Grain whisky was traditionally distilled from maize (together with malted barley), maize having been the least expensive grain, while also offering the highest level of starch (more starch means a higher yield of alcohol). However, most distillers changed to wheat in the 1980s, when EU benefits made wheat less expensive than maize, with wheat's slightly cheaper price compensating for a slightly lower yield of alcohol. Wheat divides into two types, hard and soft.
Distillers use soft wheat as it contains more starch than hard wheat (from which pasta is made).
Meanwhile, maize is still used by North British, which also reflects certain practicalities.
"The starch content of grain is influenced by annual weather patterns; therefore any variability in starch levels requires fine tuning to the process. Maize starch content is far more consistent year on year as it is less affected by weather than wheat; as a result each new harvest has little impact on the standard production regime when using maize," says Tommy Leigh, production director, North British Distillery.
The obvious question is whether the choice of grain influences the spirit. Maize can provide greater sweetness than wheat, but this also depends on whether the spirit is distilled to be richer (i.e. promoting character), or lighter (minimising character).
"The choice of ingredients doesn't affect the quality of the spirit, just the character. The difference between using wheat and maize is detectable but you can tweak the process to make them more or less identical, so it's how you use the production process that matters," says Douglas Murray, Diageo's process technology manager.
Malted barley typically accounts for 10 per cent of the total amount of grain being distilled (whereas malt whisky is distilled entirely from malted barley). Malting (germinating) the barley sees it develop enzymes which play a vital role (wheat and maize lack enzymes, not being malted). When the grain is cooked in hot water, starch cells in the grain burst, allowing the enzymes to move in and break the starch down into sugars, which dissolve creating a sugary liquid. Adding yeast ferments this liquid by converting sugars into alcohol, while also creating various flavours including esters (fruity notes). The alcoholic strength of the resulting liquid, the wash, is around 8-10% ABV.
The wash is distilled in a continuous still (made from steel) a method developed in the mid- 19th century. This is a continuous process, whereas malt whisky is distilled twice in separate batches using a copper pot still, the most historic method. Despite differences between both methods (and indeed differences in the design of continuous stills in each grain distillery), the principles are the same.
A continuous still comprises two linked columns, the analyser and rectifier. The analyser essentially 'strips' alcohol from the wash and establishes a broad flavour profile, while the rectifier increases alcoholic strength and enables the distiller to select specific characteristics for the spirit. This equates to the role of the first and second distillation when producing malt whisky.
The analyser is heated using steam, introduced at the base.
Meanwhile, wash pre-heated to around 70 degrees centigrade, or higher, enters at the top. As steam rises, wash descends through numerous perforations in copper plates spanning the column (this can mean 30-40 plates, with each perforation around 1 cm in diameter). Steam and wash meet on the plates, and the heat of the steam causes the wash to boil, 'releasing' alcohol vapours from the wash, which ascend the column accompanied by steam.
The temperature is higher at the base of the column and progressively cooler higher up, the significance of this being that different flavour compounds have their own volatility (boiling point).
Lighter flavour compounds are generally the most volatile, having a simpler molecular structure known as 'short chain,' typically comprising up to a couple of linked units. Consequently, lighter flavour compounds, including certain esters (fruity notes) evaporate at a lower range of temperatures than richer flavour compounds, such as some cereal notes. Richer flavour compounds have a heavier molecular weight, being either 'medium chain' (several linked units), or 'long chain' compounds (10 or more linked units), which require a higher temperatures to evaporate.
As lighter flavour compounds ascend the column the temperature is progressively cooler, but still sufficient for them to remain in a vapour form, and they reach the top of the analyser where a pipe conducts them to the rectifier. Meanwhile, relatively cooler temperatures see some richer flavour compounds condense on the plates and descend the analyser (all 'residue' exits through a drain at the base).
The analyser also lowers the level of sulphur compounds (created during fermentation) which span sweaty socks, meaty, vegetal notes. Copper absorbs sulphur compounds from the vapours, which explains why the plates are copper, with baskets containing copper also hanging in the analyser (copper plates and baskets in the rectifier replicate the process).
This significantly alters the resulting spirit, as sulphur compounds would otherwise mask the esters (fruityness) and prevent them from showing.
Vapours from the analyser (at around 50-55% abv) enter the base of the rectifier where it's hotter, with the temperature decreasing higher up the column.
This temperature differential, and the plates, play a more significant role in the rectifier, by separating and concentrating the desired characteristics in the vapours.
"The higher up the rectifier the lighter the spirit becomes, and the concentration of lighter components also increases," says Jane Millar, technical support team leader, William Grant & Sons.
As flavour compounds have specific boiling points, and the temperature varies from one plate to another, this means the vapours condensing on each plate result in a liquid with slightly different characteristics compared to the liquid condensing on neighbouring plates (the temperature differential between neighbouring plates is usually less than one degree centigrade).
Consequently, distillers select one particular plate, on which the alcohol that condenses comprises the desired characteristics for the spirit. Drawn from this plate, in liquid form, spirit exits the rectifier through a pipe.
The strength is usually around 94% abv, with regulations stipulating that spirit must be collected below 94.8% (malt distilleries typically collect spirit at around 70% abv, with this lower strength incorporating a broader range of characteristics).
The process is backed up by computerisation (as in malt distilleries), but technology can't replace the stillman.
"It's a very hands on role and the stillman has a lot of analytical checks and minor adjustments to make," says Jane Millar.
Douglas Murray adds: "The alcoholic strength of the wash can be slightly higher or lower than the usual figure, and if it was lower for example the stillman could increase the rate of wash going into the analyser to compensate for this." The spirit character tends to focus on fruitiness, but that still allows for variety and individuality.
"New make spirit can offer floral notes, various fruits including pear, apple, spices such as nutmeg and cinnamon, as well as cereal notes," says Richard Paterson.
Ian Palmer, general manager, Starlaw distillery adds: "Every grain distillery has a character, and it's easy to identify each one."
Distilleries Fact Box
Scotland has around 100 malt distilleries and seven grain distilleries: Loch Lomond, Cameronbridge (owned by Diageo), Girvan (William Grant & Sons), Invergordon (Whyte & Mackay), Strathclyde (Chivas Brothers), North British (a joint venture between the Edrington Group and Diageo), with Starlaw (owned by La Martiniquaise) the most recent, operational since 2010. Meanwhile, six grain distilleries have closed since 1980: Carsebridge (1983), Caledonian (1985), Garnheath (1985), Cambus (1993), Dumbarton (2002) and Port Dundas (2010).
Grain whisky is aged at least three years, the same minimum as malt whisky, and using the same types of casks. This generally means barrels previously used to age bourbon, which contribute vanilla sweetness to the character of the spirit, though Sherry casks are also used adding dried fruit richness. Different ‘fills’ of each type of cask (the ‘fill’ indicating how many times it has been used) create further variety, as each fill provides a milder influence on the resulting whisky, with casks typically used 3-4 times.
While more bottlings of grain whisky are becoming available, the vast majority of grain whisky is combined with malt whisky to produce blended Scotch.
Grain whiskies make an invaluable contribution to blended Scotch, not just for their flavours but also the way they interact with malts, seducing and bringing them into the blend as part of a loving union. No one has the right to dismiss grain whisky as being secondary,” says Richard Paterson.