E – like ester

What are esters?

Esters are chemical compounds that are formed by the reaction of an acid with an alcohol. They represent a separate class of compounds in chemistry.

How does this chemical reaction to form esters work?

Alcohols react with organic acids (e.g. acetic acid, lactic acid) or inorganic acids (e.g. sulphuric acid, phosphoric acid) in the presence of a catalyst and with the elimination of water to form a new class of compounds, the esters. This reaction is known as esterification. In chemical terms, this is a condensation reaction, as water is split off. To put it simply, you can imagine an ester as a two-component adhesive: Component 1 (= alcohol) combines with component 2 (= acid) and hardens as a mixture (= ester), displacing water.

Who discovered esterification?

The German chemist and Nobel Prize winner Hermann Emil Fischer (1852 – 1919) first described this reaction between acids and alcohols at the end of the 19th century. It is therefore also known in specialist literature as ‘Fischer esterification’.

Are there any known examples of esters?

Yes. For example, naturally occurring fats are esters, as they are made up of the alcohol glycerol and various organic fatty acids. These special esters are known as triglycerides. They are absorbed through food and are important energy stores in the body.

What special properties do esters have?

Many esters have a pronounced fruity aroma and are therefore used as flavourings. Such esters are also largely responsible for the fruity aroma and flavour of a new make spirit or whisky.

Can you recognise esters in whisky?

Yes. The often fruity notes in whisky – such as the aroma of tropical fruits (pineapple, banana), berries (blackberry, strawberry, raspberry), plums, cherries, sultanas soaked in rum, coconut, cinnamon, star anise or mint – are due to the existence of different esters.

Where are the esters formed during the production of whisky?

The esters are mainly produced during alcoholic fermentation by the yeast. However, they are also formed from existing alcohols and acids during distillation and years of maturation in wooden casks.

How does ester formation work in the distillery?

During fermentation, the yeast cells convert sugar into ethanol and carbon dioxide and produce a variety of by-products, including esters. Almost 100 different esters were identified in the resulting wash after fermentation. Other natural by-products of fermentation are various alcohols and organic acids. When these evaporate during the subsequent distillation of the wash in the pot stills and come into contact with the hot copper surface, they can react with each other to form different esters, which later end up in the distillate – the collected middle run.

Which ester is mainly formed in the cask?

Various esters are also formed during the years of maturation in wooden casks through the chemical reaction of alcohols with acids. The most frequently found ester is the compound formed from the ethanol and acetic acid. This substance is chemically known as ethyl acetate. This ester is continuously formed in the cask during maturation, with the amount of ester steadily increasing over the years in the cask. This means that the amount of ethyl acetate in the whisky is a measure of its maturation.

What flavours does ethyl acetate have?

In small quantities, ethyl acetate – the ester of acetic acid and ethanol – has a pleasantly fruity flavour reminiscent of green apples. As the whisky matures, the amount of this ester also increases and can reach a concentration range at which an unpleasant glue odour develops. The perceived flavour of an ester therefore depends on its quantity, i.e. its concentration.

Which flavours do the esters in whisky have?

Esters can create a wide range of aromas and flavours in whisky. These range from fruity and floral notes to spicy, herbal and woody tones. Examples of ester flavours in St. Kilian whisky include pear, apricot, peach, apple, vanilla, coconut and mint.

Which esters are responsible for this?

For example, an ester formed from acetic acid and propanol (= propyl acetate) is responsible for the pear flavour. The ester of butyric acid and ethanol (= ethyl butyrate) has a pronounced peach flavour as well as a pineapple aroma, while the ester of butyric acid and pentanol (= pentyl butyrate) smells of apricot. When the alcohol pentanol reacts with valeric acid, the corresponding ester (pentyl valerate) has a distinct apple flavour. An ester of benzoic acid and ethanol (= ethyl benzoate) is responsible for the odour of mint.

On which factors does the formation of esters in the production of whisky depend?

The exact composition of the fruit flavours present in a whisky depends on various factors. These include, for example, the raw materials used, the yeast strain used, the duration and temperature control of the fermentation process, the course of the distillation (temperature, speed, cut points), the storage conditions and the type of casks used in which the distillate matures. Each whisky can therefore have a unique composition of ester flavours that contribute to its individual and characteristic aroma and flavour profile.

Do you always need two separate reactants for ester formation?

Not necessarily. An acid and an alcohol are always necessary for the formation of an ester. However, this does not necessarily have to be two molecules. This is because there are also chemical compounds that have both groups – the acid and the alcohol group – in one and the same molecule. Just at different places in the molecule. Under suitable conditions, the acid and alcohol groups in this molecule can react with each other. The result is a circular ester, known in chemistry as a lactone. Such lactones do indeed exist and are found naturally in oak wood, for example. American white oak contains a lot of these special lactones, which have an aroma and flavour of coconut. These lactones are known in the specialist literature as whisky lactones or Quercus lactones.