In whisky making there are many parameters that have an influence on the final product. One of the most important factors is without a doubt the decision of the distiller when to switch from foreshot to the middle cut that will finally be filled into the casks and when to switch again to the feints at the end of a distillation run.
What is the Foreshot?
In the early stages of a distillation run, the temperature slowy rises and causes the lightest compontents of the liquid in the still to evaporate. A large portion of the early distillate is methanol, the lightest of all alcohols with a boiling point of 65°C. Unfortunately, methanol is anything but healthy. It is not very toxic by itself, but some metabolites formed in the human body – especially formic acid – can cause pretty severe toxic reactions. Improperly distilled moonshine can have significant quantities of methanol, and many fatal incidents have been reported.
Some other substances are also present in the foreshot like for example ehtyl acetate with its trademark odour of glue. Because of its higher evaporation temperature it is also present in the proper spirit, notably in bourbon where it contributes to the characteristic “bourbon nose”.
What are the Feints?
The substances with higher boiling temperatures will evaporate later in the distillation process. This is an illustrous mixture of higher order alcohols, esters, terpenes and complex aromtatic substances that are usually subsumed under the term congeners.
Most of these substances are not very dangerous to your health in small amounts. But at higher concentrations they (or their metabolites) can cause pretty uncomfortable effects like severe hangovers and may even display toxic reactions. On the other hand the feints include many substances that are favourable for the taste of a whisky.
How to Tell when to Switch
There is no fool-proof rule when to switch from foreshot to middle cut and then to the feints. The temparature in the still is constantly rising during distillation causing the concentrations of the evaporating substances to rise and decline gradually. Apart from the very early and very late stages of the distillation when the unhealthiest substances are evaporating, it is up to the distiller to decide the exact moment to turn the knobs on the spirit safe.
Every distillery has their own rules when to switch. The exact point is determined by the alcohol content of the current distillate. Because of the steadily rising temperature more and more water is evaporating over time resulting in an ever-decreasing alcohol content. Usually the middle cut is started at alcoholic strenghts in the low 70s and is collected until the strength reaches the low 60s.
Middle Cut and Character of the Spirit
The cutoff strengths for the middle cut are about as crucial to the character of the spirit as the still shape. It is even possible to counteract the effect of the still shape. Tall stills tend to give a lighter spirit than sturdy ones. But moving the cutoff point for the feints to lower strengths causes an enrichment of heavier components.
This freedom allows distillers to fine tune the chracter of their spirit to a large extent despite the constraint of the still shape.