THE WHISKY PURIST'S NIGHTMARE
Chill filtration has applications in several beverage
categories and in many industrial processes. When it comes to whisky, chill
filtration involves chilling matured whisky to between -10⁰C and +4⁰C Celsius,
then filtering by adsorption (not absorption), which is the adhesion of
dissolved particles to a surface. In the case of whisky, these particles are
things like fatty acids and proteins.
The whisky is chilled, as this helps to precipitate (clump
together) the particles so that they can be easily filtered from the liquid.
Not every distillery chill filters in the same way, and in the winter months,
some distilleries even chill filter at the high end of the range (3⁰C to 4⁰C),
without actually chilling the whisky first. Temperatures around zero are
typical, with the higher temperatures being less effective in removing all the
fatty acids and proteins than lower temperatures.
Why Do Distilleries Chill Filter?
Not all distilleries chill filter their whiskies and many
that do, still have non-chill filtered releases in their range. Regardless on
your views of chill filtration, it is fair to say that there is a passionate
and growing demand for non-chill filtered whiskies. So if most distilleries
spend money to chill filter most of their whiskies when a growing group of
people are demanding that they don’t, there must be some advantages to chill
filtration to the distillery, as indeed there are.
In essence, whisky is a mixture of ethanol, water and just a
little bit of other stuff that contributes to the colour, aroma, mouth-feel and
flavour. That ‘other stuff’ includes a host of different chemicals including
esters, ketones, congeners, aldehydes, phenols, tannins, furfurals and many
more. A typical commercial whisky is approximately 40% alcohol (principally
ethanol), 59% water and 1% other stuff. But here’s the rub. Everything organic
contains something called lipids, which are also known as fatty acids or fats.
Some of the lipid content of the barley used to make whisky persists all the
way through gristing, steeping, fermentation, distillation and maturation, and
is found in the resulting whisky. They certainly aren’t a problem in terms of
health or calories, and their contribution to flavour and aroma is part of that
argument I don’t want to get into, except to say, plenty of people love to
drink whiskies with the lipids left in.
When whisky is above 46% alcohol at room temperature, there
is no issue with lipids. But if you add enough water, if you chill the whisky
or if you do both of these by adding ice to your whisky, something significant
happens; the whisky goes cloudy. This does not mean the whisky has gone bad or
doesn’t taste as good, it just means that it loses the classic bright golden
shine that is associated with whisky. People who aren’t familiar with this
phenomenon could be excused for thinking they have an inferior or flawed whisky
if it goes cloudy when they add ice. In short, lipids are removed through chill
filtration for almost purely aesthetic reasons. Whether chill filtering
noticeably changes the flavour or mouthfeel is a subject of much debate. You
can easily do some cloud formation experiments yourself with a bottle of
non-chill filtered whisky, preferably with an alcohol percentage between 46 and
50% ABV.
Experiment 1: Pour some of the whisky into a glass
and add twice as much pure water. You should see the whisky go cloudy fairly
quickly.
Experiment 2: Put the bottle of whisky in your
freezer. After a few hours of cooling, you will see that the entire bottle of
whisky has gone cloudy, even without dilution. Leave the bottle at room
temperature and the cloudy haze will slowly disappear, with no negative
effects.
Experiment 3: While your whisky is still cloudy from
the freezer in Experiment 2, pour some of it through a coffee filter paper to
replicate the chill filtration process, and then seal it in a smaller bottle to
prevent oxidation. Once the bottle and the chill filtered sample have both
returned to room temperature, you can do your own taste comparison.
It’s worth noting that just as each whisky has different
levels of esters, aldehydes or phenols, they can also have different levels of
lipids, and some will exhibit this behavior more strongly than others. For an
incredibly stark example of the same principle, try the same experiments with
some Greek Ouzo, which goes from clear to opaque white. Absinthe also gives a
very strong result.
So Why Does it go Cloudy?
The appearance and disappearance of cloudiness in whisky
comes down to the properties of lipids, the properties of the water-ethanol
mix, temperature, and something called micelles.
Diagram 1 - Representation of a Lipid
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Fig.1 |
The lipids in whisky are basically fats, and like most fats
they have a hydrophilic (water loving) ‘head’ characterised by an electrically
charged -OH group, and a hydrophobic (water hating) ‘tail’ characterised by one
or more long carbon chains (see Diagram 1). It is the dominance of these long hydrophobic
carbon chains that prevent oil (oil being fat that is liquid at room
temperature) from mixing with water. Ethanol, on the other hand is a slightly
stranger character. It also has a hydrophilic -OH group at one end and a carbon
chain at the other, but the carbon chain is very short. The charged -OH group
is therefore able to dominate the short carbon chain, allowing it to mix easily
with water. In contrast, alcohols with longer carbon chains than ethanol, like
hexanol, do not mix readily with water.
Fortuitously, the short carbon chain of ethanol is still
sufficiently friendly with the long carbon chains of lipids to allow them to
mix together as well. So, in a mixture of water, ethanol and lipids, ethanol ensures that everything is hunky dory. But if the ethanol drops sufficiently, there will no longer be enough of it to keep the oil and water mixed and they will separate. This starts to happen when the ethanol drops below the magic number of 46% ABV at room temperature. At lower temperatures, the party mood is dampened, and the oil and water will separate even with higher concentrations of ethanol. This is what happens when distilleries chill filter, and it is also
what happened in the freezer experiment above.
As the
lipids and the water stop mixing, the lipids form something called micelles. A
micelle is basically a spherical clump of lipid molecules, where the hydrophobic
carbon chain ‘tails’ all point in to the centre, away from the water, while the
hydrophilic ‘heads’ all point outwards towards the surrounding water (see Diagram 2).
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| Fig.2 |
Though these clumps of lipid molecules are still tiny, when there are millions
of them scattering light in the same
glass, the result is a cloudy suspension of solid particles in a liquid, known as a
colloid. Incidentally, the cell walls in a human body are constructed in an
almost identical way.
Animal cell
walls have an outer layer of lipid molecules, with the hydrophilic heads
pointing outwards, and a reversed inner layer, with hydrophilic heads pointing
into the cell. The hydrophobic tails of the molecules in each layer point to
each other between the layers. Quite bizarrely, non-chill filtered whisky and cell
biology have much in common. Maybe that’s why whisky makes me feel so good!!
Does This Always Happen?
Some whisky lovers believe that if a whisky is bottled at
say 43%, it MUST be chill filtered. Others will even go so far as to say that
chill filtered whisky is an inferior product, not worth drinking. I disagree on
both counts but I will only address the former, the latter being somewhat more
subjective.
All non-chill filtered whisky has some level of lipids, and
lipids will always contribute to cloudiness when the ethanol content is low
enough. However, as mentioned above, not all whiskies have the same lipid
levels, and cloudiness does not appear en-mass when whisky first drops just
below 46%. The length of the hydrophobic carbon tail (or tails) varies between
different lipids, and it is the length of this carbon tail that determines
their solubility in ethanol. Longer carbon tails make lipids less soluble, and
these lipids form micelles just below 46% ethanol. Others need the ethanol
concentration to drop further.
Experiment 4: Take a non-chill filtered whisky and
add just a little bit of water at a time, allowing time for the micelles to
form between each addition. You will see that gradually more water brings out
gradually more cloudiness, until you reach a maximum.
The magical 46% is not a switch that flicks cloudiness on
and off; it simply marks one end of the micelle forming range as each lipid has
its own critical micelle concentration. There are a number of non-chill
filtered whiskies bottled at 43%. This is low enough for micelle formation to
begin, but sometimes for it to be less than obvious. Nonetheless, putting one of
these whiskies side by side with a chill filtered whisky of similar colour
often reveals that the 43% non-chill filtered whisky is not as bright and
shiny, a fact that may not be obvious when it is observed alone. Alcohol
strength alone is usually not sufficient to determine whether a whisky is, or
is not, chill filtered.
For those who wish to delve deeper into Chill Filtration and whether it affects people or not, check out this experiment:
