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, «-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, however, 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 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 20min before allowing the mash to stand for 1 h. The worts will then be collected before addition of a second water (70°C; 2 m3 per ton) and collection of those worts, followed by waters at 80°C (4m3 per ton) and 90°C (2m3 per 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 30min. 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 particularly 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.
The stills used in the production of whisky are of two types: batch and continuous. 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
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 adjacent 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 opposite 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 distilleries, it is mixed with spent grains and yeast, whereas in malt distilleries, it is blended with grains and thence despatched for animal feed.
Hot feints recycle
Hot spirit vapour
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