The barley is at the base of all the process. The quality of the barley has a great influence on the quality of the end product.
The barley being used for the production of whisky is carefully selected. It is after all the basic ingredient which will determine the quality of the whisky which will be sold years later. This selection was traditionally the job of the manager of the distillery.
Most of the distilleries nowadays buy their malt in a malting plant (for economic reasons), this selection is done less and less by the distillery managers, but well by the persons in charge at the malting plant. However, the maltings must respect precise requirements from the distilleries, in order to let them produce their whisky properly, and on the same way year after year.
There is no legal obligation to use Scottish barley to produce Scotch whisky. Even if some producers would like to go back to the tradition, like Bruichladdich does, most of the distilleries are not concerned by the origin of their barley. The most important thing is the highest sugar content and the lowest price. The combination of those two elements is often the only criteria in the choice of a variety of barley. A great deal of the barley used to produce Scotch whisky is coming from England or South Africa. It is not excluded that GMOare used, but it is difficult to get evidences of that. Anyway, this would perfectly conform with the productivity logic. If genetically modified barley gives better harvests with a better sugar content...
Water is another of the most important ingredients in the making process of whisky.
The quality of the whisky depends on the quality and purity of the water. Water in Scotland is famous for its great purity. The difference in taste between the whisky coming from various distilleries is partly due to the quality of water used.
Water in the Highlands is often peaty, which gives it a brownish colour. Substances, deriving from peat, are carried by the rivers which water is used to make whisky, and contribute often to the original taste of scotch whisky.
But water is certainly not the only determining factor in the taste of a malt whisky. The manufacturing process is of course very important in the final taste of whisky. Water is used in several steps during the distillation process. First of all, it is mixed to the grinded malt in order to produce the wort. It is also used for cooling the alcohol leaving the still. Last but not least, water is used to reduce the alcohol at bottling.
Yeast (brewer's yeast, often mixed with culture yeast) will start the fermentation process.
The role of yeast is capital. The choice of the yeast is part of manufacturing secret of the distilleries.