8. Beer maturation, chilling and cold storage (for chilled and filtered beer

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22 févr. 2014 (il y a 3 années et 3 mois)

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8.

Beer maturation, chilling and cold storage (for chilled
and filtered beer

8.1.

Warm maturation

8.1.1.

Why mature beer?

The traditional maturation process is particularly associated with Lager beers.

(Draught
cask beers mature naturally in cask, assisted by finings. Flavour improves and carbonation
kep constant by a slight secondary fermentation (normally stimulated with sugar primings)
and judicious cellar management.)

The word "lager
"
means "storage" in German. Many of the traditional German beers were
“lagered” for up to 9 months. This fitted the pattern of the seasons. The beer was brewed
with the new season’s malted barley and hops in the autumn when the temperatures were
mild. Afte
r fermentation the beers were stored in cellars over the cold winter to be
available fully matured as a light fresh drink during the hot summers.

Maturation starts at the end of primary fermentation, and progresses through a number of
temperature stages.

The beer is initially cooled down to between 6
0

and 8
0

C. It is then
gradually cooled to between
2
0

and
-
0.5
0

C.


Reasons
for storage (lagering) are to:



Continue fermentation to scavenge air



Continue fermentation to remove undesirable flavour substances



I
ncrease the CO
2

saturation of the beer (conditioning)



Aggregate and precipitate the haze forming compounds of protein and
polyphenols



Clarify the beer through the settlement of yeast and suspended solids



Improve flavour through slow secondary fermentation

Today few beers, enjoy the luxury of 9 months maturation.

The maturation stage has been reduced to between 1 to 4 weeks for most lager beers.
Some brewers reduce "lagering" times to as little as 3 days . New rapid continuous
maturation systems can compl
ete the required flavour changes within a few hours.

During lagering we try to optimise:



Flavour improvement and removal of unwanted taints



Oxygen control



Adjustment in carbonation



Beer sedimentation and clarification (usually prior to filtration)

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Beer st
abilisation

Warm maturation is followed by a period of cold conditioning. Cold conditioning helps to
prepare the beer for filtration and stabilises the beer. At 0 to

2
0
C there is little yeast
activity (although lager yeast can grow very slowly at this te
mperature).

8.1.2.

Flavour changes during Maturation

Once fermentation is complete most of the fermentable sugars have been converted to
alcohol. The nutrients of the wort are nearly all gone. The fermented beer is no lo
nger a
good growth medium for the yeast cells. Yeast settles out. It collects at the top of the
fermentation as in the case of ale yeast, or settles to the bottom of the fermentation for
lager yeast. However, some yeast will remain in suspension.

Yeast
plays an important part in the maturation process. Beer at the end of primary
fermentation still contains relatively high levels of undesirable flavour active compounds.
The most common compounds are diacetyl and acetaldehyde.

The levels of these compound
s can be controlled during the fermentation process. Still,
almost all beers, and lagers in particular, benefit from a period of warm maturation. A
period of warm maturation allows the yeast still in suspension to further breakdown these
compounds.

Brewe
rs use different methods to provide the necessary maturation:

8.1.2.1.

The traditional method

The traditional method is to cool the beer at the end of fermentation to around 8
0
C. The
majority of the surplus yeast is removed before transferring the beer to a warm ma
turation
tank. During the transfer the remaining yeast becomes re
-
suspended. This reinvigorates
the yeast. It starts to produce a slow secondary fermentation. This takes up the unwanted
flavour compounds and breaks them down.

In some cases the yeast remain
ing at the end of fermentation is in poor physical condition.
It will not produce an effective secondary fermentation. Under these circumstances active
yeast can be added in the form of Krausen
. “Krausening” involves adding actively
ferment
ing wort to the beer in the conditioning tank.

The secondary fermentation also generates more CO
2.

Because it is carried out at a lower
temperature (which increases the solubility of CO
2
) and often under positive back
pressure, the carbonation of the beer
increases.

8.1.2.2.

Modern method

When the main fermentation is virtually complete the beer is kept in fermenter for a further
24 to 54 hours at the fermentation temperature. This allows the yeast still in suspension to
carry out the warm conditioning before being
cooled and transferred to cold conditioning.
The success of this procedure relies on sufficient active yeast remaining in suspension at
the end of the primary fermentation.

The duration of the stand lasts until a quality specification is met.

Usually thi
s is a diacetyl specification, and hence it is often called the “diacetyl rest
”.


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8.1.3.

Oxygen and Carbon Dioxide control in Maturation

8.1.3.1.

Oxygen

Much effort is now used to prevent the pickup of oxygen from the end of fermentation. The
dissolved oxygen level at the end of fermentation should be zero.

Typical dissolved oxygen levels below 50 ppb (parts per billion) are required in bright beer
tank
in order to produce a final “in package” dissolved oxygen as low as 300 parts per
billion.

The procedures used to reduce oxygen pick up include:



Flushing the maturation tank with Carbon Dioxide



Storing the beer under a positive CO
2

top pressure



Using CO
2

for the cover flush on centrifuges (where used)



Using deaerated water with dissolved oxygen levels as low as
0.05
mg/L


dissolved oxygen for all beer/water chases



Using deaerated water with dissolved oxygen levels as low as
0.05
mg/L


dissolved oxygen for

blending



The use of large modern maturation tanks. This cuts down on surface area
absorption.



Gentle beer movements through correctly sized pipes



Purging of bends and pipes and the use of double seat valves to reduce oxygen
pick up



In
-
line monitoring of
dissolved oxygen to identify any problems and to take
immediate corrective action

8.1.3.2.

Adjustment in carbonation

At the end of primary fermentation a beer usually has a dissolved CO
2

content of between
2 and 4 g/l (3.92
-
7.84 V/V) A small pack specification may be between 5 and 5.5 g/l CO
2
(9.8
-
10.78 V/V).


In traditional maturation CO
2
was produced by secondary fermentation. Today’s shorter
maturation times usually means that CO
2
le
vel

is not high enough
.


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CO2 Injector

The modern brewery adjusts CO
2

by
injecting it in line, usually after filtration and
controlling the system with an inline CO
2

analyser.

A typical unit is the Embra CarboCheck.

The CarboCheck sens
or has a silicone
rubber membrane through which the
dissolved CO
2

permeates into a sealed,
evacuated chamber. The partial pressure of
the gas is then measured and displayed by
the analyser / control unit as a CO
2

content.


CO
2
is then injected as required.

The
injection point is a flattened pipe. This
increases absorption of the CO
2
over the
thin beer film produced.


Another device is the Haffmans carbonator.
The principle
is similar.


8.1.4.

Sedimentation

and clarification

during maturation

Until the beginning of this century most lager was sold unfiltered. It was only through
prolonged cold storage that all the suspended solids would s
ettle out. This would leave a
comparatively bright product for packaging and distribution to the consumer.

Now, it is only cask conditioned beers, and speciality beers such as wheat and lambic
beers, which remain unfiltered. These beers are allowed to s
ettle or are fined with
Isinglass to produce a "bright" product.

Most other top fermenting and bottom fermenting beers are filtered bright for packaging.

Although filtration has reduced some of the requirements of long maturation, a relatively
bright beer

is required for successful filtration. Typically:

At the end of fermentation the yeast counts should be between
5
-
10 m/ml
.

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This has to be reduced to
0.5
-
1.0 m/ml

after maturation for good filtration

The reduction of yeast counts will occur during normal
cold storage, but it can be
accelerated by:



Selecting a flocculent yeast variety



Good temperature control and cropping in fermentation vessel



Centrifugation between fermentation and storage and/or Centrifugation between
storage and filtration



Finings in
storage

The simplest method for liquid/solid separation relies on natural sedimentation. Insoluble
solid material suspended in solution is allowed to settle under the influence of gravity
(
g)
.
A number of factors influence
the rate of settling
and these ar
e expressed in
Stokes
Law.

(Don’t worry if this is too complicated, it’s not necessary to understand it. The section

concerned

is coloured green)

Rate of settling
Vg

=

d
2

(

p



l)


g






18




Where:

Vg

= sedimentation velocity (m/sec)
(how fast it settles)

.d
2


= particle diameter (meters)
(how big the particle is)


p

=⁰a牴r捬攠den獩瑹
歧/m
3
)
(How ”heavy” the particle is)




=楱u楤iden獩瑹
歧⽭
3
)
(How “heavy” the liquid is)




v楳捯獩iyf楱u楤i⡫(⸠m⸠猩s
(How “thick” the
liquid is)

g

= gravitational attraction of the earth (9.81 m/s
2
)
(The pull on the particle by gravity)

Stokes law calculates settling rate for a particular solid in a solution. Effectively speed of
sedimentation depends on particle size/density, liquid
density/viscosity.

In practice it is the actual clarification of the beer that is of principal importance.
Clarification of the beer depends on the distance through which the particle has to settle.

It therefore follows that time taken to clarify the beer

will be determined by the distance
through which the particle has to fall. The higher the beer level above the bottom, the
longer it takes for all the particles to “fall” to the bottom.

For natural gravity sedimentation the shape of the tanks is important
. Horizontal tanks will
give more rapid clarification than vertical tanks.

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From the
natural

settling velocity (Vg) for yeast of 9.81 X 10
-
6

m/s
2

it is possible to
calculate the theoretical time for yeast to settle out from two different height storage tank
s,
each designed to hold 2500 hl of beer.

V
g
h
V
g
h
Horizontal Tank
Vertical Tank


Horizontal Tank

Vertical Tank


(Vg) yeast

9.81 X 10
-
6

m/s
2

9.81 X 10
-
6

m/s
2

Beer depth

10 m

30 m

Average time for particle
to settle

24 days

73 days


Theory would favour the choice of a horizontal tank in place of vertical maturation tanks for
more rapid settling of yeast and tank bottoms.

However, other factors such as



cost of manufacture



space requirements



beer losses



ease of CIP



compacting of settled solids

have favoured vertical tanks.

In practice, yeast tends to settle out more quickly than Stokes law would suggest, possibly
due to clumping (increasing of particle size
d
).

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However, the length of time required for the yeast to

settle naturally is often greater than
the maturation time available. It is then necessary to accelerate the process by mechanical
means.

8.1.4.1.

How can we decrease the settling time
?

8.1.4.1.1.


Increase the particle size
by coagulation of proteins
.

From Stokes law it can be seen that the settling rate is proportional to the square of the
particle size
(d
2
)
.

Hence a bigger particle will settle faster. Actually, for example, a particle twice as big will
settle four times faster.

We can do this by

coagulating proteins during wort boiling and wort clarification.

8.1.4.1.2.

Add Isinglass Finings

Finings are the processed swim bladders of certain fishes. Finings have a unique collagen
structure. The bladders are cleaned, dried and processed. This product is di
ssolved in an
organic acid and a thick gel is produced.

This gel is added at suitable dosing rates to beer after fermentation. The finings react with
yeast and other particulate matter and rapidly precipitates it. The mechanism of
precipitation is related

to the high electrostatic charges on the collagen molecules and the
opposite charges on yeast and other particulate matter. Thus larger particles are formed,
which settle faster.

Precipitation of yeast and other particles before filtration, dramatically r
educes the load on
the filter.

The use of finings is not permitted in
Reinheitsgebot

beers but its use in the rest of the
world is widespread and generally accepted.

The choice of whether to use finings is normally economic. The material is expensive and
a small percentage of beer is inevitably trapped in the finings sludge. This has to be set
against the cost of kieselguhr, filter down time and residence time in storage.

8.1.4.1.3.

Increase the force due to gravity

by centrifugation


The yeas
t sedimentation rate is around
4000
times faster during centrifugation than during
natural settling under the force of gravity.

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8.1.4.2.

How a centrifuge

works.


This is an example of a self
cleaning centrifuge made by
Alfa
Laval.

Cloudy beer is pumped in at the
top. The centrifuge is packed with
thin plates. The solids are thrown
to the outside edge. The lighter
liquid is discharged (bright) from
the bottom.

At a preset time the bowl opens at
point A. This is done very quickly.
The solids are ejected through the
gap and the bowl closes again
before liquid is lost.