Dilution before Re-Distillation

Anyone who has ever done a double or even triple distillation of spirits will know that every time you run the spirits through the still it gets "stronger" - the purity increases, the ABV% percentage rises. Whatever expression your prefer.

Most Distillers (especially those that did their training with us) would also be aware (based on the Ethanol - Water Phase Diagram) that the higher your starting point ABV in the boiler, the higher your average ABV will be in your distillate. Intuitively though, it would therefore make sense to put the Distillate in at full strength when Re-Distilling it, in order to make sure it comes out as strong as possible.

Makes sense? Right? 

Unfortunately the obvious answer is not always the right answer. 

Before we look at this more in-depth, let us first consider why we Re-Distill, and how it works.

    

Does a Triple Distilled Product Taste Better than a Double Distilled Product?

Now, we know your head is going straight to Whisk(e)y, and yes, that is the most common Spirit Category where this type of Distillation is applied, but Double and Triple Distillation is not unique to Whisk(e)y and can be applied to any Spirit Category. 

The most common comment you will find regarding Multiple Distillations is that it makes for a smoother product. This is a common opinion, one one that has a tendency to be rather contentious - especially in the Whisky world. But is it even true or fair to say that?  

Let’s start with what you perceive to be ‘smooth’. This is a term generally used to describe the little 'burn' on your tongue or in your mouth at the end op the sip and swallow. The finish as we call it. The less the burn, the smoother the finish. The smoother the finish, the more drinkable the product - i.e. 'easy-drinking' and therefore - at least that is the common perception - the better the product.

But this perception is not always right. Furthermore, smoothness is sometimes associated with a specific flavor profile, for instance some individuals might say that a Smooth Whisky is a Light Style Whisky.

The Style and Level of Smoothness should not be confused. Some light-flavoured (style) whiskies can be harsh, while some heavy, robust whiskies can also be smooth. There are a number of factors that can contribute to smoothness, but fundamentally it’s all down to the number of congeners and impurities in the spirit. 

Congeners are flavour compounds present within a spirit that are usually created by the yeast during the fermentation process. They also develop and change during distillation and maturation. There are hundreds of congeners that impart different flavours in a spirit, and some are more powerful than others. 

During the distillation process the distiller has an opportunity to keep or remove these congeners depending on the style of product they want to make. Not all congeners taste great, particularly not in large quantities, so they are removed from the distillate during the distillation process. One way of doing this is - arguably the easiest way - is through increasing the amount of reflux in the still. 

What is Reflux? 

Reflux - in it's most simple sense - is the process through which vapour in a still is forced to condense and evaporate over and over again, each time gaining purity, and each time leaving behind more congeners. 

During Distillation, the fermentation being distilled is placed in the still's boiler, where heat is added into the fermentation. As the fermentation liquid (which is comprised of water, alcohol and congeners) boils, the alcohol and flavourful congeners will separate from the water, vaporise and travel up and along the stills vapour path towards the condenser.

Depending on the size and shape of the still, some of this vapour hits the copper sides of the still, condenses and runs back down along the sides of the vapour chamber or vapour path. As it moves down it reaches hot vapours rising up in the vapour path - hot enough to re-evaporate the liquid, sending it back up again.

Each time this happens the vapour is essentially being redistilled, each time separating out more congeners and impurities that are left in the still, and therefore becoming a lighter spirit. This is the process called reflux, and the lighter the spirit, the less flavor (congeners) it contains.

Now, if we are using a Pot Still or Alembic Still or Packed Column Still, we are dealing with Fixed Reflux - literally the shape of the still determines the purity of the distillate - so the taller the still, the higher the purity, the more surface area in the vapour path, the higher the purity. You can reference our Article on how Still Shape influences Spirit Quality in this regard, but keep in mind that Reflux can be increased with the use of larger stills, as well as additional features such as boil balls and upward swans necks (also called lyne arms). The bigger the surface area and the more difficult it is for the vapour to travel toward the condenser, the greater the Reflux will be.  

In Adjustable Reflux Column Stills, Fractionating Reflux Column Stills, Multi Column Stills and Hybrid Stills however, we are dealing with Adjustable Reflux - in other words the Reflux can be manipulated.

There are two ways to do this - either Distillate Flow Control (in Adjustable Reflux Column Stills) or Coolant Flow Control (in Fractionating Reflux Column Stills, Multi Column Stills and Hybrid Stills). The two techniques are also referred too as Liquid Management and Vapour Management. Both Techniques however have the same result - you can achieve any target ABV% in a single distillation. With Modular stills - like the Fractionating Reflux Column Still - you also have the option of increasing the Fixed Reflux, i.e. changing the shape, by adding additional T-Sections and Bubble Plates to the Column. This allows you to achieve and maintain higher purity faster, and run the still faster without the purity decreasing (or at least decreasing quickly).

If however, you are using a Pot Still or Alembic Still, you are limited to Fixed Reflux, and the only way to increase Purity is to redistill - to run the same liquid through the still again.

Reflux is not, however, the only factor that plays a role here.

The way the distiller operates the still, and the decisions he or she makes is equally (if not more) important.

As a rule, the most volatile congeners such as high alcohols and esters, come off the still first as the heads. The least volatile congeners are the last to distil and come through toward the end of the process as the tails, so where the distiller makes their cut points is also vitally important in creating a spirit that is light and smooth. I say as a rule, as there are certain congeners (such as methanol) which does not always follow this basic rule due to an affinity with water, similar to what we will be discussing later in this article.

So, to create a lighter new make spirit in the first place, the distiller first needs to increase the amount of reflux in the still to make the removal of congeners easier, and secondly, maintain a tight control over cuts (and for commercial production, the consistency of those cuts).

Now, as we have already said, a way to increase Reflux when using a Pot or Alembic Still is to increase the number of distillations the spirit goes through. This is where double and even triple distillation comes into the picture. In triple distillation, by adding an intermediate still in between the stripping and spirit stills, the distiller is able to drive up the level of reflux even more to separate out those unwanted flavour compounds. By adding in a third still there is also more copper contact with the spirit, which leads to the removal of more of the heavy, sulphur-containing compounds.

   

An Example of Triple Distillation

Each distillery’s process to double and triple distillation is vastly different, but for the basis of this article I will succinctly explain a standard approach.

The fermented distillers beer, at about 8 to 12% ABV, is first distilled in the stripping still - commonly referred too in the Whisky World as the Wash Still. The distillate, now called the low wines, is about 25% abv and is held in the Low Wines Holding Tank. It is then mixed with the tails from the preceding low wines distillation and distilled in the Intermediate Still.

This Intermediate Distillation is then split into two parts: strong and weak (those tails). The latter will be mixed with the next lot of low wines and redistilled in the Intermediate Still. 

The strong portion is taken forward to be redistilled in the spirit still. This runs as normal, with the heart of the run (referred too as new make spirit) being separated from the Heads and Tails, which are mixed with the distillate obtained from the Intermediate Still, and then redistilled in the Spirit Still.

Every time the spirit is distilled it becomes more pure - attains a higher average ABV% - so rather than the average strength being at 70% ABV, as in double distillation, triple distillation will produce a spirit which is closer to 80% ABV.

This also then has an impact on flavour. The stronger the spirit, the lighter its aromatic character will be.

   

But does Lighter mean Smoother?

To confuse things further, triple distillation doesn’t necessarily result in a light and ‘smooth’ spirit. Many Whisky's especially, although triple distilled, still has a heavy, meaty and rich character.   

Smoothness is obtained less through Reflux, and more by Cuts - the decisions made by the Distiller during the Distillation Process.   

I can run a 10 plate Fractionating Reflux Column, and Distill a Vodka at 96%, and still end up with a harsh product, if I did not make the right cuts. 

In the case of Whisky, Brandy and Rum though, we must mention the role barrels play as well, during maturation. Barrelling also helps create a smooth spirit by allowing for the evaporation of volatile components as part of the Angle's Share, subtracting some components left in the spirit through the cask’s charred layer, and adding flavours of its own such as vanillin. As vanillin is seen as being sweet, it is important in that sweetness contributes towards this perception of ‘smoothness’. This is also where obscuration comes in - a topic for a future article.

      

So why do we Dilute for Re-Distillation?

In a solution, alcohol molecules form stronger bonds between each other than between themselves and water. Equally, water molecules will make stronger bonds with other water molecules than with the ethanol molecules.

In a weak alcohol solution therefore, where there is a lot of water and little ethanol, fewer molecules of alcohol will have an opportunity to come into contact with each other, as there are too many water molecules present. This results in fewer, stronger alcohol-alcohol bonds, and more of the weaker alcohol-water bonds. At the same time, there is a lot of interaction between water molecules, making for a lot of strong water-water bonds. This makes it easier for us to remove the more volatile alcohol molecules from the solution during distillation, as the alcohol-water bonds are easy to break, and the water-water bonds keeps the majority of the water molecules back.

On the other hand though, the distillate obtained from this distillation, will be high(er) in alcohol, containing little or less water, and form many alcohol-alcohol bonds, making it much harder to purify the alcohol further through a second distillation.

In fact, in a water-alcohol solution with an ABV% of 95.63%, sees the number of alcohol-alcohol bonds becoming so high that the alcohol becomes difficult to vaporize as water molecules. In fact, they will have the same volatility and no more separation of the alcohol becomes possible through standard distillation methods.

Solutions like these are called Azeotropic Solutions.

When using Pot Stills and Alembic Stills, and doing Double or Triple distillations, it is usual practise to dilute the distillate obtained from the previous run first with water in order to allow easier separation during the following run. 

The percentage you dilute too (and therefore the amount of water you need to add) is calculated based on the desired alcohol % at the end of the run. 

If you are using a Pot or Alembic Still, this makes it important to know the Reflux Angle of your still so you can utilise a Phase Diagram in order to do this calculation. More on that in a future article.

With Adjustable Reflux Column and Fractionating Reflux Column stills however, this calculation is not necessary, as you can reach any purity target with any starting percentage.