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HMRC internal manual

Spirits Production Manual

Technical guidance: Stages of production: Mashing

(Mashing and fermentation diagram at SPIR4270)

Preparation of the cereal involves ‘screening’ the grains to remove debris, weighing, then the cracking of the grains in a mill (?1) to expose the floury heart of the husks and so produce ‘grist’. The grist passes through the grist hopper (?2) before being mixed with hot water on its way into a large circular vessel called a mashtun (?3). (In grain and neutral spirits distilleries, the unmalted cereals are usually boiled to rupture the starch cells for easier conversion to sugar by enzymes). The hotwater provides the heat to activate the malt diastase which converts the starch to sugar. The sugar, being soluble, dissolves into the hot water to make a sweet liquor called ‘wort’.

The wort is separated from the spent grist by being strained through the false bottom ofthe mash tun. The wort is then passed through a vessel called an underback (?4) which acts as a balance to ensure that the mash tun does not drain too quickly, and is then cooled (?5) to a temperature which the yeast can tolerate before it enters the next stage of theprocess - fermentation.

Many grain distilleries do not separate the wort from the spent grist. Instead, they allow the unfiltered porridge like mixture of wort and grist to the fermentation stage. This system is known as ‘in grains’ fermentation.