Distillation is a physical process where compounds are separated by virtue of their different boiling points. If two compounds occurred together that had the same boiling point, they would not be separable by distillation. Fortunately, very few compounds have common boiling points with other compounds.
The separation in distillation occurs when a mixture of compounds in the still is brought to boil. Compounds with lower boiling points vaporize at lower temperatures than compounds with higher boiling points. This means that the vapour, or steam, rising off the boiling mixture has a more copious amount of the lower-boiling-point compounds than of the higher-boiling-point compounds. Next, this vapour is collected and cooled to condense it back into a liquid. The resulting liquid, called the distillate, contains a considerably higher concentration of the lower-boiling-point compounds than of the higher-boiling-point ones.
In a simplified example, assume a mixture of 90% water and 10% ethanol is to be separated by distillation. Water has a boiling point of 100oC (212oF) and ethanol has a boiling point of 78.4oC (173.1oF). The ethanol will boil and vaporize well before the water, so when the vapours are collected and condensed, the resulting distillate will have a very high concentration of ethanol and comparatively little water. Of course, the distillate will not be pure ethanol because some water will vaporize at the boiling point of ethanol, even if the water itself is not boiling.
Because all the compounds in a still will vaporize to a greater or lesser extent during boiling, the separation of the compounds will not be perfect, so more elaborate stills have been developed to intensify the separation of the vapours once they have left the boiler. In modern high-separation stills this is done by employing a reflux column to manage the vapours after they leave the boiler and before they are condensed back into a liquid.
There are many different designs of stills. The most basic and rudimentary design is a "pot still" such as a closed pot, like a pressure cooker, with a pipe leading from the lid into a condenser coil. The condenser coil can either be long enough to air cool the vapours or it can be shorter and immersed in a water jacket. Such a still would afford minimum separation since there is almost no separation of the vapours once they leave the boiler. Although this design of still is not suitable for producing beverage alcohol by modern standards, it will still concentrate an 8 or 10% alcohol mash to 60% in a fairly fast run.
The next level of still is the "whiskey still", sometimes called a "gooseneck" still. This design has been in use for centuries for commercial whiskey production, and is just as popular today in modern whiskey distilleries as it has ever been. A whiskey still has a large boiler with a long broad neck rising from it. The neck bends at the top and leads to a condenser coil immersed in water. This design is very similar to the pot still design, except the tall broad neck affords enough separation to hold back most of the fusel alcohols from the distillate. This design of still is sufficient for the production of whiskey, brandy, and rum, for which it is very widely used commercially. The whiskey still is not suitable, however, for the production of vodka or gin, which requires a high-separation still capable of producing pure alcohol.
The high-separation still design is called a "column still" or a "fractionating still". A fractionating still is used to produce pure alcohol by fractional distillation for making vodka and gin, or for pharmaceutical and laboratory use. The fractionating still has already been described to some extent in the Equipment chapter. It will be useful to refer back to the diagrams and pictures in that chapter as you read on.
In a fractionating still the vapours emerging from the boiling mixture pass up a column packed with small pieces of glass, ceramic, stainless steel, copper or other material, inert to the process. In larger fractionating stills, the columns have baffle plates with holes in them instead of packing material. Each piece of packing, or the baffle plates, can hold a small amount of liquid, either internally (if they have internal crevices) or in the interstices between adjacent particles. At the top of the column the emerging vapour is condensed into a liquid by means of cold water running through a heat exchanger. The condensed liquid runs back down the column until it reaches the boiler where it is reheated, converted into vapour once more, and once again moves up the column.
At equilibrium, which may take several hours to achieve in the case of pure-alcohol production, the system consists of vapour rising up the column meeting a flow of liquid running down the column. At each vapour-liquid interface on the packing material within the column, a partial separation occurs wherein the more volatile components of the mixture go into the vapour phase and rise to the top while the less volatile components go into the liquid phase and are carried down into the boiler. At equilibrium, the many components in the mixture become stacked up in the column in the order of their boiling points, the most volatile at the top and the least volatile at the bottom.
In commercial operations, which use a continuous-run design of fractionating still, the fermented mash is fed into the boiler from a reservoir, the different components of the mixture are drawn off at various heights along the column, and the spent residue is drained off. This process can continue indefinitely as long as fermented mash is fed into the boiler. Acetone, for example, would be continuously drawn off from the top of the column while ethanol would be continuously drawn off from a point a little further down.
Very small operations such as we are concerned with here do not employ a continuous-run system. Rather, fractional distillation is carried out batchwise. After column equilibrium is established, with acetone and methanol at the top and fusel alcohols at the bottom we start to progressively draw off liquid from the top of the column. First come the acetone and then the methanol and all the other low-boiling-point compounds. Then the ethanol starts to appear, and when it does, a small portion of it is drawn off and bottled for use. The remainder is allowed to run back down the column to continue the counter-current flow and the separation process. Eventually, the ethanol will be exhausted and the higher alcohols, the so-called fusel alcohols, will start to emerge. At this point (or in practice somewhat before) the boiler is switched off.
Water is an important constituent of the fermentation substrate and, with a boiling point of 100oC (212°F), lies intermediate between the least and most volatile components of the mixture. It has one important difference from the other components, however, in that it forms an azeotrope with ethanol. An azeotrope is a mixture of two liquids with a boiling point different from either constituent. In the case of ethanol and water, the azeotrope occurs at a mixture of 97.3% ethanol and 2.7% water, and has a boiling point of 78.15oC (172.67oF), .25oC lower than the 78.4oC (173.12oF) of pure ethanol. As far as the system is concerned this azeotrope is a single compound with a boiling point of 78.15oC (172.67oF) and proceeds to separate it on that basis. The ethanol which is purified by a fractionating column is not, therefore, pure 100% ethanol but pure 97.3%, the "impurity" being pure water. No amount of redistillation under the conditions we are using will influence this percentage. 97.3% alc/vol is the theoretical maximum purity that can be derived by the above process.
If it is absolutely essential to remove all the water, for example if it is to be mixed with gasoline to produce gasohol, then special methods are available to accomplish this. For our purposes, however, where we are going to dilute the alcohol with water to 40 or 50% anyway, the presence of 2.7% water is of no consequence.
The high level of separation of a fractionating still is a function of the reflux taking place by the condensed liquid flowing down the column interfacing with the vapours rising up the column. When distillate is drawn off the still at the top of the column it is important that only about 10% is drawn off and about 90% is allowed to return down the column to maintain the reflux, and hence the high separation.
If the operator of a fractionating still wanted to reduce the level of separation afforded by the still, they could do so by drawing off a greater proportion of the distillate leaving less reflux, say 30% drawn off and 70% reflux. Or, even 90% drawn off and 10% reflux. This means that a high-separation fractionating still offers very precise control over separation level by simply adjusting the proportion of reflux. Thereby, making it possible to produce spirits that require much less separation, such as whiskey, in a fractionating still. And, it is because of this precise control over separation level that the author has chosen a fractionating still in this text as the design for making whiskey.
Whiskey Distillation: In the production of pure alcohol, a high-separation still is employed to separate out most of the water and all of the congeners (i.e. impurities) and deliver only the alcohol. But in the production of whiskey, certain proportions of the congeners need to be left in the distillate. So, only moderate separation can occur. For this reason, whiskey is usually made in gooseneck stills that give comparatively low separation. However, some modern whiskey distilleries use a form of fractionating still to maintain better control over the process.
Whiskey is distilled in two runs: a primary distillation, or beer-stripping run; and, a spirit-run. The beer-stripping run is generally done in a very crude high volume pot still called a
"beer stripper". The beer stripper is used to distill the fermented mash and concentrate the alcohol and all the impurities into a distillate of about 40 to 50% alcohol, called "low wines". The spirit-run is done in a whiskey still, either a gooseneck or a special-purpose fractionating still, called a "spirit still". The spirit still is used to distill the low wines and refine them into the finished spirit. There are actually two outputs retained from the spirit-run: the finished spirit; and, the feints (explained below).
To produce the finished whiskey, the spirit still is filled with the low wines from the beer-stripping run plus a measure of feints from previous spirit-runs. The spirit still is then heated up and brought to boil.
The distillate from a spirit-run comes out in four phases: the foreshots; the heads; the middle-run; and, the tails.
Foreshots: The foreshots are the low-boiling-point compounds that come out of the still first. They contain acetone, methanol, various esters and aldehydes, and other volatiles. Foreshots are to be considered poisonous and should be discarded.
Heads: The heads come out after the foreshots, and are almost pure alcohol, except that they are contaminated with trace amounts of unwanted congeners. The heads are retained and later mixed with the tails to make up the feints that are cycled through future spirit-runs.
Middle-run: The middle-run is the refined spirit and begins when the trace congeners of the heads fade away and yield to a pure clean spirit. The middle-run, when diluted with water, is the finished whiskey.
Tails: At some point late in the middle-run a certain family of esters begins to bleed into the middle-run. These esters are what give the whiskey most of its character and flavour. As these esters flow into the middle-run they become increasingly intense and strong flavoured. Past a certain point they become so intense and strong that they are acrid and bitter and spoil the flavour of the whiskey. The still operator has to select a point before these acrid and bitter esters prevail, to end the middle-run. Everything following this selected-end of the middle-run is called the tails. The tails are only collected until the still-head temperature reaches about 81 or 82oC (178 or 180oF). Above this temperature there are no useful congeners but just unwanted fusel alcohols. After this point the still is switched off and the spirit-run is complete.
Feints: The tails are mixed with the heads and are called "feints". Feints are saved and recycled in future spirit-runs.
As the feints are repeatedly recycled through spirit-run after spirit-run, they become more and more richly embewed with the desirable whiskey congeners, so each batch of whiskey is incrementally improved over the previous.
When distillers run their first batch, when they have no feints yet, the whiskey flavour is insipid and unbalanced and tastes of raw alcohol. But as they run subsequent batches, carrying over the feints from their previous batches, the whiskey gets better and better with each batch.
Most distillers do not recycle all the feints on-hand through subsequent spirit-runs. They include a certain measure, and each distiller's measure becomes a main part of the unique signature of that distiller's whiskey. Another important part of defining a particular distiller's unique product is how late the middle-run is allowed to bleed into the tails.
At the time that the tails bleed into the middle-run (and visa versa), the percent alcohol of the emerging distillate begins to trail off quite rapidly. Most of the middle-run will come out at close to 95% alc/vol. But, towards the end it drops quite sharply, and the distillate is generally not collected into the middle-run below about 70% alc/vol. This sharp drop in alcohol concentration is used as an indicator to identify where the middle-run ends and the tails begin for a particular whiskey. An early middle-run would end while the percentage were still 85-90%; a medium middle-run would end between 75-85%; and a late middle-run would end between 70-75%. Some Scottish malt whiskeys go below 70%, and there's the odd one that goes as low as 60%.
Unfortunately, for small operations such as we are concerned with it's too costly to measure the percent alcohol of the small samples (i.e. 2-3 ml) that would need to be measured in order to determine the alcohol content of the emerging distillate. In large-scale commercial operations, it's a simple matter to collect a 250-ml sample of emerging distillate, measure the percent alcohol, and empty it into the receiver.
So, for small operations the transition points between the phases are determined by collecting a few drops of the emerging distillate on a spoon and tasting it. This is definitely more of an art than a science and practically defies description, but once an operator has experienced these transition points two or three times it becomes very clear how this works. In order to facilitate this familiarization, tables with the times, flow rates, and volumes of each phase for actual distillation runs of batches of corn mash as described in this text is included in the Procedure section to guide a first-time distiller by indicating the length of time and volume to expect for each phase. This puts the distiller in the correct ballpark for each transition point so they can familiarize themselves with the taste changes that occur across the transitions. Even if a distiller simply duplicated the volumes on the above-mentioned tables in their run of corn mash, they would produce a reasonably good Kentucky-style corn whiskey.
Even in this modern day of advanced instrumentation, commercial whiskey distillers still rely on tasting the emerging distillate to do the final determination of the transition points.
According to North American definitions for whiskey (not official U.S. Government nomenclature), the following guidelines describe and name the different whiskey styles.
Canadian Style: If the middle-run is ended while the emerging distillate is still between 8590% alc/vol, the whiskey flavour is very mellow and smooth and lacks strong definition of the character of the grain used to make the whiskey.
Kentucky Style: If the middle-run is allowed to continue until the emerging distillate is between 75-85% alc/vol, the whiskey flavour takes on a very pronounced and distinctive character that clearly reveals the character of the grain used.
Tennessee Style: If the middle-run captures emerging distillate that's below 75% alc/vol (usually no lower than 70%), the whiskey flavour is very strong and distinctive, and has a sharp bite to it that is very much an acquired taste. However, most whiskey drinkers eventually come to prefer this style as their palate for whiskey matures. Almost all the Irish whiskies and Scottish single-malt whiskies fall into this category (although I'm certain their distillers would have a different name for the style).
Another dimension to whiskey character is whether it is "early cut", "late cut", or "narrow cut". To understand early, late, and narrow cut whiskey, picture the entire output from the spirit-run as a time-line progressing from foreshots, through heads, middle-run, and tails. Now, look at the middle-run as a "cut" from that time-line.
An early cut would be a middle-run that began earlier and ended earlier on that time-line. This would produce a whiskey with more of the early congeners and less of the late congeners.
Similarly, a late-cut would be a middle-run that began later and ended later on the time-line. The whiskey would have more of the late congeners and less of the early congeners.
As for a narrow cut, the middle-run is started later and ended earlier, making the middle-run smaller and therefore narrow on the time-line.
Early cut whiskies tend to have more of the distinctive character of the grain (e.g. corn, rye). Late cut whiskies tend to be sharper and have more bite. Narrow cut whiskies are smoother and mellower and have less distinctive flavour and less bite.
As for the dimension of whiskey flavour contributed by the proportion of feints added to the spirit-run, the more feints the more body and richness the whiskey flavour will have. In effect, the feints don't change the whiskey's flavour, but rather contribute more of the flavour.
After numerous spirit-runs, more and more feints will accumulate. At some point, the distiller can dilute the feints with water and do a special spirit-run on the accumulated feints alone. Many distillers contend that the whiskey produced by this special run is the smoothest, richest, most flavourful whiskey of all, and it is often escalated to the status of the distiller's "Special Reserve" or "The Queen's own cask".
And finally, the feature that is reputed to impart the most unique signature on the whiskey flavour is the spirit still itself. There's a certain mystique surrounding this dimension, because no one appears to have a complete explanation why there's such a profound difference between whiskey distilled in one still and whiskey distilled in another apparently identical still. It's likely to be a combination of the height of the column, the width of the column, the distribution of the heat, and so on. When single-malt whiskey distilleries fabricate new stills they replicate their old stills right down to duplicating every dinge or kink or irregularity in thickness of the copper wall, to minimize any difference a new still may have over the old stills.
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