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Dr. Krausenmacher's Lab

Home brewing blog, distilling notes, musings on wine making and other mad science

The physics of distillation

The production of distilled beverages is somewhat of a tradition in South Africa. Mampoer has a long history: legend has it that the tribal chief Mampuru (from whom Mampoer is believed to have derived its name) introduced the white settlers in Transvaal in the 1800s to a beverage produced from the spontaneous fermentation of sweet fruits. The high sugar content in the fruit made this a rather strong beverage, which the boers then put through a still to increase its potency even further.

Mampoer was traditionally made from peaches (but in more recent times also from a variety of other sweet fruits) and usually has around 45-55% alcohol by volume (ABV). Witblits ("White Lightning") is essentially a more purified version of mampoer that can be bottled at strengths up to 95% ABV (!) and has less of a fruity character. Unlike mampoer, witblits can also be produced from grapes.

The interest in home distilling has increased considerably in South Africa in recent years. More and more beginning home distillers are exploring the options that this great hobby offers. However, from the questions we get from various parts of the country it is clear that not all beginning distillers are quite clear on what is happening in their still. While one does not have to be a scientist to operate a still, a basic understanding of the principles involved will go a long way towards achieving the best results.

So let's have a look at what's going on inside that bubbling kettle.

The principles of distillation

Distillation, in its simplest form, is evaporation of a liquid into a vapour, followed by condensation of the vapour back into a liquid. This happens in nature all the time: water (from oceans, lakes, vegetation and what not) evaporates into the atmosphere when the sun warms it, and that water vapour forms clouds which condense back into water which then falls in the form of rain. That's right: Mother Nature runs the largest stills known to man, and we're living in one of them!

While the natural distillation process in planetary weather systems relies on the heat of the sun and the cold of night for evaporation and condensation, man-made stills are a little different. In the average still, we force a liquid to turn into vapour by boiling it, and we force the vapour to condense back into a liquid by forced cooling, typically with cold water.

In the above diagram a heating element supplies heat to the liquid in the boiling chamber. As a result of the boil the liquid evaporates, and the vapour is then transported through a spiral tube which is mounted in a bath of cold water. This cools the tube to the point where the vapour cools sufficiently to condense back into a liquid which then ends up in a collection vessel.

Now why would anyone do that? You start with a liquid; you end with a liquid. What's the point of evaporating and then condensing it? Well, if you were to start with a 100% pure liquid, there would indeed be no point in distilling: the final liquid would be exactly identical to what you started with. But not all liquids are pure. Ordinary water, for example, will always have minerals dissolved in it. Since these mineral do not evaporate, they will not be present in the condensed liquid (which is technically known as the "condensate") and therefore distilled water will be far more pure than ordinary water.

In practice, the liquid being distilled (boiled into a vapour and then condensed back into a liquid) is invariably impure, and the purpose of distilling it is either to purify it, or to concentrate one of its ingredients into a more pure form. Which (finally!) brings us to alcohol.

Alcohol intended for human consumption is typically produced by the fermentation of sugars into alcohol by yeast. Because there are limits to the amount of alcohol that yeast can handle, a fermented alcoholic product is not very strong: the limit to what standard fermentation can produce lies at around 15% or so alcohol by volume. So what if we want a higher alcohol content?

This is where distillation comes in. If a liquid contains 15% of alcohol, that means that 85% of it will be other stuff. Most of that is water, the rest is a whole range of ingredients that give the beverage its specific flavour. Fortunately, consumption alcohol (ethanol) has a boiling point of about 78°C at sea level, while water boils at 100°C. (At altitude, such as on the South African Highveld, both boiling points will be somewhat lower. In this discussion we will assume we are at sea level, though.) This means that alcohol will evaporate from such a mixture at a lower temperature than it takes to evaporate the water. Therefore a temperature between the boiling point of alcohol and water will create a vapour much richer in alcohol, and once this condenses back into a liquid this condensate will have a much higher alcohol concentration than what we started with.

Boiling and how it works

Boiling: it's a simple process that we all know so well. Put a pot of water on the stove and turn on the heat until it boils. But what, exactly, is happening when we do that?

Boiling is, essentially, an example of elementary physics. (Don't worry, this will not be complicated.) When we heat the water, we add energy to it. Because the laws of nature ensure that energy cannot vanish into nothing and cannot appear out of nothing, the energy we put into the water must go somewhere. And indeed it does: this energy goes into raising the temperature of the water.

But when the water reaches a temperature of 100°C, something else begin to happen. The water begins to turn into vapour. This vapour forms bubbles in the water that rise to the surface and escape in the form of steam. BUT... Turning liquid water into vapour requires energy. That energy is being derived from the same heat source that was initially used to raise the water temperature. However, the energy supplied to the water can be used only once! It now goes into turning liquid water into vapour, which means it can no longer be used to raise the temperature of the water any further! As a result of this, the temperature of the water will now remain constant, at the exact point where the heat turns it into vapour.

This is known as the boiling point of water. Simply put, boiling water will always have the same temperature. Below that temperature it simply will not boil, and it cannot have a higher temperature. If we apply more energy (by turning up the stove) the water will simply boil more vigorously and turn from liquid to vapour more quickly, but the temperature can never exceed 100°C. That's why this temperature is known as the boiling point of water in the first place.

The boiling point of an alcohol/water mixture

Fine. So alcohol boils at a low temperature and water at a high temperature, and we can use that to separate the two. Simple, right? Well... Yes and no.

First of all, many people simply assume that when the liquid in the boiler reaches 78°C the alcohol will start to boil out of the mixture while the water will just sit there. But that is simply not true. If this were the case, every still would produce 100% pure alcohol, right up to the point where all alcohol had been retrieved out of the fermented liquid and nothing but water would be left... But everyone who has ever operated a still knows that that simply doesn't happen during distillation!

What does happen is that the liquid, as a whole, will boil. Whatever you put into the kettle, once it boils, it all boils. It is impossible for the alcohol to boil and for the water not to boil at the same time! The reason for that is simple: alcohol and water do not exist separately in the kettle, but as a mixture. And that mixture has its own boiling point.

While pure ethanol boils, to all intents and purposes, at 78°C and pure water boils at 100°C, a mixture of ethanol and water will have a boiling point somewhere between those two extremes. The higher the alcohol content, the lower the boiling point, and vice versa. Since condensation may be considered the opposite of evaporation, the same is true in reverse for condensing the vapour back into a liquid: the higher the alcohol content of the vapour, the lower the temperature at which it will condense.

So if we start with, say, 15% alcohol and 85% water in the boiler, the above shows that the mixture will come to the boil at about 91°C (blue line). Below that temperature nothing will boil, and therefore no distillation will take place. At the same time it is impossible for the temperature to be higher, since a liquid cannot exist at a temperature above boiling point. Supplying more heat will simply make the liquid boil more vigorously, but it can never raise the temperature any further.

As the distillation process progresses, the amount of alcohol in the boiling mixture will decrease. This means that the boiling point will gradually increase as time goes by, and this can be used as an indication on how much alcohol is left.

Some beginning distillers worry about the temperature at which they should keep their boiler during a distillation. But that's not how it works. The temperature in the boiler is determined by the composition (alcohol/water ratio) in the kettle. So let the temperature do what it wants. Keep an eye on it to see what it does, by all means, because it will tell you how much alcohol is left in the mix. But to not try to keep your boiler at 85°C or 90°C or whatever. It simply doesn't work that way.

How boiling concentrates alcohol

The above may seem confusing, though. If a mixture of alcohol and water has a certain fixed boiling point depending on the alcohol/water ratio, the reverse should also be true, and the vapour produced at that temperature should have the same alcohol/water ratio that we started with, right? But in that case distilling would be pointless since we'd never increase the alcohol concentration.

Fortunately, that is not quite true. While and alcohol/water mixture has its own boiling point, pure alcohol boils at a lower temperature than water does. So think of it this way: liquid alcohol turns into a vapour more easily than water does. This means that, although the boiling point is determined by the alcohol/water ratio, the evaporation favours alcohol over water. At boiling point the mixture boils, but because the alcohol component evaporates more easily than water does at that same temperature, the vapour produced will be richer in alcohol than was the case in the liquid that we started with!

For example, in the above graph the blue line shows us that a 15% alcohol/water mixture will boil at about 91°C while the red line shows that at 91°C the vapour will contain about 60% alcohol. When this vapour condenses back into a liquid, this liquid will have the same alcohol content.

Still geometry and its effect on flavour

As the distillation progresses and the boiling point of the mixture in the kettle slowly increases, the alcohol content in the vapour and condensate will decrease. One of the most important decisions the distiller has to make is how far to continue with the distillation process. If you keep going all the way to the bitter end, all that will eventually come out of your still is water. But if you stop too soon, you will throw away good alcohol. So what to do?

To answer that question, we first should have a good look at what's in the kettle. In the above explanation we have assumed that the kettle contains a mixture of alcohol and water. In practice this is not the case. Whisky, for example, is essentially distilled beer. Brandy is distilled wine. Rum is distilled from a mixture of alcohol, unfermented complex sugars derived from molasses, and a complex witches brew of flavouring compounds produced by the yeast (mostly fruity esters and spicy phenols).

But wait... Wasn't the whole point of distillation to concentrate alcohol as much as possible and to remove as much of everything else as we can? So what about those flavouring components? Well, seeing as distillation is based on the principle that different substances have different boiling points and that they evaporate in different ratios at a given temperature, it is not difficult to understand that what happens to a certain flavourful ingredient during distillation depends entirely on its boiling point and how much of it will evaporate at the temperature at which we distil. For example, if one of the ingredients produced by the yeast, grain or fruit to be distilled has a very high boiling point and does not evaporate easily, it will be all but removed from the final product. If it has a very low boiling point and evaporate very easily, it will have been removed from the wash long before alcohol begins to come out of the still.

However, most of these flavour components have boiling points and evaporation properties that are between, or close to, the boiling points of alcohol and water. This means that they will be included in the vapour, to a greater or lesser degree, and that they will therefore make it into the final distilled product. However, because some flavouring components in the original liquid (the wash) will not be present in the final product and/or some flavouring compounds will not be present in the same ratio as what we originally started with, distillation can alter the flavour of the final product in comparison to that of the wash.

(Incidentally, this is why the forerunnings, a.k.a. heads or foreshots that come out of the still first should be discarded: this condensate is high in very volatile chemicals such as acetaldehyde, acetone, ketones and other ingredients of paint thinners, glue, nail varnish remover and what not. Most people believe that toxic methanol is present in these foreshots and that this is the reason why they should be discarded. This is nothing but a myth. Yeast is genetically incapable of producing more than minute traces of methanol (on the order of milligrams per litre) which means that home distilled beverages produced from a normal fermentation won't kill you. While certain fruits may naturally contain some methanol and in theory distillation could potentially concentrate this, in practice this is completely insignificant: one would have to drink entire barrels of hard liquor at once (which would kill you) before methanol toxicity could become a health hazard. The horror stories of people who died from drinking home made alcohol have invariably been traced to concoctions that contained industrial alcohols, cleaning alcohol or even disinfectant.)

To return to the question of alcohol concentration vs. flavour: let's say we try to distil our alcohol to a concentration as close to 100% as we can possibly get. Offhand, that might seem a good idea. However, the more alcohol we have in the final product, the less there will be of anything else, including the ingredients that give a beverage its flavour! If the distillation removes too much of the other ingredients that give a beverage its typical character besides alcohol, we'd essentially end up with a high strength, low flavoured product rather than with a good whisky, brandy or rum. It also means that, since the ratio of alcohol versus other (flavourful) ingredients in the vapour is determined by the boiling point, which in turn is determined by how much alcohol there is left in the mix, we might want more control over the character of the final product than we seem to have.

Fortunately things are better than that. While the vapour that bubbles out of the boiling wash depends on the physics of the boil, a lot of things happen to that vapour before it ends up coming out of the condenser in liquid form. To begin with, the temperature in the kettle space above the boiling liquid will be somewhat lower than the boiling liquid itself, and the higher above the surface of the boiling liquid we get, the lower the temperature becomes. This means that some of the vapour will condense and fall back into the boil before making it out of the kettle. The higher the kettle (or rather, the higher the dome that forms the top of the kettle) the more significant the temperature gradient will be. If the temperature in the boiling liquid is, say, 90°C, a low and wide still configuration with a shallow dome may release the vapours into its outlet at a temperature as high as 89 or 88°C, while a tall and narrow kettle with a high dome may release vapours several degrees cooler.

This means that the vapour will cool when it rises, and while it cools, some of it will condense back into the kettle. The ratio in which the various components of the vapour (known as fractions) will condense depends on the temperature gradient, i.e. how far the temperature drops as the vapours rise. As a general rule, the higher the dome of a pot still is, the more significant the temperature gradient will be, and the more alcohol and less flavour the final product coming out of the still will have. Scotch distilleries are known for their dramatic differences in still configurations and the effect this has on the whiskies they produce: the product of a distillery located in low buildings with low, squat stills has an entirely different character than what comes out of a distillery that uses narrow, tall stills with high domes.

This effect reaches its ultimate effect in a reflux still. As the animated diagram shows, a reflux still is fitted with a tall column that is packed with a material through which the vapour rises from the kettle. Because the column is so tall, the temperature gradient across the column is quite dramatic: the top of the column can easily be ten degrees or more below the temperature in the kettle. Most reflux columns are also fitted with some form of additional water cooling, which reduces the temperature at the top of the column even further. Instead of packing material, some reflux still columns (especially those used in commercial distilling) are fitted with horizontal plates that divide the column into multiple, separate sections or chambers.

The volatility of the vapours rising up through the column through the packaging material depends on their composition. The more alcohol they contain, the lower their boiling (and condensation) point will be. Less pure vapours will cool to the point where they condense back into liquid before they have reached the top of the column. The packaging material (or plates) will stimulate this condensation process considerably. Only the purest vapours with the highest alcohol content will make it all the way to the top of the column, where additional cooling (if present) will further separate out less pure fractions.

The less pure fractions that condense back into liquid before reaching the top of the column drop back down. Some may make it all the way back into the kettle, but most of them will turn back into vapour (a purer vapour than was initially the case) somewhere between the top and the bottom of the column.

Reflux stills can in theory produce distilled alcohol up to a strength of 96% by volume, however in practice most stills top out at about 90% or so. Reflux stills are typically used for the production of neutral alcohol, vodka or gin. Most reflux still columns are fitted with a thermometer or temperature probe that indicates the temperature of the vapours reaching the top of the column. Because reflux stills have a high efficiency, they can maintain a high alcohol percentage in the condensate for most of the distillation. Only close to the end of the distilling process when almost all alcohol has been boiled out of the kettle the alcohol percentage in the condensate begins to drop sharply, and the temperature of the vapours at the top of the column will suddenly spike. The distiller will then know that the distillation process is complete.

Happy distilling!

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