Malt whisky is basically a distilled, un-hopped beer. Barley is malted to convert starch into fermentable sugars. To this end the barley is germinated (traditionally in floor maltings) and then kiln dried over peated fires. In the past, peat was used because of it’s availability, but now it is used primarily to impart flavour to the malt.
The malted barley is ground in a mill and is then referred to as “grist.” The grist is loaded into a vessel called a mash tun and is warmed with water to 64°C. At this temperature the enzymes present in the malt convert the remaining starches into sugar. The solution that is drawn off of the mash tun is referred to as “wort.” The wort is pumped to another vessel called the washback. Yeast is added to the wort to ferment the sugars, producing alcohol.
When the fermentation is complete, what is now called the “wash” is boiled in a series of two, or sometimes three, pot stills. Condensate is taken off of the first still and added to the second. This process concentrates the alcohol as it has a lower boiling point than water and will condense first. The liquid that comes out of the stills has an alcohol content of between 60 and 75% which is blended and diluted to an average strength of 65% before it is put into oak barrels for maturation.
The whisky will have a strength of between 50 an 60% alcohol after the losses that are incurred by evaporation in the barrel during aging. Before bottling the whisky is further diluted to 40 or 43% alcohol.