C - like Cut Points
What is meant by cut points?
Cut points is the English term for separation points or interfaces. In distillation - a separation process based on the different boiling points of the individual components of a mixture - they represent the temperature ranges at which different fractions of a liquid mixture are separated. In this process, the cut points are determined individually by each distillery so that a distillate with the desired properties is obtained in the end.
What is separated during distillation?
In the whisky distillery, during the second distillation in the fine spirit still, the early run-up as well as the late run-down are separated from the desired middle run, the heart of the distillation.
Why are there pre-run, mid-run and post-run in the second distillation?
During the distillation process, the liquid is heated. Those components with lower boiling points evaporate first. These vapors rise up in the still, enter the condenser via the Lyne Arm, are condensed there by cooling, flow through the Spirit Safe via piping and then on to a collecting tank. This is the high-alcohol feed, which also contains toxic compounds. Subsequently, the desired components with medium boiling points boil, which is called the middle run. Later, those substances with higher boiling points pass into the gas phase and are condensed again. This is called the after-run, which contains highly unattractive compounds from an aromatic point of view. Pre-run and post-run are undesirable and must therefore be separated from the heart.
How do you recognize the transitions of the individual fractions?
At St. Kilian, the determination of the two cut points during the second distillation in the fine spirit bubble is carried out in a sensory manner. A defined quantity of distillate, which is currently flowing through the Spirit Safe, can be removed via a lever attached to the Spirit Safe. The experienced distiller can then recognize from the smell and taste whether the respective transitions from pre-run to mid-run or from mid-run to post-run have been reached.
What are the sensory properties of the different fractions?
The pre-run contains the low-boiling components of the liquid mixture and consists of chemical compounds such as acetaldehyde, dimethyl sulfide or methanol. The forerun has a pungent odor reminiscent of solvents. The middle run, on the other hand, is characterized by its fruity aromas. It contains, among other things, the potable alcohol ethanol and various esters. Esters are chemical compounds known for their fruity aroma and pleasant taste, and are therefore highly sought-after by distillers. The middle run also contains higher alcohols, so-called fusel oils, some of which also have fruity notes. The after-run contains high-boiling compounds, such as further higher alcohols and fatty acid esters. The onset of the after-run has an unpleasant musty odor reminiscent of wet cardboard.
How are these fractions physically separated from each other?
Once the distiller has identified the transition from pre-run to middle run after a sensory check, he actuates a valve by flipping a small switch and pressing a button. This valve ensures that, after passing through the Spirit Safe, the distillate no longer flows through a pipe which is connected to an intermediate receiver at the end, but now reaches the collecting tank for the desired frog (= "spirit receiver") via another pipe. The same procedure takes place when switching from intermediate to overrun. Pre-carriage and post-carriage are collected together in the intermediate storage tank.
At what alcohol levels are the cut points?
In the case of St. Kilian, the alcohol content at the first cut point, i.e. at the transition from pre-run to mid-run, is around 72 percent by volume. The transition from middle to end run occurs at about 59 percent alcohol by volume. These cut points can vary depending on the composition of the barley malt in the mash, the "mashbill," and whether mild (unpeated) or peated malt is processed.
What happens to pre-carriage and post-carriage?
The fractions of pre- and post-run still contain certain amounts of ethanol as well as desirable flavors. Therefore, they are combined with the raw spirit contained after the first distillation of the next batch of wash and then fractionally distilled in the Spirit Pot Still as described above. These quantities of separated pre- and post-distillate are approximately always the same, as a certain balance is established. This is because certain noble components from the pre- and post-run, such as ethanol and desirable aromas, pass into the heart during multiple distillation.