A – Z of Flavour – Acetaldehyde
Acetaldehyde can be a friend or foe depending on the beer it ends up in and its concentration. In modern pale lager beers, which make up the biggest volume of beer consumed in the world today, concentrations of acetaldehyde are typically 1 – 4 mg/l. At this concentration, it adds a little to the background flavour of the beer, save for a hint of apple character. It mainly imparts ‘fresh’ and ‘refreshing’ sensations to beer.
At low concentration acetaldehyde is associated with green apple, bruised apple, emulsion paint, wine (white wine), and sherry flavour notes.
Older literature on the subject often cites flavour recognition threshold values for acetaldehyde in the range of 10 -20 mg/l. These values are not applicable today (ie they are wrong!). In modern pale lager beers with low sulphite levels, flavour thresholds for acetaldehyde are in the range 2 – 3 mg/l. Actual threshold values vary with beer sulphite concentration due to adduct formation.
Beer tasters sometimes confuse acetaldehyde with the ester ethyl hexanoate, and with other aldehydes such as isobutyraldehyde. Regular training and validation of assessors is needed to minimize this risk. Formation and fate of acetaldehyde Acetaldehyde is formed and removed at several stages of the beer production process.
- It is formed during wort boiling (via Strecker degradation)
- It is formed by brewer’s yeast from fermentable wort sugars during the early to mid part of the fermentation
- It is reduced by yeast to ethanol during the latter stages of the fermentation and during beer maturations
- It is reformed in packaged beer by oxidation of ethanol after removal of yeast
Formation of acetaldehyde during wort boiling
Low concentrations (a few mg/l) of acetaldehyde are formed during wort boiling through Strecker degradation. Some of the acetaldehyde may bind to wort proteins at this stage, to be released later. Acetaldehyde in this form is resistant to reduction by yeast. Drip back of condensate in the brewing kettle can lead to higher than normal levels of acetaldehyde in wort at the start of fermentation. Regular checks on the kettle stack and condensate system are needed to guard against this.
Formation of acetaldehyde by yeast during the early to mid stage of fermentation Acetaldehyde is produced by yeast during the fermentation as an intermediate in the formation of ethanol. The amount formed relates to the efficiency of the glycolytic pathway (ieconversion of acetaldeyde to ethanol) and the activities of competing metabolic pathways (eg conversion to acetate and then to acetyl CoA for lipid synthesis). Reduction of acetaldehyde by yeast during the latter stages of fermentation and during the maturation process.
During the latter stages of the fermentation production of ethanol (and hence acetaldehyde) slows. Yeast can continue to reduce acetaldehyde concentrations in green beer after the primary fermentation is complete. Reversible binding of acetaldehyde to sulphite ions can reduce the rate at which both are eliminated at this stage. Their are several enzymes responsible for reduction in acetaldehyde concentrations:
- Several different types are found in brewer’s yeast – some have broad specificity, others act on a narrower range of substrates
- They are coded for by the ADHI, ADHII and ADHIII genes
- ADHI is the gene which is most active in fermenting yeast – its activity falls off during the latter stages of fermentation
- This mitochondrial enzyme seems to be important for elimination of acetaldehyde at the end of fermentation (being involved in conversion of acetaldehyde to acetic acid)
- It uses either NADH or NADH as a cofactor
- The enzyme itself is formed early in the fermentation in response to exposure of the yeast to wort oxygen
Formation of acetaldehyde through chemical reactions after removal of yeast
- Following removal of yeast from green beer oxidation of the beer leads to acetaldehyde formation
- This is a non-enzymic process – it does not require the presence of yeast
- It does not involve direct reaction of the acetaldehyde with oxygen, but is related to the redox potential of the beer
- With modern standards of oxygen control in breweries, formation of acetaldehyde in this way should be minimal
- Where oxygen control is poor in-process or during packaging, acetaldehyde can form in beer during storage in pack
Control of acetaldehyde concentrations in beer
- Brewer’s yeast strains differ considerably in their capability to achieve low levels of acetaldehyde in beer
- Activities of alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase (ALDH) are very important for reduction of acetaldehyde in the later stages of fermentation and during maturation
- Mitochondrial mutations are generally detrimental to the efficiency of acetaldehyde reduction
- Mutations in flocculation genes can lead to a change in flocculation character of the yeast – greater, more aggressive, flocculence means fewer cells in suspension to eliminate acetaldehyde from the beer
- Yeast variants (mutants) present in a laboratory yeast culture can inadvertently be selected for or against depending on yeast propagation processes, yeast cropping practices and yeast handling practices.
- Concentration of wort amino acids (FAN)
- Yeast flocculation-inducing factors (including premature yeast flocculation factor – PYF)
- Available wort zinc concentrations
- Wort dissolved oxygen concentrations
- Timing of wort oxygen additions
- Wort suspended solids concentrations (eg trub)
- Yeast pitching rate
- Yeast ‘health’ and ‘cell age’ (determined by yeast cropping and handling practices)
- Fermenter depth
- CO2 top pressure during the fermentation
- Fermentation temperatures
- Layering of vessel contents in the fermenter
- Tight control of suspended yeast cell count
- Control of yeast metabolic activity
- Conditioning temperatures
- Dissolved oxygen concentrations in the green beer
- Layering of vessel contents
- Minimization of dissolved oxygen concentrations during the filtration process
- Introduction of materials that have a negative effect on beer redox potential (for example beer-soluble iron from filter aid)
- Minimization of dissolved oxygen concentrations in the bright beer
- Minimization of dissolved oxygen concentrations during the filling process
- Temperature control during the pasteurization
- Note also that some types of packaging materials (specifically some types of PET bottles) can leach acetaldehyde into the beer during storage
The importance of brewery hygiene
- Poor brewery hygiene can impact on beer acetaldehyde concentrations both directly and indirectly
- Growth of acetic acid bacteria (Acetobacter spp and Gluconobacter spp) can change the redox potential of the beer leading to acetaldehyde production from ethanol
- Growth of coliform bacteria (such as Obesumbacterium proteus and Rahnella aquatilus) in fermenting wort can lead to nitrite production, reducing the yeast’s ability to eliminate acetaldehyde at the end of fermentation and during maturation
- Lactic acid bacteria (Lactobacillus spp and Pediococcus spp) can bring about premature flocculation of yeast, reducing the numbers of yeast cells available for elimination of acetaldehyde at the end of fermentation or during the maturation process
- Growth of Zymomonas spp in beer leads to formation of high concentrations of acetaldehyde – these organisms require monosaccharides such as glucose for growth so they are not common contaminants of lager beers; ales remain at risk though
The effect of acetaldehyde on yeast cells
- Acetaldehyde is toxic to brewer’s yeast
- It binds to cellular proteins and can inactivate enzymes within the cells
- It can also be mutagenic to yeasts
- Interestingly, when yeast cells are exposed to acetaldehyde many genes related to formation of volatile sulphur compounds are also expressed
Congratulations if you got through all the way to the end of the article. If you have, at least you know where to find the information you need should you ever have an acetaldehyde problem in your brewey.