Chill filtration (technical)

Many traditionalists just shudder to think that something is filtered out of the whiskey and thus removed before it comes from the barrel to the bottle. For them, every nuance, no matter how small, counts. This also corresponds to the company philosophy of the independent bottler Douglas Laing. “As natural as it gets!” Is the slogan. The family-run company from Glasgow has always bottled its whiskeys with high alcohol strength, always uncoloured and not cold-filtered. But what does cold filtration actually mean? What does it do? Does it really affect the aroma and taste of the whiskey, or is it just a cosmetic process to get a clearer or even more stable product?

In its natural state, whiskey has a tendency to form a haze and become cloudy. Over time, even light sediments can appear, which settle on the bottom of the bottle. The whole thing is a combination of temperature and ethanol content: the colder and the lower the amount of ethanol, the more cloudy the whiskey can become in the bottle. In a range between 40 and 46 percent by volume, it forms a slight cloudiness, even at room temperature, and there is hardly anything that can be done about it. Since around 90 percent of the Scotch whiskey sold worldwide are blends, most of which are not only bottled with a minimum ethanol content of 40 percent by volume, but are usually also served over ice, it is easy to imagine that this cloudiness would one day was commonplace. Consumers whose whiskey became cloudy in the bottle or in the glass did not find this spirit very attractive, to say the least. In the worst case, they would certify the whiskey to have a quality flaw or turn away from it altogether. To avoid this, the whiskey industry solved the problem with the help of cooling filtration, with Distillers Company Ltd. (DCL) took on a pioneering role around 1933. But it was the early 1970s that cold filtration was widely adopted by the Scottish whiskey industry.

What is chill filtration?

First of all, let’s look at the process itself. As the name suggests, after the whiskey has been diluted with water to drink strength and before being bottled to a low temperature, the whiskey is often cooled to around 0 ° C, sometimes even below, and the liquid is then passed through a fine filter. Tiny particles and suspended matter, which cause the whiskey to become cloudy due to cooling, are picked up and filtered out. The so-called cold stabilization time, i.e. the period over which the whiskey is cooled, can take up to a few hours. A typical filtering method used in the Scottish whiskey industry is plate and frame filtration, equipped with pulp, through which the cooled whiskey is passed at medium pressure. These filtration processes are not uniform in Scotland, but can vary in the choice of temperature, pressure and filter material.

Solubility of ingredients

Barrel-matured whiskey is normally a clear product, depending on the type and configuration of the oak barrel, straw, gold, amber and even mahogany-colored, “which consists mainly of the drinking alcohol ethanol and water. In addition, there are hundreds of different chemical compounds, some of them small Contain concentrations that contribute to the smell and taste profile as well as to the texture and mouthfeel of the spirit. Some of them are water-soluble, others in turn soluble in ethanol As a rule, the more water-friendly residues a chemical compound has, the more water-soluble it is, and the more fat-soluble residues there are in the substance (e.g. long carbon chains), the more water-insoluble or ethanol-soluble it becomes the solubility of a chemical verbi In addition, it depends on the temperature of the medium and decreases as the temperature drops.

Long-chain esters

In barrel-aged whiskey, among others Contain substances that can be composed of two chemical groups: acids and alcohols. Together, these two form a new class of compounds that draw attention to themselves with their pleasant and mostly fruity aromas: the so-called esters. On the one hand, these esters are formed by the yeast during alcoholic fermentation, but are also formed during the years of aging in oak barrels through the reaction of acids with alcohols. The acids are also products of the fermentation process, but are also formed during pass ripening through the oxidation of the alcohols with atmospheric oxygen. These acids can then react with alcohols in the whiskey, primarily with ethanol, because they are available in large quantities, to form these esters (in the case of ethanol to ethyl esters). Certain esters, i.e. those formed from long-chain acids, so-called fatty acids, are sparingly soluble in water at lower temperatures or lower alcohol contents, flocculate and cause the whiskey to become cloudy. The most important long-chain esters responsible for this haze formation are ethyl esters of lauric acid (acid with a chain of 12 carbon atoms) and palmitic acid (acid with a chain of 16 carbon atoms). An alcohol content of 45 percent by volume is the decisive limit for the precipitation of these long-chain esters at room temperature. Other compounds that are responsible for the cloudiness are high molecular fats and ethanol-soluble lignins from the oak barrel. The insolubility of ethyl esters in water increases with the length of the molecular chain, so that the ethyl esters of acids with shorter chain lengths (e.g. acetic acid , Lactic acid, butyric acid) do not cause turbidity problems in the whiskey.

Reversible and irreversible

Flocculation If whiskey is stored for a long period of time at cold temperatures, as can be the case during transport in winter, these fatty acid esters can no longer stay in solution, flocculate and cause cloudiness. However, this is reversible, i. H. it disappears again when the whiskey is warmed up. There is also an irreversible form of flocculation. This shows up as very small crystals of a lime-like compound called calcium oxalate, which slowly forms in the whiskey and settles on the bottom of the bottle when low amounts of oxalic acid in the whiskey react with similarly low concentrations of calcium ions from the water. The formation of these irreversible flakes can be largely suppressed by demineralizing the water used to reduce the whiskey to drinking strength and thus reducing the amount of calcium ions to a minimum.

Effects on taste?

The subject of cooling filtration is the subject of controversial discussions both in the whiskey industry and among consumers. While some are of the opinion that the long-chain esters are taste-critical components that count for the richer mouthfeel and the sustainability of the aroma in whiskey, others are asking whether possible taste differences are even perceptible to the human senses. In fact, there are no serious scientific studies on the subject of “cooling filtration and taste”, at least so far no meaningful results have been published. On the other hand, some blind tasting studies suggest that the taste is hardly influenced by the filtration process, even for experts. The microbiologist and master blender at Hiram Walker, Dr Don Livermore, is not critical of the effects of cooling filtration. “It is very difficult to take the filter residue from a sample and get an analysis of it because it deliquesces very easily. I have tried many times. I can give no concrete answer to what exactly forms, but from a sensory point of view we at Hiram Walker perceive the filtered residue as marginal at best. " Based on the tiny amount of filtered residue compared to the filter volume, the Canadian scientist calculates the removed amount of turbid substances in the ppb range (“parts per billion” corresponds to the amount of 1 microgram per kilogram). “So that someone can make a difference at ppb level can see, he would have to be extremely sensitive to this compound, which generally does not apply to the normal whiskey drinker. " For reasons of product clarity, all whiskeys at the Canadian company are cool-filtered before they are bottled. Those in charge at Douglas Laing act completely differently. “Single cask fillings without additional coloring and without cooling filtration enable us to offer the whiskey directly from the cask and in an unchanged form. When we cool filter we lose some of the unique properties associated with fatty acids and their esters that are in the whiskey prior to filtering. Very easily. So we don’t do it! “However, Chris Leggat admits, this decision is based on a very subjective point of view.” If we don’t cool filter, we believe we are keeping something in the whiskey. What we do know for sure is that you can With an ethanol content of 46 percent by volume in the non-chill-filtered whiskey compared to the 40 percent by volume in the chill-filtered product, ‘whiskey gets for its money’ is simply more, and that is certainly a bonus for the consumer. "