I was recently attempting to describe Glenlivet Nadurra to a lifelong Glenlivet 12 adherent, and when I remarked it had not been 'chill filtered', he stopped me with a quizzical expression. "What the hell are you talking about?" I opened my mouth hoping a coherent explanation of the chill filtration process would tumble out, but soon realized I didn't know what the hell I was talking about. Time to do some more scotch homework...
WHAT IS CHILL FILTRATION?
After cask aging is complete, the whisky is cooled down to between -10°C and 4°C. For years, American scientists have been working tirelessly to replicate this process without having to use the metric system. Once sufficiently chilled, fatty acids, proteins, and esters created during distillation begin to precipitate out of solution, and can be removed by a filter. Blended malt whisky is usually chilled to a colder temperature (-4°C) than single malt whisky (0°C) because of the former's lower concentration of fatty acids (due to the inclusion of grain whisky). If you need a deeper understanding of this so far, you might want to ask your high school chemistry teacher, or at least whomever wrote the Wikipedia article from which I am obviously paraphrasing.
The filter works by adsorption, causing adhesion of the unwanted particulate to the surface of the barrier. A Biore strip for your single malt. Barriers can made of metallic mesh, paper, or any number of microscopically porous membranes. Guinness has famously filtered their stout through isinglass (made from the swim bladders of fish) for over 250 years, but will discontinue the process next year in an effort to go vegan. Shorty afterward, they also plan to start taking Bikram yoga classes and posting their buckwheat 'meatloaf' hacks on your Facebook feed.
WHY IS CHILL FILTRATION?
Like your boss's cat-eyed wife with the dynamite rack, it all comes down to cosmetic vanity. Whisky less than 46% ABV tends to get cloudy when those same fatty acids, proteins, and esters begin to clump together at cooler temperatures (like when served 'on the rocks'). Removal of these impurities by chill filtration keeps your low alcohol malt from looking cloudy while you mistakenly drink it ice cold. I noticed this phenomenon last time I drank ouzo, going from crystal clear to milky white cotton when I poured it over ice (Greek magic trick - opa!). Anyway, at some point it was decided that cloudy scotch was ugly and must be stopped.
Whisky at 46% ABV and higher does not experience this clouding effect, and therefore has no 'need' for chill filtration. The only other reason to undergo filtering is to remove sediment or debris from the malt. It seems unlikely a barley husk would get through the evaporation and condensation processes of a pot still swan's neck and cooling coils twice, but apparently these things happen. I would more likely suspect bits of char dislodging from the bourbon cask as potential contaminants.
The chill filtration process is expensive and time-consuming, and with as much touting as a non-chill filtered malt gets on a whisky label, you would think no distiller would do this if they didn't have to. Those who do chill filter rarely advertise that fact, so I always assume it has undergone some filtering unless specified otherwise or the ABV is 46% or above.
(WHY) IS CHILL FILTRATION BAD?
Fatty acids, proteins, and esters, are also potent determinants of flavor. Fatty acids, although apparently repulsive when chemically isolated despite the luxurious richness of fatty foods, are said to be responsible for the characteristic bitter notes in coffee and beer (Forbes). The body craves protein as a source of glutamate, an essential amino acid associated with the umami flavor, not to mention the subsequent release of neurotransmitters associated with pleasure (Wired). Esters are known for imparting sweet, fruity flavors, most commonly the apple-tasting ethyl hexanoate. Unfiltered Belgian witbiers and German hefeweizens often taste like banana because of isoamyl acetate (compoundchem.com, also includes a link to an elaborate ester/flavor chart).
On top of these, there is a myriad of chemical compounds at work in your whisky: phenols (peat flavor), lactones (oak flavor), aldehydes (vanilla, nuts, spice), diaceytl (butter), phenylethyl alcohol (floral aroma), etc. Collectively, all of these byproducts of distillation are known as congeners. Removing these congeners by chill filtration should fundamentally alter the chemical composition of a single malt whisky, and therefore deprive it of its inherent complexity and full range of flavor. Well, in theory, that is...
MYTHBUSTERS
German engineer and master taster for Whisky.com Horst Luening conducted a 2014 scientific study to explore whether or not chill filtration indeed had a detectable influence on the flavor of scotch. I am fascinated by this experiment for two reasons. First, based on their criteria for participation, I actually qualify as a "connoisseur of high quality single malt whisky". That just made my day. Second and more importantly, the results show that the odds of correctly identifying a malt as chill filtered or NCF is essentially a 50/50 guess.
The study also found that as a group (111 German scotch enthusiasts in this case), there was a statistically significant preference for sherry or wine casked malts over those simply from bourbon casks, as well as peated malts over unpeated. The biggest head scratcher to me was that sherried whisky was subjectively rated higher when smoke flavor was present, but only those that had been chill filtered. Didn't see that one coming, did you? Read the study published in full on Whisky.com.
THE CHILLING CONCLUSION
Researching this was a crash refresher course in chemistry, but at least I have a better understanding of the mechanism and purpose of chill filtration. The verdict is still out for me as to whether or not this is as big deal as people make it out to be. One one hand, as a single malt purist, it is somewhat ingrained in my nature to be fundamentally opposed to the idea of removing flavor from whisky. I don't use ice, the clouding phenomenon doesn't affect me. Then again, I am faced with scientific evidence that tells me I can't tell the difference anyway, it's all in my head.
Bourbon less than 46% ABV is routinely chill filtered. Do they get online reviews denouncing its "chill castration"? Jim Rutledge, the master distiller at Four Roses, is on record as being in favor of it. A defining characteristic of 'Tennessee whiskey' (e.g. Jack Daniel's, George Dickel) is its filtration through maple charcoal, known as the 'Lincoln County Process'. Oddly enough, the only working distillery actually located in Lincoln County, TN is Prichard's, who ironically doesn't even employ the method.
Major brands of rum, gin, and tequila are often subjected to chill filtering. Vodka practically requires it. There are a few top shelf distillers of the clear liquors eschewing the process, especially at high ABV, but as far as I can tell the only consumers who seem to care are whisk(e)y drinkers. I suppose from now on, I'm going to look at chill filtration as the liquid equivalent of photo retouching. I can complain about it with mild righteous indignation without being able to actually spot it, but it probably won't stop me from perusing the SI Swimsuit Issue.
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