Yeast

A leavening agent used in doughs and batters. It usually comes in a dry, bead like form (which can be compressed into cakes), although it also exists in a fresh form. It is best activated at a temperature of 110 degrees F to 115 degrees F (the temperature of a baby bottle or a comfortable bath). Anything too cold won't activate it, too hot will kill it.

Leavening agent (microscopic unicellular plant), used for to produce malt whisky added in the fermentation step.
The yeast ferments the sugar content in the malted barley
The Scotch malt whisky is made with malted barley and water, fermented only by the addition of yeast (zymase).

The preparation of a yeast culture, in a separate vessel (Yeast Propagator), before the final fermentation step.