Critical Thinking

Sanitation releasing CO2. Larger yeasts will produce more

Sanitation is key to prevent to off flavour caused when butyric
acid is present. It is caused by a bacterial infection of sugar syrups or after
packaging. The flavour increases as the pH is lowered. The flavour can develop
further on aging if the packaged beer has bacterial contamination. This can be
prevented by smelling or tasting the syrup before using them to ensure they’re
not contaminated. It gives a cheesy, rancid flavour to the beer which also
resembles baby vomit. 9

Hydrogen Sulphide (H2S) creates a flavour in beer which
resembles rotten eggs. This is formed by the amino acids methionine and
cysteine in the malt and by certain bacteria such as Zymononas, Pectinatus, and
Megasphaera.  Yeast autolysis is also a
cause. This is when a yeast cell dies, it ruptures and therefore releases
several off flavours into the beer. 10 This particular off flavour
also occurs when the yeast is stressed and so produces more hydrogen sulphide
during fermentation than usual. Therefore it can be avoided by giving the yeast
sufficient time to ferment. The fermentation of Ale is a very vigorous process
therefore this increases hydrogen sulphide production. Hydrogen Sulphide is
scrubbed out by releasing CO2. Larger yeasts will produce more hydrogen
sulphide and so for these strains the fermentation process is less
vigorous/more mellow.

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The lightstruck reaction is described as “a non-enzymatic
light-induced reaction involving a photosensitizer compound (riboflavin), a
sulphur containing compound and isohumulones” 14 Lightstruck occurs
when the alpha acids in hop and riboflavin in the beer react with daylight or
artificial light. This creates a skunky, sulfury flavour in the brew. The
compound formed which is responsible for this is 3-methyl-2-butene-1-thiol.
This only occurs in finished beer. To counteract this effect some breweries use
chemically modified alpha acids in the brewing process. The benefit of this is
that these don’t react with riboflavin therefore the breweries can use green or
clear bottles. If these chemically modified alpha acids weren’t used extreme
care would have to be taken to always store the bottles in darkness as it would
have to be kept away from light at all times to prevent this reaction for
occurring. Storing the beer in kegs or brown bottles or using fewer hops would
decrease the chances of this reaction occurring.9

Ethanethiol is a compound which when present could create a
flavour of rotten vegetables or rotting garbage in the beer. Mercapatan
(another name for ethanethiol) is caused by using an incorrect strain of yeast.
The most likely cause of detectable levels of this particular flavour is
infection of anaerobic bacteria. Therefore it necessary to always practise good
sanitation to avoid this flavour developing in the beer. 9

Diacetyl (2,3-butanedione) adds a buttery, oily, milky flavour to
the beer and so is considered an off flavour. This is a flavour which is not
desirable in excessive quantities especially in lagers. This is because this
particular type of beer is light and so the flavour is easier to detect than in
a beer with a more robust, strong , rich flavour. Diacetyl is a by-product of
yeast during fermentation but it is usually reabsorbed by the yeast cells.
Therefore it really impacts the flavour of the beer when it is not reabsorbed which
can be because of short boiling, temperatures that a low during fermentation,
mutated yeast. When the yeast is mutated it means it has lost the ability to
reduce the diacetyl to less harmful compounds. 
It can also be formed by bacteria contamination. This is due to gram
positive bacteria Pediococcus
Damnosus and other lactic acid bacteria in cooled, young or aging beer. 9
Pediococcus damnsosus is an infection which is very common in brewing. It
generates large amounts of diacetyl. 8 To avoid
this the solution should be boiled or the appropriate amount of time. The
temperature should be raised slightly near the end of the fermentation part of
the process as this helps the yeast reabsorb diacetyl.9

 

There are certain flavours which a perceived as “faulty” when they
are tasted in beer. These flavours are commonly known as off flavours. Once the
cause is determined, It is necessary to eliminate or control these off
flavours. Some flavours are always going to be considered faulty as they are as
a result of contamination. This contamination can be caused by poor sanitation
or poor brewing practices. Poor brewing practices include not controlling the
fermentation temperatures correctly, poor bottling technique or by using the
incorrect yeast. 9

 

The most common type of ketone used in beer is
diacetyl. This gives the beer a sweet butter or caramel taste. Phenolics
produce spicy flavours. Fatty acids can make beer oxidise and grow stale
quicker and are therefore avoided by brewers.

 

The different esters found in beers also have
different smells at different concentrations. One example of an ester is Ethyl
acetate. At high concentrations this smells like nail varnish, however at lower
concentrations in beer it smells fruity. Isoamyl acetate gives a banana like
aroma, ethyl butanoate gives a scent which resembles pineapples and ethyl
hexanoate gives the beer an aroma which is like that of apples.

 

Esters also play an important part in the
flavour of a beer. Esters create fruity flavours. These flavours are often
found in the beers from England and Belgium. They are present in high
concentration in ales whereas in lagers they are present in small concentrations.
Esters form when the organic acids in hops (alpha and beta acids) react with
the alcohol in the beer and acetyl coenzyme which is also found in hops.
Certain conditions in brewing really impact the concentration of esters present
in the beer at the end of the process. These conditions include pH and
temperature. Agitation of the mixture can also have an impact on the final
concentration of esters present.

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