Why is Copper Important in the Distillation Process?
The use of copper to make stills dates back hundreds of years. Copper has a number physical properties that make it an obvious choice. Firstly it has excellent heat conductivity. It quickly absorbs and transfers heat, allowing for efficient and precise temperature control during the distillation process. Secondly, it is malleable, making it easy to shape and work with during the construction of stills. Coppersmiths can create complex and efficient designs, incorporating bends, curves, and specific shapes. Lastly, it is resistant to corrosion, especially when it comes into contact with alcohol and acidic solutions, making it well-suited for use in distillation. Despite its excellent physical properties it is its chemical properties that make it an almost essential part of spirit production. When the vaporized alcohol passes over copper in a still, several important interactions occur: catalytic reactions, ester formation and copper filtration.
Copper acts as a catalyst, facilitating various chemical reactions that contribute to flavour development. Copper ions can interact with volatile compounds, such as sulphur compounds, present in the distillate. These interactions can lead to the removal of undesirable compounds, such as sulphurous and fatty compounds, resulting in a cleaner and smoother spirit.
Esters are flavour compounds that contribute to the fruity and floral aromas in whisky. During distillation, there is an interaction between alcohol vapours and copper surfaces. The copper acts as a catalyst that promotes the formation of esters through esterification reactions. A good example examples of a reaction occurring at the copper surfaces is the esterification of acetic acid. Acetic acid is formed by the bacteria acetobacter during fermentation. During distillation the acetic acid reacts with ethanol at the coppers surface to form ethyl acetate. Ethyl acetate has a pleasant, fruity aroma reminiscent of pears or apples.
Copper can act as a filtration medium by attracting and binding certain compounds, onto its surface. This helps to remove impurities and off-flavours from the distillate. It can adsorb compounds like aldehydes, which contribute to harsh or pungent flavours, resulting in a more refined and mellow spirit. The mechanism behind copper's ability to adsorb aldehydes is not fully understood, but it is believed to involve the affinity between the copper surface and the aldehyde molecules. Copper ions or copper oxide layers on the surface of the copper may attract and bind with the aldehydes, effectively removing them from the distillate.
Various aspects of the still design can increase or decrease the amount of copper contact and affect the character of the final spirit. In the tradition wash and spirit still set up the principal factors that affect the amount of copper contact are: the size and height of the swan neck, the height and angle of the lyne arm and the construction of the condenser. Copper contact is generally greater in stills with big tall swan necks, long upward sloping lyne arms and with condensers made of copper rather than stainless steel. A modern hybrid still may have more, or less copper contact, depending on how it is constructed. A hybrid still consists of a pot with a small (4-5 plate) column. They can be made of made of steel, copper or a combination of the two. Obviously, a hybrid still in which most of the parts in contact with vapour are made of steel the amount of copper contact will be low. However, if the hybrid still is made of purely, or predominantly copper the amount of copper contact is increase significantly. A hybrid still may have more copper contact than a traditional wash and spirit still setup when the hybrid is predominantly copper and a dephlegmator on the top of the column is used. A dephlegmator is essentially a small condenser on the top of the column. The dephlegmator condenses the vapour forcing it back into the pot to travel through copper swan neck and column again and again until eventually it escapes the dephlegmator to reach the condenser.
Although a decent amount of copper contact is generally considered a good thing in spirits production it is not to say that it is a necessity to make good spirit. The formation of sulphides and aldehydes during fermentation can be minimised by careful selection of yeast strains, controlling the temperature of the fermentation and using sealed fermenters to prevent oxygenation. In addition, a spirit that is relatively high in sulphides and aldehydes can become an incredibly complex whisky with proper maturation. For example, spirit with relatively high level of sulphides can develop some extremely complex, earthy, truffle-like flavours. The flavours may start to develop after a few year but the full development and integration of these flavours can take several years or, more likely, decades.
Copper plays a vital role in spirit production due to its excellent physical and chemical properties. Its heat conductivity, malleability, and corrosion resistance make it an ideal choice for still construction, and its chemical properties are important to flavour development, contributing to a cleaner and fruitier spirit. While copper contact is considered beneficial, it is not always necessary for good spirits production, Careful yeast selection, fermentation control, and maturation can minimize undesired flavours and proper maturation can result in some complex and unique spirits, if you are prepared to wait.