2,3-butanedione
“Diacetyl , like butter, or butter popcorn”
Technical specification
Assessment
Without covering the glass, swirl the beer to release the aroma. Take a single short sniff. Repeat as necessary. Amount of flavour per capsule
130 µg Threshold
10 – 40 µg / l in beer Origins
Produced in beer from a precursor formed by yeast during fermentation. Can also be formed by contaminant lactic acid bacteria. Confusions
Butyric acid
Vanillin
Isobutyraldehyde
Importance
Desirable flavour in some ales, stouts and lagers, eg Pilsner. Off-flavour in other lager beers. Considerable efforts are made by breweries to tightly control this flavour character. CAS registry number
431-03-8 Other names
diacetyl
biacetyl
2,3-butane dione
butane-2,3-dione
2,3-diketobutane
dimethyl diketone
dimethyl glyoxal
2,3-dioxobutane
Remarks
2,3-Butanedione is one of two vicinal diketones found in beer. The ratio of diacetyl to pentanedione concentrations can be used as an indicator of bacterial contamination in beer. Threshold distribution
dimethyl sulphide
“DMS , like sweetcorn or tomato sauce”
Technical specification
Assessment
Without covering the glass, swirl the beer to release the aroma. Take a single short sniff. Repeat as necessary. Amount of flavour per capsule
150 µg Threshold
30 - 50 µg / l in beer Origins
Formed from malt-derived precursors, primarily during wort production and – to a lesser extent – during fermentation. Confusions
Methyl thioacetate
Ethanethiol
Dimethyl trisulphide
Importance
Desirable flavour in some pale lager beers and ales. Off-flavour in other beers. Excessive levels are indicative of growth of contaminant bacteria during fermentation. CAS registry number
75-18-3 Other names
DMS
dimethyl monosulfide
methylsulfanylmethane
methyl monosulfide
methane thiomethane
methyl sulfide
methyl thioether
methylsulfanylmethane
2-thiapropane
thiobis(methane)
thiobismethane
Remarks
The perception of dimethyl sulphide is influenced by the presence of aromatic higher alcohols such as 2-phenylethanol in beer. Threshold distribution
ferrous sulphate
“Metallic , like ink or blood”
Technical specification
Assessment
Take a little beer into your mouth. Move it around with your tongue for a few seconds then swallow it. To confirm, rub a little of the beer onto the back of your hand and check for metallic odour. Amount of flavour per capsule
8.2 mg Threshold
2.7 mg / l in beer Origins
Derived from contamination of beer with metal ions, either from brewing raw materials or from corrosion of brewery plant. Metallic odours can also be produced by lipid oxidation. Confusions
Sulphitic
Citric acid
trans -2-Nonenal Importance
Taint and occasionally off-flavour in beer. Primarily affects beer mouthfeel but occasionally beer odour can also be affected. CAS registry number
7782-63-0 Other names
ferrous sulfate
iron(2+) sulfate
iron(2+) sulphate
ferrous sulphate heptahydrate
ferrous sulfate heptahydrate
iron(2+) sulfate heptahydrate
iron(II) sulfate heptahydrate
iron(II) sulphate heptahydrate
Remarks
Copper, manganese and tin can also generate metallic notes in beer. Such metals stabilize beer foam and shorten beer flavour life. Threshold distribution
hop oil extract
“Hop oil , like hoppy ale”
Technical specification
Assessment
Without covering the glass, swirl the beer to release the aroma. Take a single short sniff. Repeat as necessary. Amount of flavour per capsule
1.25 mg Threshold
250 µg / l in beer Origins
Contributed to beer by addition of hop oils in the brewhouse, fermentation or post-fermentation. The intensity and nature of the character depends on the hop products and brewing practices. Confusions
Kettle hop extract
Geraniol
4-Vinyl guaiacol
Importance
Positive flavour character imparted to specialty ales by addition of hop oil. Different hop varieties in combination with different beers give rise to a variety of hop flavour characteristics. CAS registry number
8007-04-3 Other names
hop oil
target hop extract
hop essential oil
Remarks
Not a reference standard in the strictest sense, since the flavour is derived from a complex natural mixture of many compounds. Threshold distribution
hydrogen sulphide
“H2 S , like boiled or rotten eggs”
Technical specification
Assessment
Without covering the glass, swirl the beer to release the aroma. Take a single short sniff. Repeat as necessary. Amount of flavour per capsule
18 µg Threshold
4 µg / l in beer Origins
Produced by both ale and lager yeast during fermentation and maturation. Amount depends on wort composition, yeast strain and fermentation conditions. Confusions
Sulphur dioxide
Ethanethiol
Methanethiol
Importance
Present in all beers. Concentrations vary considerably from beer to beer. Off-flavour in most beer styles. Signature flavour character in Burton ale. CAS registry number
7783-06-4 Other names
H2 S
hydrogen sulfide
dihydrogen monosulfide
dihydrogen monosulphide
hydrogen sulfuric acid
hydrogen sulphuric acid
sulfur hydride
Remarks
Can contribute to a perception of ‘freshness’ in beer when present at low levels. Threshold distribution
isoamyl acetate
“Isoamyl acetate , like bananas or pear drops”
Technical specification
Assessment
Cover the beer with your hand and swirl the glass to release the aroma. Remove your hand and take a single long sniff. Repeat as necessary. Amount of flavour per capsule
3.5 mg Threshold
1.1 mg / l in beer Origins
Produced by both ale and lager yeast during fermentation. Amount depends on wort composition, yeast strain and fermentation conditions. Confusions
Ethyl hexanoate
Ethyl acetate
Ethyl butyrate
Importance
Present in all beers. Concentrations vary considerably from beer to beer. Key flavour impact character in some lagers and ales. Signature flavour character in German-style wheat beer. CAS registry number
123-92-2 Other names
3-methylbutyl acetate
acetic acid 3-methyl butyl ester
acetic acid isopentyl ester
isoamyl ethanoate
3-methyl butyl acetate
beta-methyl butyl acetate
2-methyl butyl ethanoate
3-methyl butyl ethanoate
3-methyl-1-butanol acetate
3-methyl-1-butyl acetate
3-methylbutyl acetate
isopentyl acetate
isopentyl alcohol acetate
isopentyl ethanoate
Remarks
Disappears during ageing of beer in pack, contributing to a loss of freshness. Threshold distribution
3-methyl-2-butene-1-thiol
“Lightstruck , like a skunk or freshly-brewed coffee”
3-methyl-2-butene-1-thiol
100 ng
Technical specification
Assessment
Best detected by smelling the beer at a distance. Hold the glass at arm’s length and take short sniffs to prevent the risk of adaptation. Amount of flavour per capsule
100 ng Threshold
4 – 30 ng / l in beer Origins
Formed as a result of exposure of beer to daylight or artificial light. This initiates a reaction involving hop bitter acids, vitamins and sulphur compounds. Small amounts are also formed in beer production. Confusions
Sulphur dioxide
cis -3-HexenolEthanethiol
Importance
Off-flavour associated with exposure to light of beer packaged in clear or green glass. Consumers are very tolerant of this off-flavour. Many successful beers contain this flavour at the point of consumption. CAS registry number
5287-45-6 Other names
lightstruck
sunstruck
prenyl mercaptan
3-methyl but-2-ene-1-thiol
methyl butene thiol
3-methyl-2-butene-1-thiol
3-methyl-2-butenyl mercaptan
3-methylbut-2-ene-1-thiol
prenyl thiol
Remarks
Beers which have been bittered with chemically-modified hop extracts do not develop this character. Threshold distribution
trans -2-nonenal“Papery , like cardboard or oxidized beer”
trans -2-nonenal
Technical specification
Assessment
Without covering the glass, swirl the beer to release the aroma. Take a single short sniff. Repeat as necessary. Amount of flavour per capsule
730 ng Threshold
50 – 250 ng / l in beer Origins
Formed in malt and wort production where it binds to malt proteins. Carried through the brewing process in this bound form. Released during storage of finished beer in pack. Confusions
2,4,6-Trichloroanisole
2-Ethyl fenchol
Isobutyraldehyde
Importance
Off-flavour in beer associated with ageing. Formation of this flavour is more pronounced when precautions have not been taken in relation to minimizing process oxidation. CAS registry number
18829-56-6 Other names
(E)-heptylidene acetaldehyde
(E)-3-hexyl acrolein
(E)-3-hexyl-2-propenal
(E)- non-2-enal
trans-non-2-enal
(E)-2-nonen-1-al
trans-2-nonen-1-al
(E)-2- nonenal
trans-2- nonenal
Remarks
Sulphur dioxide in beer reversibly reacts with trans-2-nonenal to reduce its flavour impact. Threshold distribution
2,4,6-trichloroanisole
“Musty , like corked wine or a damp cellar”
2,4,6-trichloroanisole
580 ng
Technical specification
Assessment
Without covering the glass, swirl the beer to release the aroma. Take a single short sniff. Repeat as necessary. Amount of flavour per capsule
580 ng Threshold
10 – 500 ng / l in beer Origins
Associated with use of recycled wood and cardboard. Can migrate across packaging materials to contaminate raw materials, filter aids, and beer. Confusions
2,4,6-Tribromoanisole
Acetaldehyde
2,6-Dichlorophenol
Importance
Taint in beer. Associated with a high degree of consumer rejection, even at low levels. Often described by consumers as ‘chemical’ or ‘contaminated’. CAS registry number
87-40-1 Other names
methyl 2,4,6-trichlorophenyl ether
1,3,5-trichloro-2-methoxybenzene
TCA
Remarks
2,4,6-Trichloroanisole is one of several chloroanisoles that can taint beer. The flavour produced by this compound is typical. The same compounds are also responsible for ‘cork taint’ in wine. Threshold distribution
4-vinyl guaiacol
“Phenolic , like cloves or wheat beer”
Technical specification
Assessment
Cover the beer with your hand and swirl the glass to release the aroma. Remove your hand and take a single long sniff. Repeat as necessary. Amount of flavour per capsule
900 µg Threshold
300 µg / l in beer Origins
Low levels of this flavour are produced during wort production. High levels result from the use of speciality yeast strains or contamination of beer with wild yeasts. Confusions
Isobutyraldehyde
Sulphur dioxide
Vanillin
Importance
Key flavour impact character in some ales and stouts. Off-flavour in lager beers when it is associated with a moderate degree of consumer rejection. Signature flavour character in German-style wheat beer. CAS registry number
7786-61-0 Other names
4-vinyl-2-methoxyphenol
4-ethenyl-2-methoxyphenol
2-hydroxy-3-methoxystyrene
4-hydroxy-3-methoxystyrene
4-hydroxy-3-methoxyvinyl benzene
2-methoxy-4-vinyl phenol
varamol
para-vinyl catechol-ortho-methyl ether
p-vinyl guaiacol
vinylguaiacol
para-vinyl guaiacol
Remarks
4-Vinyl guaiacol (4-VG) is produced by yeasts which possess the phenyl acrylate decarboxylase (PAD) gene. The enzyme associated with this gene produces 4-VG from barley-derived ferulic acid. Threshold distribution
