ANNANDALE DISTILLERY IS COMMITTED TO RESPONSIBLE DRINKING

Chill Filtration - Sacrilege Or Saviour?

Copper - It's Effect On Single Malt Scotch Whisky Flavour

Small Casks - Why Don't We Use Them?

Making Single Malt Scotch Whisky is part science and part art. As a scientist, I rejoice in the artisan aspect of whisky-making and sincerely hope that the human senses and the human mind will forever be the final arbiter of sensory maturity and sensory excellence rather than some mechanical device. Nonetheless, over the past 50 years especially, science has revealed some of the mysteries of whisky making and exploded some of the unhelpful myths without, I hope, ruining the mystique and the majesty of Scotland’s Single Malts.

This section of Annandale’s website comprises a series of short Technical Notes on various aspects of Single Malt Scotch Whisky production, from barley through to the mature product. The topics are presented alphabetically, and the series will be expanded over the coming months.

I’m deeply grateful to the late Dr Jim Swan and to Dr Gordon Steele for educating me in the science of whisky making and to Mark Trainor (Head of Production at Annandale Distillery) for schooling me in the practical subtleties of making the very finest Single Malt Scotch Whisky.

Professor David Thomson

Co-Founder – Annandale Distillery

When whisky is chilled, either through storage at low temperatures or the addition of ice in-glass, it may form a cloudy haze. This is considered undesirable by some whisky drinkers, although not all. Chill filtration removes the compounds responsible for haze, leaving the whisky crystal clear.

To understand how haze is formed, and subsequently removed, it’s necessary to know something about two types of chemical compound formed in whisky; fatty acids and esters.

Fatty acids

Yeast and lactic bacteria (to a lesser extent) produce fatty acids during the fermentation of wort (the sugary solution produced when starch is converted into sugar during the mashing of barley) into wash (alcoholic beer-like liquor). Fatty acids, along with another substance called glycerol are the principal constituents of fat. The ‘body’ of a fatty acid is formed from carbon atoms linked together in a chain. These chains of carbon atoms can be short, long or somewhere in between.

Esters

Esters are formed when a molecule of alcohol and a molecule of acid react together (known as esterification). Esters are fruity in character and make a significant contribution to the sensory characteristics of Scotch whisky.

Two basic types of ester occur in Scotch Whisky: Acetate esters formed by esterification of acetic acid and alcohol, and ethyl esters formed from ethanol and a fatty acid. As ethanol (ethyl alcohol) is the principal alcohol formed during brewing, ethyl acetate is the most commonly occurring acetate ester in whisky. This has a characteristic sweet, pear drops smell. As most of the alcohols formed during fermentation have short carbon chains, the corresponding acetate esters also have short carbon chains and therefore are not greatly implicated in the formation of haze. However, because ethyl esters are formed from long-chain fatty acids, they can be quite large molecules. As the chain length of a fatty acid or an ester increases, volatility decreases. Consequently, fewer of these larger fatty acids and esters will make their way into the distilled spirit. Nonetheless, some do, and this is how the problem of hazing arises.

Micelles

Molecules of long chain fatty acids and esters have polar (charged) and non-polar (non-charged) entities, due to the nature of the various different functional groups that form part of the molecule’s structure. Polarisation causes the molecules to clump together to form structures known as micelles, where the non-polar (neutral) parts of the molecules aggregate towards the centre of the micelle and the polar (charged) parts project out into the solution. As the proportion of water in whisky is increased (the critical point seems to be about 46% alcohol by volume), the micelles begin to come out of solution and are precipitated. It’s these precipitated micelles of long chain fatty acids and esters that form haze in whisky. Lowering the temperature of the whisky further reduces the solubility of the micelles which exacerbates the problem of haze. Consequently, adding a few chunks of ice to a non-chill filtered cask-strength whisky could reduce the ABV below 46% and chill the liquid sufficiently to cause the micelles to come out of solution and cause haze. The most important esters in haze formation are ethyl laurate, a C14 (14 carbon chain) molecule, and ethyl palmitate and ethyl-9-hexadeanoate, both of which are C18 molecules. Removing these could potentially diminish the flavour of the whisky, albeit to a very modest extent

Process

As the name suggests, chill filtering removes haze from whisky by first chilling it (between ambient and -10oC) and then filtering it through a plate and frame filter containing filter sheets made of low-calcium kieselguhr (a type of diatomaceous earth). Chill filtering is actually a very mild process that removes about 80% to 90% of the fatty acid/ester micelles from Scotch whisky without causing any collateral damage.

To chill filter or not to chill filter?

With distilleries such as Annandale, that produce Single Cask, Single Malt Scotch Whiskies at cask strength, chill filtering is utterly pointless and not something we are likely to entertain. People that buy Annandale’s whiskies generally prefer truly authentic whisky and they’re unlikely to be put-off by haze. Indeed, for many, it is a hallmark of authenticity.

However, chill filtering isn’t quite the ‘evil’ that some people make it out to be, especially when implemented properly. By virtue of its very purpose, chill filtering will inevitably remove some of the fruity esters and this could potentially impact on the fruity character of the whisky, but probably only to a very modest extent. It shouldn’t remove colour. Nonetheless, as haze is construed as a fault or an impurity by some whisky drinkers, chill filtering is probably justifiable for mass market blended Scotch whiskies and vatted Single Malts……but not for Single Cask, Single Malt Scotch whiskies such as Annandale’s Man O’Sword or Man O’Words.

Rule of thumb

When buying Single Malt Scotch whisky, check the alcohol by volume (ABV) on the label. If the ABV is 46% or above, it probably hasn’t been chill filtered. If it’s below 46% ABV it almost certainly has been chill filtered. Most Single Malts are sold at either 40% ABV or 43% ABV, the latter in the USA and duty free. These will have been chill filtered. Most Single Casks Single Malts are sold at cask strength (typically 55% - 61% ABV, depending on age) and will not have been chill filtered (as with Annandale).

The choice is yours!

I always buy non-chill filtered whisky and add a small amount of water and/or ice to reduce the ABV to approximately 35%. Watching the whisky form a haze is, for me, part of the sheer pleasure of drinking Single Malts.

David Thomson

Co-Founder – Annandale Distillery

Acknowledgement:             Dr Gordon Steele

Copper pot stills are the iconic distillation vessels found in all Single Malt Scotch Whisky distilleries. The choice of copper as a construction material for the stills and condensers was initially governed by its malleability, which makes it relatively easy to form into the complex shapes, and by its ability to conduct heat efficiently. The fact that it happens to play an important role in spirit quality is a happy coincidence. This positive effect on flavour is attributed to the ability of copper to reduce the level of sulphur containing compounds in the distillate and to the act as a catalyst in the formation of esters (flavour compounds that contribute positively to the flavour of Scotch whisky). Surprisingly, given the importance of copper, details of how it influences whisky flavour are not fully understood. However enough is known for distillers to appreciate the importance of copper during distillation, in terms of determining and refining spirit flavour and also in terms of the consistency of spirit quality.

Sulphur containing compounds, at high levels, are generally considered to give the spirit unpleasant, undesirable odours often described as vegetable, rotten egg, grassy, rubbery or struck matches. Importantly not all sulphur compounds are undesirable; at lower levels they can make a positive contribution to the complexity of the whisky improving the feintyness*, and meaty aromas. (*’Feints’ is the name given to the third fraction of distillate produced in the spirit still. It has a characteristic smell and flavour {‘feintyness’} which can be desirable, but only in very small amounts. Excessive feintyness is very unpleaant.)

The most common sulphur based compounds in whisky are dimethyl sulphide (DMS), dimethyl disulphide (DMDS) and dimethyl trisulphide (DMTS), although there are others. All three of these sulphur compounds have an undesirable odour. They are formed when methanethiol reacts with hydrogen sulphide. These compounds derive from the amino acids methionine and cysteine, respectively, which ultimately derive from the yeast and barley used in fermentation.

Copper is involved in both increasing and decreasing the levels of sulphur compounds, although it is the latter that’s most important. Copper salts, which are produced by corrosion of copper on the inside surfaces of the stills, promotes the formation of DMTS from methionine. Conversely, native (i.e. uncorroded) copper lowers the concentration of DTMS. Perhaps the simplest way in which copper removes undesirable sulphur compounds is by absorbing them onto the inner surfaces of the stills and condensers during distillation and subsequently releasing them when exposed to air as the distillation equipment is emptied at the end of the process. Of greater significance, sulphur compounds are also reduced via chemical reactions that transform them into less flavoursome substances or by forming complexes with copper which don’t ultimately distil over into the new make spirit.

The role of copper in the formation of esters is not well understood. The majority of esters in Scotch whisky are produced by yeast during fermentation. They are formed when alcohol (principally ethanol in this case) reacts with carboxylic acids (principally fatty acids but also acetic acid) and impart flowery and fruit flavours to the whisky. Copper is a catalyst for ester formation which occurs when the reactants meet native copper on the inner surfaces of the stills and condensers. It is well known that after cleaning the inside of copper stills and condensers, undesirable flavour changes may occur until a patina has built up on the copper surface which changes the properties of the copper as a catalyst.

It is important to recognise that while copper plays a crucial role in influencing the quality of new make spirit, flavour reactions can only occur when the liquid is in contact with the copper. Thus, it is important during the short period of copper exposure to maximise the desired effects of reducing sulphur containing compounds and esterification. This is achieved via process management techniques and by optimising various distillery design features. Process management parameters include distillation time, reflux rates (the average number re-condensation - re-volatilisation cycles achieved before the spirit leaves the still), boil rates, cut points and condenser temperatures, all of which control the amount of time the liquid remains in contact with copper. These parameters are used to optimise new make spirit flavour and thereafter, by adhering to the optimised recipe and process conditions, ensuring production of a consistent spirit with the desired favour profile.

Distillery design, particularly the design of the copper stills and the copper innards of the condensers, is fundamental to spirit quality (as described above). Once built, making changes is prohibitively expensive so, from the moment of conception, it’s important to design the stills and the condensers to produce the desired style of spirit. Equipment designed to give enhanced contact with copper tends to produce lighter, fruitier spirit. Lesser contact with copper yields spirit which is heavier and meatier in character. Tall stills with long lyne arms (the wide pipe that connects the top of the still to the condenser) and tube and shell condensers, all will give lighter, less sulphury and fruity spirits.  In contrast, short stills with traditional worm tub condensers which give more complex spirits with a heavier, meatier character. Using twin spirit stills to distil the same volume of spirit (as with Annandale Distillery) produces increased copper contact because the surface area to volume ratio reduces as still size increases. Consequently, two smaller stills will give greater copper contact versus one big still with the same combined volume.

The location of copper in the distillation apparatus is also important for reducing sulphur compounds. Interestingly, there are opposite effects in the wash still and spirit still. With the wash still, the copper pot is least effective in reducing sulphur compounds whilst the copper innards of the condenser have a much greater influence. The opposite happens in the spirit still, where the pot has the greatest influence and the condenser has the least. Consequently, the most effective sections of the copper distillation apparatus are the wash still condenser and the spirit still pot. It is noteworthy that these tend to be areas of higher copper corrosion, possibly due to a relatively acidic environment. Higher acidification also facilitates greater removal of sulphur compounds. It may be that the environment in these sections of the distillation apparatus makes them relatively susceptible to acid corrosion which in turn improves their ability to remove sulphur compounds.  Once again having more but smaller stills (as with Annandale) will improve these processes by effectively increasing the surface area to volume ratio. It’s noteworthy that distilleries renowned for their light fruity spirit (e.g. Glenfiddich) have chosen to increase the number of stills rather than the size of the individual stills, when increasing production capacity.     

David Thomson

Co-Founder – Annandale Distillery

Acknowledgement:             Dr Gordon Steele

We’re often asked why we don’t mature our whisky in quarter casks (125 litres), octaves (50 litres), and firkins or blood tubs (~30-40 litres). (200 litre ex-bourbon barrels are the smallest casks we fill at Annandale Distillery.)

To address this question, we need to consider the role of oak in whisky maturation:

Oak adds to maturing whisky as well as taking something from it. If the cask has previously held bourbon, Scotch whisky, sherry, wine, port, rum, cognac, etc., some of this maturing liquid will have become impregnated in the staves (staves are narrow lengths of wood with chamfered edges that form the body of a wooden cask). This liquid remains, impregnated in the stave after the cask is emptied. When new-make spirit is filled into the emptied cask, some of the previous occupant will be extracted from the staves, adding flavour and colour to the maturing liquid, often quite quickly.

The maturing spirit also extracts various naturally compounds from the oak, including vanillin (vanilla/sweet/confectionery flavour), tannin (astringency and golden brown colour – as in black tea) and various sugars (principally xylose but also arabinose, galactose, ribose rhamnose and glucose).

Whisky casks are usually charred inside. Charring has a three-fold effect: 1.) It produces a layer of carbon on the inner surface of the stave. Carbon is very porous, which allows it to adsorb (as opposed to absorb) various compounds from the maturing spirit. Some of these adsorbed substances may be undesirable flavour and odour compounds. 2.) It increases the porosity of the surface of the stave which encourages deeper penetration of the spirit into the oak. 3.) It induces chemical reactions in the oak that are implicated in the conversion of hemicellulose (one of the principal structural components of oak) into the aforementioned sugars (although glucose arises from elsewhere).

The sum total of all of these effects is known as ‘cask effect’ (or sometimes ‘oak effect’).

Probably the most important consideration when choosing casks for whisky maturation, is the extent to which the distiller wishes the flavour of the final whisky to be influenced by cask effect. The nature and extent of the cask effect depends on three factors:

The ‘freshness’ of the oak (i.e. how often the cask has been filled previously and for how long). The fresher the oak, the greater its potency in terms of delivering vanillin, tannin, xylose and other oak components.

The nature of the previous occupant(s). Sherry is likely to have a fairly potent effect. However, if the previous occupant had been Scotch whisky, the impact would typically be much less.

The ratio of the inner surface area of the cask to the volume of liquid held within the cask. The smaller the cask, the greater the ratio of the inner surface area of oak to volume of liquid inside, as shown in the bottom row of the table below. The smaller this ratio, the bigger the cask effect.

Firkin - Volume/litre 40 | Surface area/square metre 0.82 | Volume/square metre (litre) 48.8

Barrel - Volume/litre 200 | Surface area/square metre 2.23 | Volume/square metre (litre) 89.7

Hogshead - Volume/litre 250 | Surface area/square metre 2.59 | Volume/square metre (litre) 96.5

Sherry Butt - Volume/litre 500 | Surface area/square metre 4.03 | Volume/square metre (litre) 124.1

By way of example, a 40 litre firkin has the equivalent of 48.8 litres of spirit per square metre of inner cask surface area (i.e. a ratio of ~50:1) whereas, with a 200 litre barrel, it’s 89.7 litres/square metre (~90:1). In other words, there’s much more oak contact in a firkin than in a barrel (x 1.84) and therefore the ‘cask effect’ is likely to be bigger. The difference between a barrel and a sherry butt is also considerable (89.7 versus 124.1 litres/square metre, respectively). There’s even a noteworthy difference between a barrel and a hogshead.

Make no mistake, cask effect is often used quite deliberately, as means of expeditiously bringing Scotch whisky to an acceptable level of sensory maturity, at an early age. There’s nothing wrong with this as such (in my opinion) provided the resulting whisky remains within the boundaries of what is an acceptable/credible Single Malt Scotch Whisky and provided the defining character of the distillery’s spirit is still evident. (These are the criteria applied at Annandale Distillery). However, it’s totally unacceptable if some sort of cask effect (e.g. ‘finished in Madeira wood’) is presented as an intriguing adjunct, when the real reason is to mask bad, imbalanced, rough or otherwise immature spirt. Sadly, this happens. I’m always very suspicious when multiple cask effects are used to finish a whisky, especially a young whisky.

Amongst purists, there’s even a point of view that once-used ex-bourbon barrels (i.e. barrels that have only been used once to mature bourbon) exert too much cask effect on the flavour of the whisky. Whilst this point of view may be a little extreme, it’s certainly possible to ‘over bourbon’ Scotch whisky (again, in my opinion).

To summarise, at one end of the spectrum, the most benign maturation vessel for Scotch whisky (i.e. with minimum cask effect) would be a several-times-used ex-bourbon cask (ideally of known provenance). The whisky may take longer to mature but the intrinsic character of the distillery’s spirit will surely shine through (warts and all). During the first three years of production, Annandale Distillery laid down ~50% of its stock in several-times-used ex-bourbon casks with a view to longer-term maturation. The other end of the cask effect spectrum would be represented by a never-used-before, 40 – 50 litre cask made from virgin Spanish oak and conditioned for two years with either oloroso of Pedro Ximinez sherry. In this case, it’s entirely possible that the maturing spirit will be ‘over-oaked’ (i.e. excessive cask effect) before reaching the legal 3 year minimum, and any evidence of distillery character will have been overwhelmed.

At Annandale Distillery, we’re entirely confident in the quality of our new-make spirit*, so we don’t want to (and we don’t need to) obliterate its intrinsic character using excessive cask effect. That’s why we don’t mature our whisky in small casks.

(*We’re so confident about the quality of our new make spirit that we bottle it at 63.5% ABV and sell it as Rascally Liquor®. Why not give it a try?)

David Thomson

Co-Founder – Annandale Distillery