The Chemistry of Beer Flavor
Beer is comprised of hundreds of different chemical compounds that contribute to the overall flavor and aroma perception of the beer. Different styles of beer have different flavor and aroma characteristics that vary depending upon the many factors associated with the production of the beer. The specific types and amounts of taste and aroma compounds within a particular beer depend upon the types and amounts of raw materials used in making the beer, the strain of yeast used to produce the beer, and how the important processing variables are managed during wort production, fermentation and post-fermentation processing and packaging.
Although there are myriad potential taste and aroma nuances within beer, there are several broad flavor descriptor categories in which we have an understanding at the molecular level regarding the compounds that are responsible for producing the associated flavor perceptions.
Broad flavor compound categories include fruity/ester-like, floral/sweet, citrus/terpenic, grassy, spicy/herbaceous, caramel/nutty, woody/smoky, and roast/burnt. Details regarding the compounds associated with these taste and aroma families will be discussed in this story and also summarized in Table 1 on page 85.
Fruity/ester-like aromas
Fruity/ester flavors and aromas are common in many beer styles including English ales and wheat beers. Fruity/ester-like aromas are commonly characterized as being similar to the sweet aromas generally occurring in ripe fruits such as apples, bananas, pears, melons, etc. The typical chemical families for compounds that produce these aromas are esters and lactones, but ketones, ethers and acetal compounds are also contributors to these aromas.
Fruit-specific aroma compounds include isoamyl acetate, which is responsible for a sweet (and banana) aroma, 2,4-decadienoic acid ester which has the characteristic aroma of pears, and 3-methylthio propionic acid esters which have the characteristic aroma of pineapple. Some tropical fruits contain specific sulfur compounds (such as the methyl and ethyl esters of 3-methylthio propionic acid) that impart an “exotic” fruity aroma at appropriate concentration levels. These fruity/ester-like aroma compounds can also work in combination with other compounds to impart different flavor characteristics. Other compounds with non-specific fruity characters but generally described as sweet and fresh (e.g., lactones, aromatic aldehydes, and terpenoids) can also provide additional fruity notes.
Floral/sweet flavors
Floral flavors are characteristic of the compounds emitted by flowers. These sensations also typically include a sweet, green, fruity or herbaceous character, and are potentially derived from hops used to make beer. Compounds contributing these characteristics are not all associated with a particular family of compounds, but rather may include substances from different chemical classes. Important compounds that contribute to a floral/sweet flavor include phenyl ethanol, geraniol, beta-ionone and some esters such as benzyl acetate and linalyl acetate. At higher concentrations these compounds may produce an unwanted perfume-like
flavor perception.
Citrus/terpenic flavors
Citrus/terpenic and piney flavors are typically derived from hops used to make beer. Citrus-like perception is associated with the typical flavor and aroma occurring in citrus fruits and plants (e.g., lemon, orange, grapefruit), but there are also certain terpenoid components associated with these fruits. One terpenoid component with a very strong aroma impact is called citral. Citral is a mixture of geranial and neral. Another important terpenic component is nootkatone. Nootkatone is perceived as very bitter. It is found in grapefruit and is associated with their very bitter taste and aroma character.
Grassy, spicy and herbaceous flavors
Grassy flavor and aroma is associated with the smell of freshly-cut grass, ground leaves or vegetable matter.
Typical substances that impart grassy flavors include short chain, unsaturated aldehydes and alcohols such as trans-2-hexenal and cis-3-hexenol. Esters and heterocycles such as alkyl-substituted thiazoles and alkoxy pyrazines also belong to this group. Other esters, acids and terpenoid compounds such as hexyl 2-methylbutyrate and alpha-pinene also contribute additional green, piney notes to flavor and aroma.
Spicy/herbaceous flavors are characteristic of specific spices and herbs. Aromatic aldehydes, alcohols and phenolic derivatives are typical constituents with their strong flavor impact effects. Specific compounds include anethole (anise), cinnamaldehyde (cinnamon), estragole (fennel), eugenol (clove), d-carvone (dill), and thymol (thyme).
Caramel/nutty flavors
Caramel and nutty flavors are typically developed within barley that has been kilned as a part of its processing. These flavors are developed during heating when chemical and physical changes occur to sugars, starches, amino acids and proteins. These changes are the results of the formation of Maillard reaction products. The slightly bitter and burnt flavor of roasted nuts can also be produced. Compounds such as corylone, maltol and furanol are contributors to a caramel/nutty flavor profile.
Woody/smoky flavor
Woody and smoky flavors and aromas are caused by substituted phenol compounds (e.g., guaiacol), ionone derivatives and, if in relatively low concentration, by some aldehydes (e.g., trans-2-nonenal). These compounds contribute warm, woody, sweet and smoky flavors. Other compounds that contribute to woody taste and aroma perception include vanillin and lactones.
Roasty/burnt flavor
Roasty and burnt flavors within beer are usually derived by using highly-kilned malt (e.g., unmalted, roasted barley or black patent malt) to make the beer. Roasty or burnt flavors are generally the result of compounds in the pyrazine family. A broad range of roasty/burnt flavors result from having different combinations of alkyl, acyl or alkoxy functional groups on the base pyrazine ring structure. Burnt, roasted, green, earthy or musty aromas and flavors may be produced, depending upon the specific combinations of functional groups.
Practical conclusions
Brewing is both an art and a science. Knowledge of the chemical basis for flavor in beer, although interesting, is not enough to enable a brewer to make good beer. To make the best beer possible, a brewer should not only have a working knowledge of the chemical basis for flavor in beer, but also the cause and effect relationship between ingredients and the finished beer. A brewer must also understand how ingredients interact with each other and within the brewing process to produce a specific taste and aroma profile in the finished beer. As homebrewers, we should care about the chemical basis for flavor because it helps us have a deeper understanding about how our ingredients affect the finished beer.
Reference:
Kuentzel, H. & Bahri, D., “Food Flavorings”, 1991, p.115-157
FLAVOR & AROMA COMPOUNDS
Fruity/Ester-like
trans-2, cis-4-deca-dienoic acid ethyl ester: fruity, pear
isoamly acetate: fruity, sweet (banana)
6-undecalactone: fruity, creamy
hexyl acetate: fruity, “tuti-fruiti”
3-methylthio pro-pionic acidtester: fruity, pineapple
4-(p-hydroxyphenyl)-ethyl 2-butanon: fruity, floral(raspberry ketone)
acetaldehyde diethyl acetal: fruity, refreshing
Floral/Sweet
Phenylethanol: floral, sweet
geraniol: floral, flowery
p-Ionone: floral, fruity, berry
benzyl acetate: floral, fruity
linalyl acetate: floral, fruity, citrus
geranyl acetate: floral, sweet, fruity
Citrus/Terpenic
Geranial/Neral mixture (citral): lemon
Trans-alpha-Sinensal: orange
Nootkatone: grapefruit, bitter, fruity
Octanal (orange aldehyde): orange, sweet, fruity
Decanal: orange, bitter
Linalylacetate: citrus, fruity, floral
Grassy
cis-3-hexenol: green, grassy
trans-2-hexenal: green, fresh
cis-3-hexenal: green, leafy
trans-2-hexenol: green, fruity
2-sec.butyl-3-methoxy-pyrazine: green, earthy
2-pentyl-4,5-dimethyl-thiazole: green, floral
Spicy & Herbaceous
trans-anethole: herbaceous, sweet, anise
trans-cinnamaldehyde: spicy, warm, sweet, cinnamon
estragole: herbaceous, warm, fennel
eugeno: spicy, warm, buring, clove
thymol: herbaceous, sweet-medicinal, warm, spicy, thyme
Caramel/Nutty
2-hydroxy-3-methyl-2-cyclopenten-1-one: nutty/maple-like
