Chill Filtration – What is it, why do it and does it make a difference?
You’re sitting outside on a chilly evening, the firepit is roaring, the skies are clear and the stars are twinkling away. You turn to your glass of single malt and notice, with a feeling of concern, that it isn’t the normal crystal clear, golden nectar, it’s cloudy!
What’s wrong? Has it gone off? Can whisky go off? (to all intents and purposes – no it can’t).
Actually, it’s normally a good sign as it means your whisky hasn’t been chill-filtered. Occasionally though, you get “irreversible floc” which causes cloudiness and is a flaw (we’ll talk about that later).
But what is chill-filtration? What does it do? Is it purely there to make the whisky look better, or does it have an impact on taste?
The Background – what’s in your whisky?
To understand what is going on we have to have a look at what compounds make up whisky. There are three key groups:
- The main ingredients of your whisky are alcohol (ethanol to be precise) and water. The combination of these decide the alcohol by volume (ABV) of your dram.
- Then there are several hundred chemical components that contribute to the taste, aroma and mouthfeel of your drink. This is what makes whisky so complex.
- Finally, you get to the culprits of the haziness, the various fatty acids that naturally occur in whisky (ethyl esters of long-chain fatty acids, and larger alkyl esters to be technical). These are mainly created during the fermentation of the yeast to make the alcohol, though they are also created during the maturation of the whisky in the oak casks.
The combination of these three groups impact not just the ABV, but everything else about your drink: the colour, flavour, aroma, taste, finish and mouthfeel.
How does this make my drink cloudy – and what IS “flocculation”?
Let’s start with flocculation (or “floc”) – not some weird ailment but the term used in chemistry to describe when soluble particles come out of a liquid and form sediment. It’s this process which makes your whisky cloudy in certain conditions.
In whisky you get irreversible floc and reversible floc. Irreversible floc is caused when small crystals of calcium oxalate form in the whisky and settle to the bottom. They are harmless (but not pleasant in your mouth) and are caused when oxalic acid in the oak barrels react with calcium in the water used to reduce the whisky ABV. It’s prevented by using demineralised water.
Reversible floc is what we are interested in today though and for this we go back to the fatty acids.
The issue here is that these fatty acids aren’t soluble in water and so if they are mixed with water they will remain undissolved and the water is cloudy. Add alcohol though and they will dissolve, the higher the alcohol level the more likely they are to dissolve. So, as the ABV in your whisky is raised, the more likely the fatty acids are to dissolve and your whisky become clear.
The final factor is heat, the warmer a solution is the more likely compounds are to dissolve in it. The simple example of that is your mug of instant coffee. Add coffee granules to boiling water and they’ll dissolve, add them to cold water and they won’t. The same happens with those fatty acids.
What IS going on?
It’s all about the combination of heat, alcohol content and fatty acids. As a general rule, if your ABV is 46% or over, then the fatty acids in the alcohol will stay dissolved at room temperature and your whisky will be clear.
But immediately the temperature drops, the fatty acids will drop out of the solution and your whisky becomes cloudy. That can be caused by something as simple as adding ice.
Warm the whisky back up, even just holding it in your hand will often be enough, and the fatty acids will dissolve again in the alcohol. And the higher the alcohol content the colder it has to get before flocculation happens.
Why is cloudiness a problem?
It’s the aesthetics basically. Customers spending a decent amount of money on their whisky don’t want it to be cloudy. Distilleries are very aware of this and are desperate to make sure the displays of their whisky in the shops aren't marred by being cloudy – customers simply won’t buy it.
As around 90% of whisky sold is blended, generally 40% ABV and often served over ice - the problem is real and having cloudy whisky could mean financially ruin for a distillery.
In 1933 the Distillers Company introduced chill filtration as the solution and by the 1970s it had been widely adopted across Scotch whisky production.
How does Chill Filtration work?
In simple terms, the distillers reduce the temperature of the whisky, the fatty acids come out of solution and form clumps and they are then filtered out of the whisky by forcing the whisky through using super fine filters under pressure. Different distilleries chill to different temperatures (between -10 and 4 degrees Centigrade, with most around 0 degrees), use different sorts of filters and push the whisky through those filters at different pressures.
With the fatty acids removed, your whisky won’t then go cloudy when the temperature drops.
Does it make any difference?
In malt whisky, both blended malt whisky and single malt whisky, being able to say that your whisky is “non-chill filtered” has become a key selling point. The argument is that by not chill filtering the customer gets a dram that is closer to the natural cask spirit.
As to whether it makes any difference, the discussion has been fierce for many years. It hasn’t been helped by the fact that there has been no published serious scientific study as to its effects.
Many blind tastings have been done over the years though, with the overall results being that, even for experts, it’s difficult to taste any differences.
The one common piece of feedback is that there is a noticeable impact on the mouthfeel (the fatty acids removed by chill filtration are the ones that contribute to this)
Well, there you are, chill filtration is used for aesthetic purposes to prevent your whisky looking cloudy.
But from a taste point of view, it appears to make very little difference whether you chill filter or not.