Chill-filtration is an industrial process designed primarily to remove esters in whisky. These flavour compounds can form hazes and deposits when stored at low temperature.
At Bruichladdich we do not chill-filter any of our whisky, and here is the reason why:
The principal components responsible for a haze are the ethyl esters of lauric, palmitic and palmitoleic acids. These three esters are formed by reacting ethanol with the relevant fatty acid, and are soluble in alcohol, but insoluble in water. This is why Bruichladdich is bottled at the higher strength of 46% - any lower can result in cloudiness as a result of this insolubility. The addition of water in the glass to reduce the % alc. below 44% will result in a haze. The insolubility of ethyl esters in water tends to increase with molecular chain length, thus the ethyl esters of acids of shorter chain length (e.g. ethyl acetate, hexanoate, octanoate) do not cause haze problems in whisky.
Solubility is also dependent on temperature. When spirits with relatively high concentrations of esters are subjected to low or fluctuating temperatures, hazes can appear in the spirit. Ethyl esters can be reduced by refrigeration followed by pumping through a plaque filtration system - the process known as of chill-filtration. Whilst some esters are relatively minor influences on whisky flavour, chill- filtration also removes more flavour critical components, as well as fatty acids that count for the richer ‘mouth feel’ and ‘persistence’ of flavour.
There are around 100 individual flavour compounds, grouped in four categories, responsible for the taste, texture and aroma of Bruichladdich: esters, fusil oils, fatty acids and aldehydes. Esters are the largest group of aroma compounds (36) giving pleasant, very intense aromas. Esters of fatty acids are first formed by enzyme reaction of yeast during fermentation, and then the reaction between fatty acids and alcohol. Lightest aroma fraction are the fruit esters, the middle aroma fraction fortifies the lighter, while the high boiling fraction is the smallest part, and heaviest.
The most common, though not the most aromatic, is ethyl acetate: ethanol (an alcohol) oxidises toacetaldehyde (an aldehyde) oxidises to acetic acid (a fatty acid) which reacts with alcohol to become ethyl acetate (an ester). Ethyl acetate is present at 420 PPM. Maturation can increase the ester levels by 50%. This is commonly removed by chill filtration to prevent haze developing in cold temperatures or on the addition of water.
There are 22 volatile fatty acids present in whisky that account for various aromas in whisky. Acetic acid accounts for up to 95%, the remaining 21 being very pungent - Capric acid, the second largest compound, followed by caprylic acid, palmitoleic acid (higher if yeast present at distillation) and butyric acid - the most dominant aromatic. The level of fatty acids increases on maturing at least three fold to 0.32g/litre alc. These give the ‘persistence’, the length of bouquet, and viscous, richer mouth feel of natural whisky. These are often inadvertently removed by chill-filtration when tying to remove ethyl acetate.
Aldehydes are highly reactive, volatile aroma compounds obtained by the oxidation of primary alcohols. The aldehydes produced during fermentation are unpleasantly pungent, but they are reduced by 50% during distillation to around 80mg/litre. These flavour compounds can be distilled over and concentrated, mainly in the more volatile, first fraction (heads) of the distillate. The concentration level of these compounds in the distillate can be significant if the still has not been operated properly or their concentration were already high in the starting material.
These increase during aging though the micro-oxidation reaction of alcohol during maturation, where acetals are formed which soften the pungent aroma; 20% of aldehydes can become acetals. Acetaldehyde (oxidised ethanol - by far the most abundant alcohol) is the predominant aldehyde. Most of the alcohols in whisky can be oxidised to form an aldehyde. Further oxidation results in an acid (often referred to as a Fatty Acid). In the case of acetaldehyde, the acid formed is acetic acid. It is the reaction of an acid with an alcohol that yields an ester. Rectification reduces the level of aldehydes at distillation. Other aldehydes include aromatic congeners vanillin, ethanol lignin, syringaldeyde, coniferaldehyde, sinapaldehyde, and furfural – the ‘malty’ aroma. In the glass these react with air and water to produce bouquet. These are often removed by chill-filtration.
Fusel Alcohols are the higher aliphatic alcohols than ethanol, and produce aroma. They are produced through the metabolisation of nitrogenous compounds by yeasts. The term ‘fusel’ comes from an old German word "fuseln", which means "to bungle" and was applied to bad spirit. However, many off these compounds are collected as part of the distillate, imparting their own unique flavour complexity, but can be quite obnoxious if in excess. Their harsh attributes require extended maturation time to integrate and mellow, especially amyl alcohol. While not strictly ‘oils’ they have an oily consistency. Fusels predominantly consist of higher alcohols - a mixture of amyl alcohols, propanol and butanol, esters, fatty acids and some specific aldehydes formed from distillation. The non-volatility and oily nature of higher alcohols lies in their high molecular weight and structure. 4 main fusel alcohols, most pleasant is phenylethyl alcohol. Propanol, isobutanol, isoamyl & pentanol are the major contributors. The total content of fusil alchohol, rather than the individual quantities, determines the strength of the aroma, usually about 1g/litre in whisky – 100% more in Bourbon, 50% less in Irish whisky.
So if you want more flavour, don't chill-filter. We don't at Bruichladdich. We would rather a haze in the glass than a lose the flavour and texture created all those years ago during fermentation and ameliorated over years of maturation.