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Sanitation is key to prevent to off flavour caused when butyricacid is present.

It is caused by a bacterial infection of sugar syrups or afterpackaging. The flavour increases as the pH is lowered. The flavour can developfurther on aging if the packaged beer has bacterial contamination. This can beprevented by smelling or tasting the syrup before using them to ensure they’renot contaminated.

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It gives a cheesy, rancid flavour to the beer which alsoresembles baby vomit. 9Hydrogen Sulphide (H2S) creates a flavour in beer whichresembles rotten eggs. This is formed by the amino acids methionine andcysteine in the malt and by certain bacteria such as Zymononas, Pectinatus, andMegasphaera.  Yeast autolysis is also acause. This is when a yeast cell dies, it ruptures and therefore releasesseveral off flavours into the beer. 10 This particular off flavouralso occurs when the yeast is stressed and so produces more hydrogen sulphideduring fermentation than usual. Therefore it can be avoided by giving the yeastsufficient time to ferment. The fermentation of Ale is a very vigorous processtherefore this increases hydrogen sulphide production.

Hydrogen Sulphide isscrubbed out by releasing CO2. Larger yeasts will produce more hydrogensulphide and so for these strains the fermentation process is lessvigorous/more mellow.  The lightstruck reaction is described as “a non-enzymaticlight-induced reaction involving a photosensitizer compound (riboflavin), asulphur containing compound and isohumulones” 14 Lightstruck occurswhen the alpha acids in hop and riboflavin in the beer react with daylight orartificial light. This creates a skunky, sulfury flavour in the brew. Thecompound 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 usechemically modified alpha acids in the brewing process. The benefit of this isthat these don’t react with riboflavin therefore the breweries can use green orclear bottles.

If these chemically modified alpha acids weren’t used extremecare would have to be taken to always store the bottles in darkness as it wouldhave to be kept away from light at all times to prevent this reaction foroccurring. Storing the beer in kegs or brown bottles or using fewer hops woulddecrease the chances of this reaction occurring.9Ethanethiol is a compound which when present could create aflavour 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 isinfection of anaerobic bacteria. Therefore it necessary to always practise goodsanitation to avoid this flavour developing in the beer.

9Diacetyl (2,3-butanedione) adds a buttery, oily, milky flavour tothe beer and so is considered an off flavour. This is a flavour which is notdesirable in excessive quantities especially in lagers. This is because thisparticular type of beer is light and so the flavour is easier to detect than ina beer with a more robust, strong , rich flavour. Diacetyl is a by-product ofyeast 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 whichcan 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 toreduce the diacetyl to less harmful compounds.

 It can also be formed by bacteria contamination. This is due to grampositive bacteria PediococcusDamnosus and other lactic acid bacteria in cooled, young or aging beer. 9Pediococcus damnsosus is an infection which is very common in brewing. Itgenerates large amounts of diacetyl. 8 To avoidthis the solution should be boiled or the appropriate amount of time.

Thetemperature should be raised slightly near the end of the fermentation part ofthe process as this helps the yeast reabsorb diacetyl.9 There are certain flavours which a perceived as “faulty” when theyare tasted in beer. These flavours are commonly known as off flavours.

Once thecause is determined, It is necessary to eliminate or control these offflavours. Some flavours are always going to be considered faulty as they are asa result of contamination. This contamination can be caused by poor sanitationor poor brewing practices. Poor brewing practices include not controlling thefermentation temperatures correctly, poor bottling technique or by using theincorrect yeast. 9 The most common type of ketone used in beer isdiacetyl.

This gives the beer a sweet butter or caramel taste. Phenolicsproduce spicy flavours. Fatty acids can make beer oxidise and grow stalequicker and are therefore avoided by brewers.  The different esters found in beers also havedifferent smells at different concentrations. One example of an ester is Ethylacetate.

At high concentrations this smells like nail varnish, however at lowerconcentrations in beer it smells fruity. Isoamyl acetate gives a banana likearoma, ethyl butanoate gives a scent which resembles pineapples and ethylhexanoate gives the beer an aroma which is like that of apples. Esters also play an important part in theflavour of a beer. Esters create fruity flavours. These flavours are oftenfound in the beers from England and Belgium. They are present in highconcentration in ales whereas in lagers they are present in small concentrations.

Esters form when the organic acids in hops (alpha and beta acids) react withthe alcohol in the beer and acetyl coenzyme which is also found in hops.Certain conditions in brewing really impact the concentration of esters presentin the beer at the end of the process. These conditions include pH andtemperature. Agitation of the mixture can also have an impact on the finalconcentration of esters present.

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