Pretty much every whiskey lover knows that without the addition of yeast to the cooked mash, all you would have is a pretty tasty, multi-grain porridge. After the starches in the corn, rye, and malted barley are converted to sugar by the enzyme amylase released from the malted barley, a multitude of single-celled fungi are added to the cooled mash. They proceed to gobble up those sugars, resulting in the production of alcohol and the release of carbon dioxide gas.

You can see the bubbles of CO2  in the fermentation tanks. Your tour guide will issue a friendly warning not to lean too far over the tank for too long, not because the gas is toxic, because it is not. But the cloud of CO2 has crowded out any oxygen above the tank. In short, you could pass out from lack of oxygen. (And wouldn’t you feel simply terrible about ruining a fermentation batch if you fell in?)

But more is being produced by those busy little yeasties than alcohol and carbon dioxide. One of the chemical by-products of the activity is a class of aromatic compounds called esters. These are the molecules that lend fruits and flowers their various characteristic scents. For example, bananas smell like bananas because they contain the ester pentyl acetate. Oranges contain octyl acetate. And apples get their odor from methyl butyrate. The essential oils of herbs and sweet spices also get their scent from esters. Rosmarinic acid is found in rosemary. Cinnamon oil’s is ethyl cinnamate.

The reason bourbons vary in their detectable aromatics is that even though all are fermented by the same species of yeast, Saccharomyces cerevisiae, each distillery uses its own proprietary yeast strain. Slight genetic variations within S. cerevisiae mean that different strains produce different esters.

For example, a lot of people can detect the scent of banana on the nose of Old Forester expressions. Peach is a commonly identified aromatic on the nose of Elijah Craig. Famously, Four Roses Distillery has five different proprietary yeast strains, variously characterized as delicately fruity, lightly spicy, richly fruity, floral, and herbal.

In short, if you change the yeast strain you change the whiskey.

Most of the major distilleries have laboratories where they employ microbiologists to propagate their yeast strains, which are stored in cryogenic freezers at minus 80 degrees Celsius. They are also usually stored at multiple sites in case of a prolonged power failure at one. Since all forms of life have a pesky habit of evolving over generations, yeast, which like other microorganisms can produce multiple generations in a matter of hours, not decades, the distilleries’ scientists also monitor the yeast populations for mutations.

Smaller distilleries simply buy their yeast from laboratories that specialize in developing and propagating different strains that produce different esters once they settle on the one with the aromatics they are looking for in their whiskeys.

Photos Courtesy of Maggie Kimberl

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