What Are The Differences Between Sweet and Sour Mash Whiskeys?

Whether making bourbon, rye, wheat, or any other style of American whiskey, all distillers are faced with a choice between using sour or sweet mash. But what exactly are they, and what is the difference between the two?

First, a quick review of the initial steps in whiskey making. Milled grains are blended with water to create what’s called a mash. The grain element has natural sweetness, and into this mix goes yeast, which touches off fermentation to turn that sweetness into alcohol. The resulting beer-like liquid—the mash—is then distilled into whiskey.

Generations ago, distillers discovered the idea of adding something to the mash called backset—liquid strained from the previous batch. Whiskeys made without backset are sweet mash whiskeys, and those with backset are sour mash whiskeys. Why would anyone choose to add this strange acidic substance to a mash? Sour mash distillers use the backset to protect against bacterial contamination during fermentation, and to achieve consistency of flavor. But sweet mash distillers say leaving out backset gives them more control over a whiskey’s flavors and allows them to release great whiskeys at a younger age.

Sour Mash Is Most Prevalent

Sour mash whiskeys are by far the more prevalent style used by major distillers and some larger craft distillers, and are usually distilled on column stills; heavyweight brands like Jim Beam, Jack Daniel's, and Evan Williams. Many smaller distillers focus on sweet mash whiskeys, with Indiana-based Hard Truth and Kentucky’s Wilderness Trail, Kentucky Peerless, and Castle & Key being among the more prominent. Though all four distill on column stills, sweet mash is also the choice for most distillers using pot stills.

When backset is added to the mash, it helps it reach the right pH (level of acidity) for fermentation, which is ideally between 4.8 and 5.0 on the pH scale when using a sour mash, according to Greg Metze, Old Elk master distiller. “The main concept for using a sour mash for whiskey production is the prevention of bacteria growth,” said Metze. “Souring is accomplished through the addition of acid to the mash, which consequently reduces the pH. Utilizing backset stillage is the preferred method as it is a free source of acid, primarily lactic and acetic resulting from fermentation.” In addition to promoting clean fermentation, the sour mash process aids in consistency between batches of whiskey. The flavor of each new batch is influenced by the flavors in the added spent mash from the previous batch.

The trade-off, according to Hard Truth and Wilderness Trail, is control. Both Hard Truth’s master distiller Bryan Smith and Wilderness Trail co-founder and master distiller Shane Baker say sweet mashes give distillers complete control over the flavors imparted from fermentation, whereas sour mash whiskeys will carry the DNA from the prior batch due to that included backset.

The advantages of the sour mash process make it particularly useful for distilleries seeking absolute consistency to match an established flavor profile. While both Smith and Baker note differences in flavor between sweet and sour mash whiskeys, neither point to one method as superior. “I definitely can find identifiers when tasting whiskeys, but it’s really more of a differentiation between the processes.” says Smith.

While product consistency might be the name of the game, some distillers have taken an experimental tack by adding backset from a completely different style. New Riff Backsetter, first released in 2020, added backset from a previously distilled peated 100% malt whiskey to mashes of both rye and bourbon, creating distinctly different styles of whiskey.

While the specific origin of the sour mash process is unclear, it’s often attributed to Scottish-American chemist and distiller Dr. James C. Crow (of Old Crow fame), who rose to prominence distilling for Oscar Pepper at what’s now Woodford Reserve Distillery but once was known as Labrot and Graham. Crow, according to Kentucky historian Whitney Todd, “brought innovations to the production of corn whiskey by bringing scientific methods to the industry. These new production techniques helped make a higher-quality whiskey more consistently.”

But there’s evidence that Crow, along with Pepper, merely popularized it, with Cheri Daniels and Deana Thomas of the Kentucky Historical Society pointing to an 1818 sour mash whiskey recipe from Catherine Carpenter, one of the early women of American spirits. Consistency was the key to Crow’s success, however, and the methods attributed to him endure today as the mash of choice for most whiskey distillers, especially those producing on a large scale.

Sweet Mash Devotees

The sour mash process was an important step forward during the early days of American whiskey making, giving distillers better control over bacterial growth and allowing for greater consistency from barrel to barrel. Sweet mash whiskeys, first popularized by the pre-Prohibition rye whiskey distillers of Pennsylvania and Maryland, give modern distillers a more hands-on level of control in developing a whiskey’s flavors. “The sweet mash process gives us complete [creative] control over our batches from start to finish,” says Baker. “When we started Wilderness Trail in 2012, we immediately knew we wanted to use sweet mash. We understood the benefits and how it increased control of every batch’s flavor profile, created a softer taste, and also required less aging time to create a great product.”

Smith points to the softer distillate and greater complexity at younger ages as the key advantages of sweet mash whiskeys. “When Pat [Dr. Patrick Heist, co-founder of Wilderness Trail] was explaining from a microbiological standpoint how mashing affects the distillate, I was listening closely, but then they took us into their rickhouse and we tasted,” says Smith, recalling an early visit to Wilderness Trail. “That whiskey at 2 years old blew my mind. It was so complex and so soft at such a young age.”

There’s no scientific proof that sweet mash whiskeys are more complex at younger ages, but releases from many craft distillers, especially Kentucky Peerless, have received high marks for young whiskeys from our tasting panel, with one, a 2 year old rye, landing on our Top 20 Whiskies of the Year list in 2017.

As for the potential growth of harmful bacteria, both Smith and Baker say that’s a manageable challenge, addressed by sanitizing the tubs, pipes, and other equipment between batches. At Wilderness Trail, an area of focus is keeping track of what bacteria may begin to grow. “With our background in fermentation and our new state-of-the-art equipment, we’re able to not only keep bacteria contamination under control, but to culture and catalog each,” which helps identify potential future problems, says Baker.

While sweet mash whiskeys take up less shelf space at retail, Baker sees growing interest from distillers as whiskey drinkers reach for wider variety. For now, however, the new guard of sweet mash distillers stands out together.

Sweet Mash Whiskeys to Try

While there are many distillers producing sweet mash whiskey, here are three examples

93 points - Hard Truth Sweet Mash Barrel Strength rye (Batch 005), 46.5%, $40
Hard Truth offers an accessible entry point into sweet mash whiskeys. This 94% rye whiskey delivers cloves, spiced orange, cinnamon, and cracked pepper.

91 points - Kentucky Peerless Small Batch rye, 53.7%, $90
Peerless’s rye received accolades from our tasting panel for its fruity complexity and powerful peppery spice.

89 points - Wilderness Trail Small Batch Bottled In Bond bourbon, 50%, $53
This spicy and musty whiskey is another excellent value among sweet mash whiskeys, with a balanced palate of fruit, oatmeal cookies, and pepper.