Skip to 0 minutes and 6 secondsBeer is one of the most popular and largely consumed alcoholic drinks in the world. The total world's beer production amounts to around 200 million kilolitres. It is a complex alcoholic beverage, containing many flavour-active compounds, present in a wide range of concentrations, including compounds called humulones. Beer flavour is a delicate balance of all these compounds, and for the brewers, the challenge is to produce their products with consistent flavour, and to maintain the flavour balance for as long as possible in the market place. Malting and brewing technology have remained very traditional, but the efficiency of the process has improved through a better understanding of the technology and the underpinning chemistry.

Skip to 0 minutes and 53 secondsInnovation in the brewing industry is driven by cost reduction, for example, by more efficient use of the raw materials and lower energy consumption, and the need for improved quality and safety of the final product. There are hundreds of compounds present in the hops used to produce beer, of these, the most interesting are the humulones, specifically the non-bitter tasting (-)-humulone. Humulones have a carbon atom bonded to four different groups. The two ways of arranging the four substituents around the chiral centre, shown as (-)-humulone and ( )-humulone, are mirror images of each other (and so they are enantiomers); however, only (-)-humulone is present in hops.

Skip to 1 minute and 36 secondsDuring the brewing process, (-)-humulone undergoes a rearrangement reaction to form cis- and trans-isohumulones; the compounds responsible for the majority of the bitterness in beer. In cis-isohumulone, the -OH and the alkene (Me2C=CHCH2) groups are arranged on the same side of the planar five-membered carbon ring, whereas, in trans-isohumulone, the -OH and the alkene Me2C=CHCH2) groups are arranged on opposite sides. The rearrangement reaction of (-)-humulone is known as an isomerisation reaction. This is because cis- and trans-isohumulones are both structural isomers of (-)-humulone; they all have the same structural formula (C21H30O5) but the atoms are arranged in different ways. Isohumulone is the compound responsible for the main bitterness in beer and is formed from boiling hops during the manufacturing process.

Skip to 2 minutes and 27 secondsBitterness units (BU) are used to describe the extent of the bitterness of a beer. A single bitterness unit is equal to one milligram of isohumulone per litre of beer. The bitterness units in different beers ranges from around 10 bitterness units in a standard lager, to around 45 bitterness units in certain dark, malty beers, where higher levels of bitterness are needed to counter the sweetness of lots of malt. As well as the bitterness unit, the flavour components of beer can be measured in units called flavour units. A flavour unit of a flavour-active compound is measured in multiples of the flavour threshold (the lowest concentration of detection by aroma or taste) and is recorded as a concentration.

Skip to 3 minutes and 11 secondsBeer has primary, secondary and tertiary flavour compounds. Primary flavour compounds are found in beers at concentrations greater than two flavour units, these include isohumulone (ranging from 2.0 flavour units in standard larger to 12 flavour units in dark, malty beers), ethanol (found at about 2.9 flavour units in a standard beer), CO2 (at concentrations between 3.0 to 5.5 flavour units) and various other related hop compounds (2.0 to 12 flavour units). Secondary flavour compounds have concentrations ranging from 0.5 to 2.0 flavour units; it is the secondary compounds that create the distinct flavour of a beer. These compounds include fruity-flavoured esters (such as, 3-methylbutyl ethanoate, or isoamyl acetate, and ethyl hexanoate), alcohols with carbon-chain lengths greater than two, and polyphenols.

Skip to 3 minutes and 48 secondsTertiary flavour compounds are found in the concentration range of 0.1 to 0.5 flavour units; an example being ethyl octanoate, which imparts an apple flavour to the beer. The removal of a single tertiary flavour compound would not directly affect the resultant flavour of the beer, but a combination of all the tertiary flavour compounds, working together, does influence the final taste. Overall, with a beer containing hundreds of organic compounds and around 2000 ales, lagers and keg beers being produced in the UK alone, it is important for breweries to ensure they are aware of flavour and bitterness compounds to develop unique and desirable brews.

The organic flavour components of beer

Different beer types

Originating in the Middle Ages or later, the majority of beers that are drunk today are considered to be recent inventions. There are several-dozen beer styles currently in acceptance (though many new brews still fight for a chance to be accepted), and we are going to look at the different families that beers are divided into.

Flavour, strength and the appearance of the head and colour of the beer, are all relevant in determining the characteristics of a beer style; and are influenced by the quantities and types of the ingredients used in the brew, as well as the brewing processing conditions.

General classifications of beer style families are split by the town of manufacture and the colour of the resultant brew, however chemistry-wise this is very unrevealing; hence, the following classifications attempt to subjectively sort beers by their flavour characteristics.

The difference between lager and ale

To a drinker, the look, smell and taste describe the difference between a lager and an ale. Whereas ales have a fruity 'estery' taste, lagers are described as more 'crisp'.

However, the main differences between the two styles of beer come down to the brewing process. In 'Stages of Brewing Beer' we discussed the two different types of yeast that can be used in the beer making process: top or bottom fermenting yeast.

When brewing ales, top fermenting yeasts are used and the barrels are kept at the cellar temperature (warm). Top fermentation means that the ethanol concentration can reach up to 12%, much higher concentrations than the bottom fermenting yeast would allow.

Conversely, lagers use bottom-fermenting yeasts and ferment under cold temperatures, which would make the top fermenting yeast stop working. After fermentation the beer is 'lagered'; this is the period of time after the yeast has been added where the beer is left in the cold for several weeks in order to age, the slow process of fermentation at cold temperatures is what results in the 'clean' taste.

Bitter ales: For bitter ales, a moderate colour is seen. The bitterness can stretch from moderately high to very intense, whereas the maltiness is generally low for this style.

Malty ales: This family of beer styles incorporates ales that have the dominant flavour of maltiness; the maltiness is a product of heat treatments (where the Maillard reaction occurs during browning, as we will see later), which also results in most malty ales having a dark colour.

Phenolic ales: This class of beer is described as having the flavour of cloves, and is usually characteristic of wheat-based and barley-based ales. The phenolic flavour is considered to be an off-flavour in other beer styles.

Bitter lagers: The malty flavour in many lagers is very unnoticeable, as the dominating flavours in the brew is bitterness.

Speciality beers: The speciality beer family encompasses all beers with flavours not usually found in beers; such as spices, smoky flavours, and fruit beers among others.

Overall, there are many different types of ales and lagers, allowing beer drinkers to pick and choose the brews that they find most pleasant tasting.

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Exploring Everyday Chemistry

University of York