feed and the manufacture of yeast extract.
Lager fermentations take longer due to the lower temperature. The
name lager originates from the German word for store and describes the
period of secondary fermentation (storage) at low temperature which
these products undergo to improve yeast settling, clarification and CO2
dissolution. In the past, this could last for several months, but with
modern techniques such as centrifugation and artificial carbonation it is
less protracted and is now complete within one to two weeks.
Depending on the product, beer from the fermenter can be subjected
to a variety of downstream processes. It may be run to casks where
priming sugars are added to stimulate the secondary fermentation neces-
sary in cask-conditioned beers, or it may be filtered prior to pasteu-
rization and kegging. Bottled and canned beers usually undergo a
combination of filtration and centrifugation before packing and
pasteurization.
Brewing is quite a robust process microbiologically due to a combi-
nation of factors such as low nutrient status, ethanol content and low pH
and there is a limited range of micro-organisms of concern to the brewery
microbiologist. Members of the Enterobacteriaceae such as Obesumbac-
terium proteus, Klebsiella and Enterobacter species are sensitive to the
low pH and ethanol of beer but can grow in wort producing off-odours
like dimethyl sulfide which can persist through to the final product. They
also contribute to the production of nitrosamines by reducing wort
nitrate to nitrite. Obesumbacterium proteus is particularly associated
with top-fermenting yeasts but can be controlled by acid washing.
Acetic acid bacteria of the genera Acetobacter and Gluconobacter can
be found throughout the brewery. As obligate aerobes they are partic-
ularly associated with cask-conditioned beer where they cause spoilage as
a result of turbidity, ropiness and the oxidation of ethanol to ethanoic
(acetic) acid. Zymomonas mobilis is an anaerobic, Gram negative rod
which can ferment sugars to ethanol. It causes more problems in ale
brewing where it grows in the primed beer producing turbidity and off-
flavours.
Lactic acid bacteria of the genera Lactobacillus and Pediococcus can
grow widely in the brewery environment and in beer where they produce
acid, diacetyl, which gives beer a sweet butterscotch flavour, and
Table 9.10 Possible taints in beer
Taint Associated Flavours Possible Cause
Acetaldehyde Apples, paint, grassy Bacterial spoilage: Acetic acid
bacteria/Zymomonas
Sulphur Bad eggs, drains Formed during fermentation,
wild yeasts/Zymomonas
Cloves Herbal phenolic Bacterial spoilage/wild yeasts
Musty/fungal Mouldy, stale, hessian Mould or bacteria: Usually in
water
Fruity Estery, pineapples, solvent,
bananas, pear drops
Wild yeast/Brettanomyes: Wort
bacteria/Enterobacter
agglomerans
Ethyl Acetate Solvent-like Wild yeast/Hansenula anomala
Diacetyl Toffee, butterscotch, honey Formed during fermentation/
lactic acid bacterial spoilage:
Lactobacillus/Pediococcus
DMS Sweetcorn, jammy Bacterial spoilage: Hafnia/
Obesumbacterium. Wort
bacteria
TCP Acetic Acid Medicinal, antiseptic Sour,
vinegar
Bacterial spoilage: Wort bacteria
Bacterial spoilage:
Acetobacter/Gluconobacter
Acidic Sourness and creaminess
Sourness and apples
Bacterial spoilage: Lactobacillus
Acetobacter
Courtesy L. Hargreaves

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Bài gửiTiêu đề: 666666666666666   Thu Jun 02, 2011 12:16 pm

polymeric material known as rope. Yeasts that are not used for the
fermentation can cause hazes and off-odours and are known as wild
yeasts. Normally these are described as being Saccharomyces and non-
Saccharomyces. Saccharomyces wild yeasts can be detected using a
medium containing copper sulfate to inhibit the brewing yeast. Non-
Saccharomyces yeasts such as Pichia, Hansenula, Brettanomyces and
others can be detected with a medium containing lysine as the sole
nitrogen source which Saccharomyces cannot utilize.
Some of the possible taints in beer and their causes are presented in
Table 9.10.
9.11 VINEGAR
Vinegar is the product of a two-stage fermentation. In the first stage,
yeasts convert sugars into ethanol anaerobically, while in the second
ethanol is oxidized to acetic (ethanoic) acid aerobically by bacteria of the
genera Acetobacter and Gluconobacter. This second process is a common
mechanism of spoilage in alcoholic beverages and the discovery of
vinegar was doubtless due to the observation that this product of
spoilage could be put to some good use as a flavouring and preservative.
The name vinegar is in fact derived from the French vin aigre for ‘sour
wine and even today the most popular types of vinegar in a region
usually reflect the local alcoholic beverage; for example, malt vinegar in
the UK, wine vinegar in France, and rice vinegar in Japan.
In vinegar brewing, the alcoholic substrate, known as vinegar stock, is
produced using the same or very similar processes to those used in
alcoholic beverage production.Where differences occur they stem largely
from the vinegar brewers relative disinterest in the flavour of the
intermediate and his concern to maximize conversion of sugar into
ethanol. In the production of malt vinegar for example, hops are not
used and the wort is not boiled so the activity of starch-degrading
enzymes continues into the fermentation. Here we will concentrate on
describing the second stage in the process, acetification.
Acetification, the oxidation of ethanol to acetic acid is performed by
members of the genera Acetobacter and Gluconobacter. These are Gram-
negative, catalase-positive, oxidase-negative, strictly aerobic bacteria.
Acetobacter spp. are the better acid producers and are more common
in commercial vinegar production, but their ability to oxidize acetic acid
to carbon dioxide and water, a property which distinguishes them from
Gluconobacter, can cause problems in some circumstances when the
vinegar brewer will see his key component disappearing into the air as
CO2. Fortunately over-oxidation, as it is known, is repressed by ethanol
and can be controlled by careful monitoring to ensure that ethanol is not
completely exhausted during acetification.Most acetifications are run on

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