There's a saying: "The shape of the still determines the character of the distillery." While the appearance of the equipment can be imitated, the real essence lies in the know-how behind its design and operation. Simply judging the flavor of a whisky based on the still's shape is like the blind men and the elephant-misleading without full understanding. Let's explore how distillation technology connects and contributes to the quality of whisky, even in century-old distilleries.
In any distillery, the golden copper stills are always a sight to behold. Whether tall, short, fat, thin, gracefully curved, pot-bellied with a flat top, or long-necked and slender, each shape has become a symbol of a distillery's unique flavor. But before diving into the details of still design, one important question must be answered: Why use copper?
Beyond its excellent ductility, thermal conductivity, and corrosion resistance, copper plays a critical role in purifying the spirit. During fermentation, compounds in the grain and the metabolism of yeast produce congeners, including sulfur compounds that can give off undesirable aromas such as boiled cabbage, sulfur, or rotten eggs. These off-notes are often disliked, but copper can help eliminate them during distillation by reacting with these compounds and neutralizing them.
The various shapes of pot stills are primarily designed to control how much contact time alcohol vapors have with the copper walls, which directly affects the final spirit character.
For instance, a short still allows vapors to condense and collect more quickly, reducing copper contact. This results in a spirit that retains more intense flavors-richer, oilier, and more complex, though sometimes rougher in texture.
On the other hand, a still with a large, round swan neck-like those used at Ballindalloch-allows alcohol vapor to expand, partially condense, flow back down, and re-evaporate multiple times. This reflux process increases copper contact and results in a lighter, cleaner, and smoother spirit.
Copper also plays a catalytic role in transforming undesirable compounds. For example, it can convert thiols (which are sharp and unpleasant) into less pungent carbonyl compounds. However, its effect on ester formation (which contributes to fruity aromas) is limited, and it can also reduce phenolic content, which impacts smoky or medicinal notes.
From the perspective of macroscopic physics, all matter exists in three states-solid, liquid, and gas-depending on changes in temperature and pressure. Distillation utilizes heat energy to convert a liquid (the distillate) into vapor, which is then cooled and condensed back into liquid form. The key principle behind distillation lies in the fact that different substances have different boiling points, allowing for separation based on volatility.
Water: approximately 86%–94%
Alcohol (ethanol): approximately 6%–14%
Volatile congeners: flavor compounds that evaporate with steam, around 0.1%
Non-volatile substances: a small amount, which do not evaporate during distillation
Water: approximately 5%–35%
Alcohol: approximately 65%–95%
Congeners: approximately 0%–0.5%
The graph below illustrates how the alcohol concentration by volume changes with temperature. The two curves define the transition between the liquid phase, the gas-liquid mixture phase, and the gas phase.
Since alcohol has a lower boiling point than water, the alcohol concentration in the vapor is always higher than in the original liquid mixture. As distillation continues, the alcohol content in the liquid gradually decreases, the temperature required for vaporization slowly increases, and the alcohol concentration in the vapor (and thus the condensed distillate) also decreases over time.
When the alcohol concentration in the vapor drops to around 1%, the remaining alcohol in the liquid mixture is typically only about 0.1%. At this point, extracting the rest would require a disproportionate amount of energy, making it uneconomical. Therefore, most distilleries choose to stop distillation at this stage.
Because the distillate contains not only water and alcohol but also many other compounds-even in trace amounts-the temperature–concentration curve becomes more complex. However, the underlying theory remains the same, and it is this principle that guides distillers in controlling the final alcohol yield.
In the next article, we will introduce to you the influence and role of various parts of the distillation equipment in the distillation process.