conversion is the process by which starches in grain are changed into fermentable carbohydrates. In order for this to happen, the cereal needs to be physically broken down in a mill and the starch gelatinized, making it available to enzymes. The enzymes convert the starch into sugars and dextrin, while proteins are broken down into amino acids and nitrogen-rich elements, which will be required during fermentation.
Only endogenous enzymes (contained within the grain) are permitted in scotch whisky, which is generally based on enzyme-rich barley malt. Exogenous enzymes (i.e., not contained in the grain) are permitted to be added in Canadian, American, and Irish whiskies, all of which are primarily based on raw (i.e., unmalted) grains (as is scotch grain whisky).
In malt whisky distilling, the conversion takes place in a mash tun. The malted barley is ground into grist, then hot water at 63.5° C is added. This gelatinizes the starch while the enzymes within the malt hydrolyze it. The liquid is then drained off as wort.
In scotch grain whisky production (much as with other whiskies that are not made primarily from malt), conversion takes place immediately after the main cereal component has been cooked. Should this grain slurry be allowed to cool, the starch can revert, making it impossible for the enzymes to work. It is then mixed with the malt slurry (making up 10 percent of the total mash) either in a stream flowing through a continuous tube convertor or in a conversion tank where it is held for up to 30 minutes at 62–65° C for the conversion to take place. This temperature is critical. Too low and full conversion will not take place. Excessive temperature will break down some enzymes and adversely affect the early stages of fermentation where they still continue their activity.
enzymes and mash.Buxton, Ian, and Paul S. Hughes. The Science and Commerce of Whisky. London: Royal Society of Chemistry, 2014.
Russel, Inge, and Graham Stewart. Whisky: Technology, Production and Marketing, 2nd ed. Kidlington, UK: Elsevier, 2014.
By: Dave Broom