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ACETALDEHYDE 6he first compound we are going to looM at is acetaldehyde. Acetaldehyde is most commonly described as green apples however other descriptors include bruised apples and when in really high concentrations solvent liMe 6he descriptor of beer tasting like apples is not to be taken lightly as there is a diHHerence when describing a beer with a green apple and red apple flavour 6he Hormer is acetaldehyde and the latter is in fact a descriptor Hor the ester ethyl heZanoate something completely different. #cetaldehyde can find its way into your beer in several ways. Very minimal amounts of acetaldehyde are formed during wort boiling through Strecker degradation – a chemical reaction that converts amino acids to aldehydes. This acetaldehyde may bind to wort proteins and subsequently be released later this is not a good thing as acetaldehyde in this form is resistant to reduction by yeast). Brewers avoid acetaldehyde build-up by ensuring a vigorous boil in which the acetaldehyde evaporates off and that none of the condensate drips back into the kettle. This is another good reason to keep that lid off if you’re homebrewing. Not only will it let your dimethyl sulphide &/5 which causes a sweetcorn liMe oHH flavour evaporate away but some of this acetaldehyde will also disappear. The primary cause of acetaldehyde in beer is yeast. During fermentation acetaldehyde arises as an intermediate product in the formation of ethanol alcohol #cetaldehyde levels are increased in green beer by a range of factors: rapid Hermentation certain yeast strains high Hermentation temperatures Hermentation under high pressure reduced wort aeration and or inHection lactobacillus and pediococcus can bring about premature flocculation oH yeast reducing the number of yeast cells available for elimination of acetaldehyde at the end of fermentation or during the maturation process). Once the Hermentation stage is complete the yeast continues to reduce acetaldehyde concentrations. Beer that has had the yeast removed too quickly will therefore be high in acetaldehyde and most likely diacetyl as well. The third way in which acetaldehyde can find its way into your beer is oZidation although acetaldehyde forming in this instance is not really associated with immature green beer but aged beer. Beer oxidation leads to acetaldehyde formation it also leads to other oHH flavours such as sherry cardboard and honey as a result oH oxidation of ethanol. This is a non- enzymatic process and therefore doesn’t require yeast in order to maniHest 9ith modern standards of oxygen control in breweries Hormation oH acetaldehyde in this way is oHten minimal although can be more common Hor homebrewers 9here a brewery has poor oxygen controls in- process or during pacMaging acetaldehyde can form in beer during storage in the bottle Meg or can DIACETYL The second compound we’re looking at is diacetyl 9ith flavour descriptors liMe butterscotch movie popcorn and toHHee liMe itos an aroma that many donot find unpleasant but it is rarely appropriate in notable levels in beer. The most common way diacetyl finds its way into your beer is once again Hrom the yeast during fermentation and it is produced during every fermentation in varying amounts. The highest concentrations are found in the initial stages oH Hermentation however it is subsequently reabsorbed by the yeast in the final phases *ow much oH this initial diacetyl is left is determined by a range of factors including the particular yeast strain p* amino acids yeast health stress yeast pitching rate Hermentation time and temperature. If yeast is to clean it up then brewers must give it time. A fermentation that is completed in 72 hours may be finished in terms oH consuming all the wort sugars available however it is yet to clean up all the intermediary products produced during fermentation – primary of which is oHten diacetyl as well as acetaldehyde and removing the yeast too early would leave these flavours behind +n lager brewing where things are colder and slower brewers will often do a ‘diacetyl rest’ which is the process of letting the temperature of the beer rise so that the yeast can warm up and get to work rapidly on reducing the diacetyl levels in the beer before being cooled once again. Diacetyl could also be a symptom of infection by bacteria. A particularly notorious little gremlin is Pediococcus damnosus a lactic acid bacteria that is an obligate anaerobe with a nasty habit of producing large amounts of diacetyl. Key here is that it is an nobligate anaerobeo meaning that it thrives in an environment void of any oxygen. It does particularly Diacetyl is actually manufactured to add to products like margarine and popcorn so if you would like to try and replicate diacetyl at home hunt down some butter flavouring and add it to a beer. Diacetyl is acceptable in low levels in certain styles such as some 'nglish ales Yeast doesn’t actually produce diacetyl it produces α -acetolactate which spontaneously decarboxylates to diacetyl. FACT If you want to try recognising this flavour at home you can buy a green apple extract to dose a beer or taste commercial examples where minimal amounts of acetaldehyde on the aroma are acceptable such as #merican light lagers. #side Hrom finding a rotten egg you can also have a whiff of potassium or sodium metabisulphite although be careful if you have an allergy to sulphur). Beers where sulphur is appropriate are generally restricted to those beers produced in $urton with Marston’s Pedigree being the best example. TRY AT HOME ACETALDEHYDE DIACETYL HYDROGEN SULPHIDE TECH TALK 50 | Summer 2018 | ontapmag.co.za