Scotch Grain Whisky

Malting is the process of encouraging barley to germinate. The barley is steeped in water to increase its moisture content to over 40%. The barley is then transferred to a malting box, where the controlled conditions of moisture and temperature allow it to germinate - a process normally taking five to six days. During germination, barley produces large amounts of enzymes - natural chemicals used by the seed to convert starch into sugar. The barley begins to grow, and small rootlets appear. At this stage, the grain is called green malt.

As well as malted barley, whole cereals such as maize or wheat are used in the production of grain whisky. Cereals are selected for their starch content, normally in the range of 65% to 70%. At full capacity, the distillery uses around 3,000 tonnes of cereal per week in addition to green malt.

Because the enzymes in the green malt can only work on dissolved materials, cooking the cereal is essential to the starch and makes it soluble in water. To reduce cooking times, the cereal is cooked under pressure. The cooker is filled with water and milled cereal. Steam is injected directly into the mixture, and the pressure and temperature rise to 4.3 bar / about 150°C respectively. The mixture is held under these conditions for about 30 minutes. Finally, the cooked cereal, which resembles a thick porridge, is discharged under its own pressure to a holding vessel.

The mashing process converts the starch in the cereals into sugar using the natural enzymes in the green malt. The malted barley is mixed with hot water in a mash tun, to which the cooked unmalted cereals are added. The mash is complete when the temperature reaches 63°C, the optimum temperature for the conversion of starch to sugar. The sugary liquid created by the mashing process is known as wort. Each mash consists of 30 tonnes of cereal and about seven or eight tonnes of green malt. The wort is filtered, cooled and pumped to the next stage – fermentation.

In the Tun Room, the sugar solution (wort) is fermented by yeast into a weak alcohol solution called wash – which is why the vessels in the Tun Room are called washbacks. The total capacity of the North British Tun Room is around 10 million litres. We have 24 washbacks each of 200,000 litre capacity, plus 14 washbacks each of 400,000 litre capacity. During fermentation, yeast breaks down sugars to produce alcohol and release carbon dioxide. The process takes minimum of 72 hours, and produces wash at around 8% alcohol. Each washback is cleaned at over 80°C between use to ensure optimum hygiene conditions for fermentation.

Grain spirit is distilled in a Patent or Coffey still, each made up of an analyser and a rectifier. Unlike malt whisky, which is produced in batches, the production process for grain spirit is continuous. Four stills give us the capacity to produce the equivalent of 500 bottles of spirit per minute. Grain spirit is extracted from the still at a strength of 94.5%.

For spirit to qualify for the title of Scotch whisky, it must be matured for at least three years in an oak cask, in Scotland. The spirit arrives from Edinburgh to Muirhall in a road tanker at a strength of 94.5%. It is pumped into reducing vats, where demineralised water is added to reduce the strength to a customer specification before being filled into casks. Evaporation losses – poetically called The Angels’ Share – amount to around 2% per year. As the spirit matures in cask, it mellows and develops further flavour characteristics, along with its distinctive colour.