The Influence of “Terroir” in Bourbon Production

I’ve been doing some research on bourbon recently for a writing project I was hired to do. One question I wanted to answer was whether the idea of terroir (or some similar concept related to the “taste” of a place) plays a role in quality differences. My conclusion is that location does play an important role although it differs significantly from the role terroir plays in wine.

Basically there are five factors that influence the flavor of bourbon: water, the grain recipe, fermentation, distillation, and maturation in oak.

By law bourbon must be made from at least 51% corn and typically includes some rye, barley or wheat as well. The precise ratio of grains (called the mashbill) matters a great deal but I doubt it matters where the grain was grown as long as the plants were healthy. This is the main difference between wine and whiskey with regard to terroir. For bourbon, the main ingredient could be grown anywhere.

Water however is often mentioned in Bourbon lore. Kentucky distillers insist water resting on the limestone soils of Eastern Kentucky are ideal for making Bourbon. But as far as I can tell, they are inflating its importance. What matters is that the water used to make the mash be mineral rich, alkaline (i.e. high PH) and low in iron. There are countless places around the world that would satisfy those criteria. In any case you could add calcium carbonate to the water to elevate the PH. Water is important but I doubt it is water from a specific location that matters. Famed Kentucky producer Brown-Foreman uses treated municipal water for their celebrated Old Forester bourbon.

The strain of yeast used influences the flavor of bourbon. Although many producers use commercial yeast, some cultivate their own strains and take great pains to make sure it is consistent from year to year. But that is not quite the same as using whatever ambient yeast happens to be around in the environment as  some natural winemakers do. As it is with wine, this is an under-explored dimension of terroir. But again it isn’t obvious that location is the variable here.

The distillation process differs from distillery to distillery and differences in equipment can make a substantial difference. But this has nothing to do with location.

That brings us to the most important dimension of bourbon-making—the aging process. Depending on who you ask, 50-80 percent of the flavor of Bourbon comes from the barrel. By law, Bourbon must be aged in 100% charred, new oak barrels. The minimum aging requirement is two years for a spirit to be called straight bourbon.  In addition to the degree of char, the type and quality of the wood matters a great deal and bourbon producers are careful about where they source their wood. Thus, I suppose you could argue the terroir of the wood is a factor in bourbon quality.

But the source of the wood is not the most important factor in the aging process. The most important differentiator among styles of Bourbon has to do with the location of the warehouse as well as the location of the barrels inside the warehouse. Bourbon is almost always aged in warehouses without much temperature control. Thus, the ambient temperature influences how the bourbon ages. Barrels subject to more heat will have more oak character because heat advances the chemical reactions going on in the barrel. Whiskey aged in cooler areas will age more slowly and can be aged for a longer time. Hot, dry conditions will increase evaporation thus increasing the proof of the alcohol, cool or humid conditions will inhibit evaporation keeping the alcohol levels more moderate. This applies not only to the region of the country where the aging takes place but the location in the warehouse as well. The top floors are typically warmer than the bottom floors. Many producers have a “favorite floor” where they expect to get their best whiskeys. Of course, most bourbons are blends of many barrels, a process that would diminish the influence of warehouse location. (Perhaps that is an argument for more single barrel bottlings.)

This is a clear case where location matters, not because of soil characteristics but because of ambient temperatures during aging.