Volatility and Cut Points in Distillation
Distillation is essentially the separation of compounds based on volatility (the tendency of a substance to vaporize). A substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure, and is thus more volatile. In the context of distillation, only compounds that can vaporize in the still are considered volatile, whilst those that cannot are considered non-volatile. New-make distillates should contain only volatile compounds.
The composition of the distillate is not uniform during the run of the batch. At any given point in the run, the vapour in the pot will contain wide array of volatile compounds of varying degrees of volatility. However, the more volatile compounds will vaporize preferentially, and are thus present in very high proportions at the beginning of the run. As the run proceeds, the availability of these compounds in the pot decreases causing the boiling point of the mixture to rise (see Raoult’s Law). The internal temperature of the still is thus gradually increased, causing progressively less volatile compounds to vaporize in larger quantities. As ethanol (beverage alcohol) has a higher volatility than water, the ethanol concentration of the distillate is highest at the beginning of the run, and then gradually decreases as the run progresses. A distillation run usually continues until it is no longer economical to extract any ethanol from the remaining mixture in the pot.
In the context of spirits distillation, the term congener refers to anything that is not ethanol or water. Although they constitute only a small portion of the spirit by mass or volume, congeners are responsible for the spirit’s flavour and character. Not all congeners are desirable in the distillate, and by careful selection of cut points, the distiller can remove many of them it. The high, volatility fraction that comes off first is known as the heads or foreshots, while its lower-volatility counterpart at the end of the run is known as the tails or feints. These fractions contain high levels of undesirable compounds and are either discarded or recycled into the next batches for further rectification. Great care must be taken when recycling the foreshots and feints, and it is important to do so very consistently, otherwise, the resulting distillate will be inconsistent. Substrates with a high pectin content are conducive to methanol production, so particular care must be taken in how heads are recycled.
Between these two fractions is the heart of run. This is the product stream. There is no set rule as to exactly where the distiller should cut for his middle or heart. Very generally speaking, taking a high cut (favouring the high-volatility compounds) will generally produce a fresher, lighter bodied spirit, but risks becoming solventy. Conversely, a low cut will yield a deep, full-bodied spirit, but at the risk of being feinty. Taking a narrow cut allows the distiller to home in on smaller group of congeners, while a wide cut yields a greater diversity of congeners.
Author: Matt Spinozzi