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. From a chemical point of view, this is a condensation reaction, as water is split off. In simple terms, an ester can be thought of 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 flavorings. Such esters are also largely responsible for the fruity smell and taste of a New Make spirit or whisky.
Can you recognize 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, raisins 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?
Esters are mainly formed during alcoholic fermentation by the yeast. However, they are also formed from existing alcohols and acids during distillation and years of maturation in wooden barrels.
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 have been 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 barrel?
Various esters are also formed during years of ageing in wooden barrels through the chemical reaction of alcohols with acids. The most commonly found ester is the compound formed from the alcohol ethanol and the acid acetic acid. This substance is chemically referred to as ethyl acetate. This ester is continuously formed in the barrel during maturation, with the amount of ester in the barrel increasing steadily over the years. As a result, the amount of acetic acid ethyl ester in the whisky is a measure of its maturation.
What flavors does ethyl acetate have?
In small quantities, ethyl acetate - the ester of acetic acid and ethanol - has a pleasantly fruity aroma 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 sticky smell develops. The perceived aroma of an ester therefore depends on its quantity, i.e. its concentration.
What aromas do the esters in whisky have?
Esters can produce a wide range of aromas and flavors in whisky. These range from fruity and floral notes to spicy, herbal and woody tones. Examples of ester flavors 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 (= acetic acid propyl ester) is responsible for the pear aroma. The ester of butyric acid and ethanol (= butyric acid ethyl ester) has a pronounced peach aroma as well as the smell of pineapple, while the ester of butyric acid and pentanol (= butyric acid pentyl ester) smells of apricot. If the alcohol pentanol reacts with valeric acid, the corresponding ester (valeric acid pentyl ester) has a distinct apple aroma. An ester of benzoic acid and ethanol (= benzoic acid ethyl ester) is responsible for the smell of mint.
On which factors does the formation of esters depend during the production of whisky?
The exact composition of the fruit flavors 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 distillation process (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 aromas that contribute to its individual and characteristic aroma and taste 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, these do not necessarily have to be two molecules. There are also chemical compounds that have both groups - the acid and the alcohol group - in one and the same molecule. Just in 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 ring-shaped ester, known in chemistry as a lactone. Such lactones actually exist and occur naturally in oak wood, for example. American white oak contains a lot of these special lactones, which have an aroma and taste of coconut. These lactones are referred to in specialist literature as whisky lactones or Quercus lactones.