The chemical processes which take place during the manufacture of grain spirit are broadly similar to those which occur when malt whisky is made. Both the raw materials and the equipment are different, however. The mash from which grain whisky is made uses unmalted cereals - usually wheat or maize, it doesn't matter which, so distilleries can buy at the best price - together with a small amount of 'green'malt (barley which has germinated but not been kilned). Usually about 16% of malt is added: it has to be there to convert the starches in the other cereals into sugar, so it can be turned into alcohol by the yeast.
Mashing and Fermenting
The cereals are finely milled then cooked at high temperatures in a pressure cooker.The pressure is provided by the direct injection of steam, and the purpose of the cooking is to soften the husks of the grain and dissolve the starch into solution.The slurry thus created is cooled and transferred to the mash tun, where a measured amount of green malted barley is waiting, and the whole lot is mashed (i.e., converted into a sugar solution), as for malt whisky.
Oils in the maize
The wort, and any solids it contains, is drained off after an hour or so, cooled and pumped to a fermentation vessel, where yeast cream is added. Fermentation continues for forty-eight hours, during which time the sugar is converted into alcohol (at between 6% and 7%) and carbon dioxide.The wash for grain whisky does not bubble quite so vigorously, because of the oils in the maize. It is also lower in alcohol.