What Is Bourbon Whiskey?

I can’t think of a whiskey that is more closely associated with America than the bourbon… but I also can’t think of a whiskey with more common misunderstandings about how it’s made. So today, let’s take an in-depth look at this iconic American spirit and dissect how it gets created, step by step.


The legal definition for a bourbon is pretty straightforward:

“Bourbon whisky”, “rye whisky”, “wheat whisky”, “malt whisky”, or “rye malt whisky” is whisky produced at not exceeding 160° proof from a fermented mash of not less than 51 percent corn, rye, wheat, malted barley, or malted rye grain, respectively, and stored at not more than 125° proof in charred new oak containers; and also includes mixtures of such whiskies of the same type.

27 CFR § 5.22 (B)(1)(i)

It’s a short but simple template. What makes bourbon interesting is the variations on a theme — the choices that distillers make in the production process which, in turn, creates different flavors and characteristics.

Raw Materials

In either the Scotch or the Irish tradition, malted barley is commonly the principle ingredient. But bourbon is different, and that’s all thanks to a little bit of science. (And exceedingly frugal distillers.)

Whiskey is a grain based spirit, which presents a small problem. Grains don’t actually contain any sugar — it’s a tiny seed that contains a store of complex carbohydrates that are intended to support a new plant as it starts to sprout and grow. But in order to make spirits, we use an organism called yeast to create our alcohol, and yeast only eats sugar. So before we can start making any alcohol, first we need a way to convert the complex carbohydrates in the grains into simple sugars for the yeast to consume.

This is usually where malted barley comes in. Barley is a grain like any other, and by moistening it and allowing it to partially sprout (called “malting”), the seed creates enzymes that break down complex carbohydrates into simple sugars. These enzymes are then combined with the rest of the grains (including unmalted ones) and cooked, which releases the sugar we need for the rest of our process.

Back in the day, this malting process was the only way for distillers to reliably get the enzymes needed to convert carbohydrates into sugars, which is why those older traditions of scotch and Irish whiskey relied heavily on them for their spirits. In fact, the flavor profile of malted barley became so synonymous with those styles that their respective governments legally mandated the use of malted barley going forward.

Bourbon came on the scene much more recently — whiskey in the United States was only legally defined in 1909 when President Taft clarified some provisions from the 1907 Pure Food and Drug Act. Starting in 1862, lab produced enzymes were available and quickly became cheaper to use than the time consuming process of malting barley, which led to many American distillers using them nearly exclusively.

No longer tied to using malted barley, American distillers decided to start turning towards the cheapest and most widely available crops for their grains. Here in the United States, the native corn plant became wildly popular with farmers and as a result became the favorite choice of distillers.

All of these developments led to the codification of bourbon being a grain-based whiskey in which at least 51% of the grains are corn. That left a ton of room for distillers to make choices about what else to throw in the mix, from malted barley to quinoa, all of which is thrown together with these lab grown (exogenous) enzymes in a cooker to convert the complex carbohydrates into sugary liquid for the yeast to eat.


Now that we have a soup of sugary liquid and grain remnants, it’s time to start the fermentation process. This is where yeast (a single cell organism) eats the sugar in the solution and produces ethanol (alcohol), carbon dioxide, and heat.

Before we add the yeast, there’s something unique that many American bourbon distillers will do, and that’s to add something called “backset” into the mix. We went over this process in depth in our What Is Sour Mash Whiskey article, but the general idea is that yeast isn’t the only thing that wants to eat the sugary liquid we produced. Bacteria also want to get in there, which can cause unwanted flavors or reduce the amount of alcohol we get out of our mix.

“Backset” is the leftover remnants from a previous distillation run and can be particularly acidic. Which is fine for yeast — they can survive in an acidic environment, and will even create their own — but it’s terrible for bacteria. Distillers will add as much as 30% of the backset from their previous distillation runs into their newly created mash and then add the yeast of their preference.

Typically, distillers will use cultured lab-grown yeast for their fermentation process. These cultured yeasts are grown and selected for their ability to create alcohol and for their specific flavor profiles, ensuring that the flavors are consistent from one distillation run to the next, but there is no restriction on using wild (ambient) yeast for distillers.

Often, distillers will have a specific yeast strain that they prefer, something that creates just the right flavor profile that they want for their whiskey and will stick with that formula. The combination of those factors — grain bill, yeast formula, and whether or not backset is used — is called a “recipe”.

Fermentation will typically take a couple days and once complete what we end up with is a relatively awful tasting and very chunky beer that, appropriately enough, is called “distiller’s beer.”


American whiskey is all about innovation, and that continues through to distillation. Where Irish and Scottish spirits are best thought of as coming out of a traditional pot still, American distillers will often turn to a column still, or a “continuous still”, for their distillation needs.

“Nancy” – the eponymous column still at Still Austin

With a traditional pot still, you can only distill what’s in the pot at the start of the run. There’s no way to add more liquid and keep going. It also doesn’t do a very efficient job at distilling the alcohol, only increasing the ABV or Alcohol By Volume of the liquid at the end of the first run to somewhere in the 35% ABV range. Which means you need multiple runs in the pot still to get the quantity and quality you’d need, especially with a bourbon.

A column still solves both of those issues.

The overall design of a column still has multiple copper “plates” stacked one on top of another in a single very tall column. I’ll get into it in detail at some point, but the simple way of thinking about it is that each plate is basically its own pot still. Some of the liquid will condense on the pot, get re-vaporized with the addition of steam heat at the bottom of the column, and continue to move up the column with a higher alcohol content. And those portions that are undesirable typically move down the column until they hit the bottom and are removed (this is where we get the “backset” for sour mashing).

This re-distillation process using plates means that the only limit to the alcohol content of your spirit is the number of plates in your still. Add more plates and you can distill a higher proof spirit.

As the still runs, if the inputs and outputs remain constant (the same flow rate, the same temperature, the same consistency, etc) then the distiller will start to see a consistent quality and alcohol content on the liquid that forms on specific plates and be able to capture that specific spirit. This process can continue indefinitely (hence “continuous still”) as long as new distiller’s beer and steam is added to the still.

One way that bourbon producers will ensure a consistent temperature for the still is by pre-heating some of the distiller’s beer as it is fed into the column. Usually, a device called a “beer heater” will be added to the tubing for the newly selected finished whiskey coming off the still, where tubes carrying the super heated vapor will be passed through a chamber holding the fresh and cold distiller’s beer. This not only heats up the beer, but also starts to condense the newly made spirit.

Something interesting here that I didn’t talk about is “cuts.” Because, for bourbon, you usually don’t see any. With a pot still, you can selectively remove the heads and the tails from the run, because those fractions boil off the mixture earlier and later (respectively) than the alcohol we want to capture. Since we’re using a column still, that same process won’t work. Whatever comes off the still at that point is what goes into the barrel.

The last problem to discuss here is that, with a column still, there’s a tendency to get as much alcohol content out of the still as possible. Distillers make money by the alcohol content, so getting a more highly purified stream of alcoholic liquid would theoretically mean higher profits at the end. But higher alcohol content also means there’s less character and flavor in the spirit (if everything is alcohol then there’s no room for anything else), which makes for a less delicious experience. This is a problem that the government decided to step in and fix by setting a legal distillation limit, so bourbon cannot be distilled to any higher than 160 proof (80% ABV).



By law, bourbon must be aged in charred new oak barrels. That means a barrel (watertight, but not airtight, container) that is made of oak (any kind, not just American oak), which has never been used before, and where the inside has been charred by fire.

There’s a lot of things this doesn’t specify, though.

First thing you might notice is that there’s no restriction on the size of the barrel. Barrel size actually is a somewhat important consideration, as smaller barrels provide more surface area per drop of bourbon for the whiskey to interact with and therefore will add more character and flavor more quickly. Similarly, large barrels will provide less character to the whiskey, but be more cost efficient to the distiller.

There’s also no restriction on time. By law, a whiskey that has been in a charred new oak barrel for as little as a day can be labeled as a “bourbon.” But that’s where the sub-categories come into play, as those actually do mandate a minimum time in the barrel.

  • Straight Bourbon means that the whiskey has been in a charred new oak barrel for a minimum of two years (four years if it doesn’t specify on the label).
  • Bottled-In-Bond means that all of the whiskey came from a single distilling season and has been aged for a minimum of four years.

There are other considerations to take into account as well, such as the temperature of the climate where the barrels are aged and the kinds of warehouses used, but now we’re getting into excruciating detail that even I (a self professed whiskey nerd) am finding overly detailed for this kind of an article.

Something interesting that’s happening more often these days with maturation is barrel finishing. This is a concept that was started with scotch whisky and seems to be catching on elsewhere, in which a fully mature whiskey is placed into yet another barrel with different or interesting characteristics for a period of time. The idea is that the whiskey will pick up some of the flavors and characteristics of these new barrels and add something interesting and unique to the flavor profile. Everything from maple syrup barrels to blackberry brandy barrels has been used to varying degrees of success.

One of the reasons why this is becoming popular is the explosion in “craft whiskey” and the dominance of MGP. Based in Indiana, MGP is a massive distilled products company that makes all kinds of whiskey which other companies can then use in their own products. With their high quality whiskey being so cheap, many companies simply take that MGP bourbon, slap it in a barrel for a few more years, and then sell it as a new product. In some cases, the results are fantastic, but in others it can be a pretty disappointing experience.

In any case, for a bourbon the whiskey can enter the barrel at no more than 125 proof. Just like we saw with distillation, the higher the alcohol content, the less flavor and character the spirit will have in the end. Some distillers go lower than 125, but capping the alcohol content at 125 means the whiskey drinking public can be sure to get a good flavorful experience in their bottle.


After maturation it’s finally time to put this whiskey in a bottle and send it on its way.

Sometimes you might see a “single barrel” expression of a bourbon, such as Blanton’s (as seen in the top image for this article). But more often what you’re getting is a blend of different barrels from the same batch of bourbon. Every barrel will taste slightly different, and blending allows the producer to create a more consistent and delicious flavor profile for their end product.

Good to note here is that there is no definition for the term “small batch.” So even though your bottle might claim to have come from a small batch of bourbon, their definition of “small” could expand to multiple warehouses.

Bourbon is also commonly “chill filtered” at the end of the process. When you create a whiskey, one of the byproducts of alcoholic fermentation is fatty acids and oils that are created by the yeast cells. These compounds can add flavor to the spirit, especially as they interact with alcohol compounds while aging in the barrel, but they can also make the whiskey appear cloudy or impure — especially when it gets cold. Chill filtration is a process where the whiskey is chilled, allowing the oils to precipitate out of solution and get skimmed off while the rest of the product heads to bottling.

The very last step in the process is adding some water to the bourbon to reduce the strength of the spirit. Not only does this make a batch of whiskey last longer, but it also can improve the flavor profile of the whiskey and make it much more interesting.

  • For standard Bourbon, the spirit must be bottled at 80 proof (40% ABV). Only water and caramel coloring can be added.
  • For Straight Bourbon, it must also be bottled at 80 proof but caramel coloring is forbidden.
  • Bottled-In-Bond bourbon must be bottled at 100 proof (50% ABV) and caramel coloring is also forbidden.

Now that you can decipher what some of those words on the bottle mean, it’s time to start trying out the different varieties and find a favorite! Even with all the requirements, there are plenty of variations and choices for distillers to make, meaning you can always find something unique and interesting in just about every brand you try. Check out our full list of reviews here — and happy exploring!


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