The role of copper in spirit production

Elemental copper has unique properties making it ideally suited for the production of whisky. The metal is both soft and malleable but extremely tough. It is also an excellent conductor of heat. For these reason the earliest illicit whisky makers made use of it very early on. Copper has a lot more to offer however as it has a significant influence on the character of the new make spirit.

Copper

The chemical element copper (lat.cuprum) has the symbol Cu on the periodic table and is found at the top of Group 11 above silver and gold. Group 11 known as Transition Metals. The coinage metals, copper, silver, and gold, have held great importance in societies throughout history, both symbolically and practically. Copper was first used by man over 10,000 years ago and a copper pendant discovered in northern Iraq has been dated about 8700 B.C. Copper is one of the oldest metals known to man. The extraction of solid, i.e. pure copper from its ores was made more than 7,000 years ago. The heavy metal counts together with gold, silver, lead, iron, mercury and tin to foundational metals of antiquity. North and South Americaare the largest copper producers in the world today. One of the most important properties of the material Copper is the very good electrical conductivity.

Copper for whisky making

Despite being a comparatively soft metal when hammered, copper hardens. This property alone would make it ideal for distillation as it can be easily shaped and so could be brought into the desired shape by the distiller with ease. In addition, the metal’s properties as a fast heat conductor have made it ideal for the creation of alembic (pot) stills but the most important reason copper is used to make whisky today is its interaction with sulfer. When copper ore is mined the ore is treated with dilute sulphuric acid. This trickles through the ore dissolving the copper and creating a weak solution of copper sulphate. The copper is then recovered by electrolysis in a process known as SX-EW (solvent extraction/electrowinning). This process, or reaction also has a role in whisky production.

Sulfur compounds

Another extremely important property of copper is its great affinity for sulfur and its various forms. The fermented wort (wash) contains organo-sulfur compounds, some of which come from the grain itself, the remained are created during the fermentation process. These are mainly Ethanethiol, dimethyl sulfide, dimethyl disulfide and Dimethyl trisulfide, but may also be other as yet unidentified compounds that make a significant contribution to the undesirable sulfur flavors. The biosynthesis of sulphurous Amino acids (cysteine, methionine) and water-soluble sulfate salts are natural sources of sulfur. The latter in the course of fermentation are converted to hydrogen sulfide and other sulphides. Organo-sulfur compounds are sometimes very volatile and can be extremely odorous. Hydrogen sulfide has a characteristic rotten egg smell which can be detected at very low levels. Beyond this in higher concentrations these will create rubbery, burnt, or fleshy notes as well as aromas of boiled cabbage, steamed vegetables. In addition to these armoas being penetrating, uncomfortable and generally undesirable they also overpower far more fragrant compounds - such as fruity sweet esters and sweet grain notes. Most of these flavours will be removed during the distillation process but not all. In the collected main cut or heart some will be collected with a low enough odour threshold or sufficiently high concentration to alter the flavour of the new make spirit.

Reaction with copper

During the distillation, the alcoholic liquid is heated and begins to evaporate. Every time the hot vapours come into contact with the copper surface on their way to the condenser, various chemical reactions take place there. The previously volatile sulfur compounds react with copper to form copper sulfides, which are deposited in the form of black deposits on the inside of the pot stills. Likewise, the carbon dioxide formed during fermentation and dissolved in the wash reacts, which is bound in salt form as green-blue copper carbonate. From time to time these odorless salts must be loosened from the copper surface in order to again expose fresh copper for further reactions. To do this, the lid of the round hole in the still - the so-called manhole - is opened for about 15-20 minutes to let in fresh air. The adhering copper salts flake off and fall to the bottom of the still, from where they are discharged with the distillation residue.

Influence on copper contact

The size and shape of a pot still, which vary greatly depending on the distillery, make a decisive contribution to the aroma of the later whisky. The reason for this is the different contact times the rising vapours experience with copper during the distillation. The resulting different reaction times of determined the quality and quantity of the respective sulfur compounds created through contact with the metal. The longer the copper contact, the lighter the new make spirit. A longer, wider still neck offers the rising vapours a larger copper surface than a shorter, narrower neck. Another way to control the degree of copper contact is the rate of distillation. Slow heating means a cooler temperature in the throat of the distant copper still and has the consequence that the rising steam condenses earlier, runs back again and evaporates again (commonly called reflux). If this process is repeated several times it thus promotes a longer copper contact. At Glengoyne distilling is extremely slow at a rate of 14 liters of spirit per minute in the wash and 5 liters per minute in the spirit still, creating a light as well as fine distillation. On the other hand, a higher supply of heat and a higher temperature increases the rate of distillation and increases the density of the vapours that rise above the throat. As a result, some vapours reach the condenser without (long) copper contact and thus the proportion of sulfur notes in the fine spirit increases.

Condensation

The time the spirit remains in the condenser is also important. The tube bundle condenser common today basically form a kind of chamber with numerous copper tubes through which cold water is passed. The vapours from the still condense on these tubes and form a film of liquid that drains from top to bottom around each tube. Due to their tubular structure, these condenser guarantee a much more intensive copper contact and thus cause a greater reduction in sulfur compounds. In comparison, the externally cooled worm tubs produce a heavier and more full-bodied new make spirit, as the vapours condense in them right at the beginning - i.e. when they enter the copper tube - and then come into little contact with copper as a trickle flowing down.

Reactions in the cask

Copper also plays an important role in the breakdown of remaining sulfur compounds during the cask maturation process. Due to the interaction of three substances - naturally occurring tannins in oak, oxygen dissolved in the distillate or from outside the barrel, and dissolved copper ions form hydrogen peroxide. This highly reactive compound is able to oxidize the odour intensive sulfur compounds through a chemical reaction and thus “olfactorily defuse them”. Since many of the sulfur oxides resulting from this are more or less odorless. The strong oxidizing agent hydrogen peroxide can also use the further oxidize any alcohols present to the corresponding acids. This is of particular importance because esters are formed from alcohols and acids. Copper is also involved here, as it acts as a catalyst to accelerate the chemical reactions and thus contributes to the formation of these mostly fruity and aromatic flavors in the later whisky.

Wear and tear

The copper wears off as a result of the reactions with different ingredients from the wash or the low wines. As a result, the thickness of the stills away from the copper decreases over the years, and these have to be replaced for safety reasons as soon as the value falls below a critical value, which is determined by means of ultrasound. The residue in the wash still - the pot ale - contains a significant amount of copper (approx. 40-140 mg/kg) in the biomass, consisting of yeast residues, soluble proteins and carbohydrates. There are also high concentrations of largely soluble copper ions in the Spent Lees - the residues after the 2nd distillation. Using a combination of ion exchangers and electrolysis, valuable copper can be recovered. Laboratory tests have recently shown that copper is better able to remove sulfur compounds in certain parts of the still than in others. Using the example of dimethyl trisulfide that smells rotten vegetables, the level of these unpleasant aromas in the condenser of the wash still and in the pot of the spirit still was best reduced.

Sulfer & copper transformations

The intese odours of Hydrogen sulfide, Dimethyl-(di,-tri) sulfide, Ethanethiol & Carbon dioxide are allbroken down into odourless copper sulfides & copper carbonate. Likewise the compindation of Tannins & oxygen with strong smelling Hydrogen-peroxide creates odourless sulfur oxides, the adition of alcohol results in the creation of acids. These acids and sulpher dioxides together create fruity and aromatic esthers. None of which would exist without the use of copper!