The flavour of beer comes not just from the ingredients, but also the processes involved in brewing.   Yeast's main task for the brewer is converting glucose into alcohol and carbon dioxide, but tiny amounts of other compounds are formed also.   These compounds have very low flavour thresholds on the human palate, so a few parts per million (or even trillion) can impart strong flavours in the finished beer.   Each yeast variety produces a different amount of these compounds and different compounds under different conditions.   Thus producing many varieties of beer.

Esters 

An ester is a molecule with the functional group R-CO-O-R, the product of an organic acid (R-COOH) and an alcohol (R-OH) reacting where R is the main body of the organic compound.   Esters are the key ingredients in purfumes, pheromones and fruit fragrances.   Many, many esters are present in beers but the most important three are;

Ethyl Acetate (Ethyl ethanoate) - CH₃COOHC₂H₅ - gives a generally fruity flavour with solvent-like qualities.

Isoamyl Acetate (3-methyl-1-butyl acetate) - CH₃COO(CH₂)₂CH(CH₃)₂ - gives a flavour remeniscent of bananas or pear drops.

Ethyl Caproate (Ethyl Hexonoate) - C₅H₁₁COOHC₂H₅ - has an apple-like flavour.

Others produce flavours and smells like stawberries, pineapples and apricots.   The actual smell & taste of bananas or strawberries will be a mixture of many esters and other compounds forming a tremendously complex flavour profiule.   Similarly the smell & taste of a beer will contain many compounds, and some will overlap with the ones found in these fruit, giving a suggestion of their flavour.

Higher Alcohols

The alcohol we think of in alcoholic drinks is ethanol, CH₃CH₂OH.   Alcohols are named after the functional group -OH they carry.   A different carbon backbone will produce related compounds which may be referred to as higher alcohols due to their higher molecular weight.   These may also be called Fusel Alcohols.

Propanol - CH₃CH₃CH₂OH - butanol - CH₃CH₃CH₃CH₂OH - and Isobutanol (2-Methylpropan-1-ol) - (CH3)2CHCH2OH - give an alcoholic flavour even in low concentrations.

Isoamyl alcohol (3-methyl-1-butanol) - CH₃CH₃CH₂(CH₃)CH₂OH and Active Amyl Alcohol (2-methyl-1-butanol) - CH₃CH₂(CH₃)CH₃CH₂OH - produce a banana, solvent-like flavour.

Phenoethyl Alcohol (2-phenylethanol)  - (C₆H₅)CH₂CH₂OH - and Tyrosol (4-(2-Hydroxyethyl)phenol) - CH₃CH₂(C₆H₅)OH - have a smell & flavour described as medicinal-like or of roses.   The C6H5- group is a benzene ring, giving the characterisitc properties to its smells / tastes.

Ketones

Ketones have the functional group R-CO-R, where the carbon has a double bond to the oxygen and each R is a carbon chain.   They are found in perfumes and solvents.   The important ketones in beer are;

Diacetyl (2,3-Butanedione) - CH₃COCOCH₃ - this is an important flavour compound in beer, usually seen as an off-flavour with a butterscotch taste.   This, alongside acetoin gives butter its characteristic taste.   Has a flavour threshold at around 0.1ppm.

alpha-acetolactic acid - CH₃COCOH(CH₃)COOH - this is a precursor to diacetyl in yeast metabolism.   It escapes the yeast cell and oxidises to diacetyl in the beer.   Given time this diacetyl is reabsorbed by the yeast and reduced to 2,3-butanediol (see below).

acetoin (3-hydroxybutanone) - CH₃COCH(CH₃)OH - found in butter, apples, asparagus, blackcurrants and blackberries among other foods.   Has a buttery smell and taste.

2,3-butanediol - CH₃COCOHCH₃ - has no strong odour or flavour.

2,3-pentanedione - CH₃COCOCH₂CH₃ - is another buttery, cheesey tasting compound but is less influential than diacetyl.

Acids

Yeast produces organic acids such as lactic and pyruvic as it metabolises sugars but the ones which greatly affect flavour are the fatty acids.   The carboxylic acid functional group is a characteristic of these R-COOH.

Caproic acid (hexanoic acid) - CH₃(CH₂)₄COOH - has a characterisitc barnyard odour.

Caprylic acid (octanoic acid) - CH₃(CH₂)₆COOH - has a goaty, fatty, coconut odour.

Capric acid (decanoic acid) - CH₃(CH₂)₈COOH - soapy

The names above derive from the Latin "caper" meaning "goat".

Beer Styles

Many of these flavours will be considered off-flavours but certain beers will have a little of these characteristics as part of there style definition;

Bavarian Weissbeer has a banana, clove-like flavour sometimes with a small amount of phenolic taste.   This comes from the esters ethyl ecetate & isoamyl acetate and the higher alcohols isoamyl alcohol & phenoethyl alcohol.

Belgian lambics may have a barnyard flavour from caproic acid with other acids such as lactic being generated by associated bacteria.