Single malt whisky offers the greatest variation and depth of flavour of all spirits, despite being made from just barley, yeast and water. A single malt whisky must be produced at a single distillery, distilled from malted barley and water. The mash must be processed at the distillery and converted into a fermentable substrate using only endogenous enzyme systems and fermented only with the addition of yeast. The spirit must be aged for at least three years and one day and have nothing added to it save for water and spirit caramel. Whisky must be distilled to no more than 94.8% alcohol by volume and bottled at no less than 40%.