Whisky (spelled this way for Scotch, but as whiskey for Irish and other forms of the product) is a distilled beverage made from cereals and normally matured in oak. It is subject to a great deal of legislation and custom.
EU regulations state that it can be made from any cereal aided by starch-degrading enzymes with distillation to less than 94.8% ABV, with ensuing maturation in wooden casks of less than 700 L in volume for a period in excess of 3 years for sale at a strength in excess of 40% ABV. UK legislation dictates that Scotch whisky must be produced in Scotland, the enzymes must be entirely derived from malt and the only permitted addition is caramel. The United States, Japan and Canada have their own legislative peculiarities that will not be discussed here.
The major cereals used for the manufacture of whisky are barley, wheat, rye and corn (maize). Malted barley is employed as a source of flavour and enzymes, which are not only responsible for converting the barley starch but also that of adjuncts to fermentable sugars. The main analytical criteria for whisky malts are their diastatic power,α-amylase and extract, especially when they are being used alongside adjunct. The malts may be ‘peated’, that is, flavoured with the smoke from peat burnt on the kiln. Such malts are classified on their content of phenols.
Rye (Secale montanum) is quite widely used in Eastern Europe and former USSR, and is sometimes malted. Wheat (Triticum vulgare) has largely replaced corn in Scotch grain whiskies as the cost of importing grain from the United States became prohibitive and it is also used in some American whiskies.
However, in the United States, corn (Zea mays) is especially widely used.
Malt is essentially mashed as in the case for beers, with clear wort being important to prevent burning on the stills. Wort from unmalted grain, how-ever, is not separated from the spent grains because modern continuous distillation processes do not demand it. Fermentation and distillation are effected with all of the grain materials still present.
For malt whisky, mashes of water : grist ratio of 4 : 1 will be mixed in at 64.5◦C, the malt having been broken in a roller mill. Although modern malt Copyright © 2005 by Blackwell Publishing Ltd
distilleries are changing over to the use of lauter tun technology (cf. brewing, Chapter 2), traditional distillery mash tuns feature rotating paddles to mix the mash and these will be employed for approximately 20 min before allowing the mash to stand for 1 h. The worts will then be collected before addition of a second water (70◦C; 2 m3per ton) and collection of those worts, followed by waters at 80◦C (4 m3per ton) and 90◦C (2 m3per ton). The first and second worts are cooled by a paraflow heat exchanger to approximately 19◦C and diverted to a fermenter or washback. The third and fourth worts are pooled as part of the mashing water for the next mash. Unlike for the brewing of beer, there is no boiling of worts.
The initial processing in the production of grain whiskies is significantly different from that of malt whiskies. Indeed it is not unheard of for distilleries to work with unmilled grain, in which case prolonged cooking is a necessity.
For the most part, however, the first stage in production is the hammer milling of the cereal. The desire is fine particles that are readily extracted by water.
The cereal is mashed with 2.5 parts water (or recycled weak worts or ‘backset’, which is a portion of the stillage from the distillation process that has had its solids removed. The latter is felt to deliver yeast nutrients). The mash, typically at 40–45◦C, is agitated to ensure that there is no sticking together of grist (‘balling’). Some malted barley is likely to be included as a source of enzymes.
The slurry is now pumped to a cooker (pressure vessel) wherein the mash is mixed and injected with steam, to achieve gelatinisation of the cereal. The temperature will be raised to 130–150◦C and held there for a relatively short period of time. Mixing is essential to avoid charring and excessive browning (Maillard) reactions. The contents of the cooker are now discharged to a flash cooling vessel, the sudden fall in pressure being referred to as ‘blow-down’. The impact of this is to release any residual bound starch from the grain matrix.
The temperature falls rapidly to around 70◦C. The slurry is mixed with a separate slurry of malt (10–15% of the total grist bill) that may be at 40◦C, but alternatively may be at the conversion temperature for starch (65–70◦C). The malt enzymes then catalyse not only the hydrolysis of the malt starch but also that from the cooked grain. Food grade enzymes will also be added – and to some extent there may still be the use of green (unkilned) malt as a source of enzymes. Mashing will typically be for up to 30 min. Although the wort was formerly separated from the grains, this tends not to be done now in grain distilleries, and the whole mash contents are transferred to the fermenter.
There is no boiling, so enzymes can continue to work. Furthermore, it also means that the fermenter contents can be more concentrated than would be the case otherwise . The downside to this is the risk of fouling of stills.
Fermentation of whisky was formerly performed widely with the surplus yeast generated in brewery fermentations. However, specific strains particu-larly suited to whisky production have been developed and these are supplied by yeast manufacturers in bulk for commercial use. Hybrids emerged not only from the ale strain Saccharomyces cerevisiae but also from the ‘wild yeast’ Saccharomyces diastaticus, which produces a spectrum of enzymes fully
capable of hydrolysing starch to fermentable sugar. Thus, the distilling strains enable high alcohol yield. The strains may also be selected on the basis of their ability to produce esters.
Yeast is supplied either as compressed moist yeast, as ‘cream yeast’ (see Chapter 12) or, increasingly, as dried yeast. Quality considerations of the yeast (viability, etc.) are just as for brewing (see Chapter 2).
Fermentation on a small scale may be in closed wooden barrels, but on a larger scale, it will be in stainless steel vessels known as washbacks. Unlike in breweries, there is little temperature control during fermentation, other than to target the initial temperature, which may typically be in the range 19–22◦C. The temperature may go as high as 34◦C during fermentation, hence the need for ale-based strains rather than lager-based ones. Typically the fermentation is complete within 40–48 h. Some advocate holding a few hours prior to distillation in order to ensure that the endogenous lactic acid bacteria have an opportunity to enhance flavour.
Distillation
The stills used in the production of whisky are of two types: batch and contin-uous. Batch (or pot) stills employ double or triple distillation and generate a highly flavoured spirit. Continuous stills provide lighter flavoured spirits that are mostly employed in blending.
Pot stills are traditionally of copper, which may reduce the sulphuriness of the whisky (Fig. 6.1). The still comprises three major parts: the pot, which holds the liquid to be distilled; a swan neck and lyne arm; and a condenser.
The precise design of the swan neck/lyne has a considerable impact on the reflux pattern obtained and hence on the flavour.
The pot is heated either directly or indirectly. In the former case, an agitator may be present to prevent charring. Pots can be of diverse shapes, but in traditional Scotch whisky production, there are two stills: the wash still and the spirit still. All of the fermenter contents (the ‘wash’ will typically be 8% ABV) are transferred to the wash still and boiled for between 5 and 6 h to render a distillate known as ‘low wines’ which has an alcohol strength of 20–25% ABV.
This is subsequently transferred to a smaller spirit still. The spirit coming over from this can be divided into three components: the foreshots, the middle cut and the feints. The charge to the spirit still is a mix of foreshots and feints and low wines to a net alcohol concentration of less than 30% ABV.
The foreshots emerge first from the still, the feints last. They contain the undesirable highly volatile and least volatile components, respectively. They are recycled for re-distillation. The foreshots represent perhaps the first 30 min of the distillation and are collected in the feints receiver until the opening distillate strength of 85% has fallen to 75%. At this point, the spirit is judged to be potable and is collected in the spirits receiver. Collection proceeds for up to 3 h, with the alcohol dropping to 60–72% ABV. Thereafter the flow is
Condenser Water jacket
Sight glass
Swan neck Head
Water jacket Tailpipe
Siphon
Water Water
Water
Shell
Condenser tubes
Steam coils Manhole Lyne arm
Charging line Air vent
Fig. 6.1 A pot still.
diverted once more to the feints receiver and collection may continue until the alcohol reaches as little as 1% ABV.
Continuous distillation takes place in column stills, the most famous of which being that designed by Aeneas Coffey (Fig. 6.2). It comprises two adja-cent columns. The wash is preheated by passing it through the tube in the second column (rectifier). Thence it is fed into the first column (analyser) near the top and steam is passed in at the base of the column. As the wash falls, volatiles are stripped from it and these emerge from the top of the column, passing to the rectifier column. Alcohol separates from water at the base. The spirit is removed towards the top of the rectifier. The final cut is taken off from the base of the column. Foreshots (from the top) and feints (from the base) are recycled into the top of the analyser.
Inside the column is a series of plates with holes that permit the upwards flow of vapour. The plates are linked by downcomers that alternate on oppo-site sides of the plates such that the descending liquid is obliged to flow across each plate. After distillation, new distillates are diluted (e.g. to 58–70% ABV) before filling in oak casks.
The residue from the distillation process is called ‘pot ale’. In grain distil-leries, it is mixed with spent grains and yeast, whereas in malt distildistil-leries, it is blended with grains and thence despatched for animal feed.
Drain
Downcomer Plate
Steam Hot spirit vapour
Hot wash Exhaust
Cold wash
Hot feints recycle
Spirit collection
Rectifier column
Analyser column
Fig. 6.2 A Coffey still.
Whiskies are matured in oak casks. Whereas American bourbon and rye whiskies are put into new oak casks, Scotch, Irish and Canadian whiskies are filled into casks that have previously been employed for Bourbon or for sherry. For the most part they comprise 50 L butts. Whisky casks are either of American white oak (which are used for Fino and Amontillado Sherries) or Spanish Oak (used for Oloroso Sherry). The bourbon casks used for Scotch whiskies must be filled at least once with bourbon and the whiskies must have been in the cask for at least 4 years. Ageing of whisky in most countries must be for at least 3 years. There is a significant loss of alcohol by evaporation in this time, referred to as the ‘angel’s share’. In the maturation there is the development of mellowness and a decrease of harshness. Flavours associated with mature whisky are vanilla, floral, woody, spicy and smooth. The unde-sirable flavours that dissipate are sour, oily, sulphury and grassy. Various components are extracted from the wood, including those developed by wood charring. The major flavour components of whisky are listed in Table 6.1.
Usually the lighter bodied spirits generated on a continuous still are blended with a range of heavier bodied spirits coming either from batch stills or by dis-tillation to lower ethanol concentrations in column stills. In the decantation process, the various whiskies are decanted into troughs by which they flow
Table 6.1 Flavour constituents of whisky.
Main congeners Ethyl octadecanoate Acetaldehyde Ethyl octanoate Ethyl acetate 4-Ethyl guaiacol Isobutanol 2-Ethylphenol
Methanol 4-Ethylphenol
2-Methyl butanol Ethyl undecanoate 3-Methyl butanol Eugenol
n-Propanol Furfural
Furfuryl formate Other congeners Gallic acid Acetyl furan Guaiacol Benzaldehyde Hexadecanol
Butanol Hexanol
Coniferaldehyde 5-Hydroxymethyl furfural m-, o-, p-Cresol Isoamyl acetate
Decanoic acid Isoamyl alcohol
Decanol Isoamyl decanoate
Diethoxypropane Isoamyl octanoate Diethyl succinate Cis-Oak lactone Dimethyl disulphide Trans-Oak lactone Dimethyl sulphide Octanol
Dimethyl trisulphide Phenol Dodecanoic acid Phenylethanol Dodecanol Phenylethyl acetate Ellagic acid Phenylethyl butanoate 3-Ethoxypropanal Scopoletin
Ethyl butanoate Synapaldehyde Ethyl decanoate Syringealdehyde Ethyl dodecanoate Syringic acid Ethyl hexadecanoate Tetradecanoic acid Ethyl hexadecenoate Triethoxypropane Ethyl hexanoate Vanillic acid Ethyl lactate Vanillin Ethyl nonanoate 3,5-Xylenol
to a blending vat wherein they are mixed by mechanical agitator and com-pressed air. Then ‘de-proofing water’ is added to take the product to its final strength.
In Scotland, the final products may be a blend of whiskies from more than ten grain distilleries and up to a hundred malt distilleries. There is an astonish-ing interaction and cooperation between separate companies to enable this.
The blending is deliberately complex so that the unavailability of one or two whiskies in any single blending will not be noticeable. In other countries where there are far fewer distilleries, batch-to-batch variation must be achieved by varying conditions within the distilleries themselves – for example, the grist or the fermentation and distillation conditions.
Most whisky is filtered. Insoluble fractions, notably lignins and long chain esters of fatty acids, are removed by cooling to as low as−10◦C and filtration, typically in plate and frame devices with diatomaceous earth as filter aid.
Whiskey variants
Bourbon (United States) is made principally from corn (maize) plus added rye and barley and is aged in charred barrels. A close relative is Tennessee whiskey (United States), which is produced using a sour mash process. Canadian whisky (Canada) is a light product from rye and malted rye, with some corn and malted barley. Corn whiskey (United States) is from maize and is aged in barrels that have not been charred. Rye whiskey (United States) is from rye mixed with corn and barley and is aged in newly charred oak barrels.