What Is a Sour Mash Whiskey?

There are some phrases you’ll see come up again and again with whiskey. Some of them are intuitive (like “single barrel”), others are a little more obscure. “Sour Mash” is one of those terms that appears on a wide array of bottles but isn’t always clearly explained or widely understood — so today I want to take some time to explain what it means and what effect it has on the spirit.

Let’s start from the beginning.

American whiskey is, by definition, a distilled spirit that uses grains as a raw material. While growing, plants create sugar through photosynthesis and the energy that they don’t use immediately is stored in the form of starch (a complex carbohydrate) which is most highly concentrated in grains and seeds. Distillers take those grains, grind them up, cook them, and add some enzymes to convert those complex carbs into simple sugars that yeast can eventually use and turn into delicious alcohol.

(For any sufficiently annoyed food scientists reading this vastly oversimplified description, that’s “modification”, “gelatinization”, and “enzymatic hydrolysis”, respectively. )

The problem we get into at this point is that yeast isn’t the only organism that wants to eat the newly created sugar — there are other things out there that want to get in on the action, with bacteria being one of the bigger problem areas. Bacterial growth in the sugary liquid can be helpful for creating interesting and complex flavors, but that’s really only helpful after the yeast has eaten it’s fill. Otherwise, we’re losing out on alcohol, which is how we make money.

Helpfully, though, yeast can survive in environments where bacteria can’t — specifically, environments that are more acidic. Which is where the “sour mashing” comes in.

Skipping ahead for a minute to the end of the process, once the distillation is run and finished, there is still something leftover. Not everything in the mash is converted into alcohol — and even then, not all of the alcohol is recovered during the distillation process. What’s left over is called “backset” in American whiskey parlance, and is a slightly acidic hodge-podge of compounds.

Some distillers just discard the backset. With a “sour mash” process, a portion of this backset is taken from the end of the previous distillation and added to the next fermentation’s newly cooked grains.

This accomplishes two things. First, the fact that the backset is acidic means that we’re making the mash a less hospitable environment for bacterial growth. Sure we’ll still get some, especially if the distiller chooses to do what’s called a “secondary fermentation” or a longer fermentation, but that bacterial growth won’t negatively impact alcohol yield as much as it would without that acidity.

What we also get is some blending of the flavors. Backset contains a lot of what are typically referred to as “heavier” compounds — things with a higher boiling point that are less volatile than alcohol and rarely (if ever) make it out of the still. Adding those compounds back into the mash allows them another chance to enhance the flavor of the whiskey, adding some character and complexity that otherwise wouldn’t be possible.

So, what can you expect from a sour mash whiskey?

From a distiller’s perspective, you should expect a better yield of alcohol from the mixture. But it also adds flavors that would otherwise be lost. Like most things it’s a choice each distiller makes, not a hard and fast requirement for the production of any kind of spirit. But given the effects it’s something that is a useful trick in the distiller’s bag.

Now, there are also other ways to achieve a similar result. (Some distilleries add off the shelf chemicals to increase the acidity of their mash, for example.) But the sour mashing process is a time honored tradition — so next time you see this phrase on a whiskey bottle, you’ll can appreciate the work and history behind it.

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