Making corn mash for bourbon starts with cooking cracked corn in hot water to release its starches, then cooling the mixture so enzymes from malted barley can convert those starches into fermentable sugars. The process takes several hours from start to finish, but each step follows a clear logic: break open the corn, convert the starch, then let yeast turn the sugars into alcohol. Here’s how to do it right.
The Grain Bill
Federal standards require bourbon mash to contain at least 51% corn, but most bourbon recipes use 60% to 80% corn with the remainder split between flavoring grains and malted barley. A classic starting point is 70% corn, 15% rye, and 15% malted barley. Swapping rye for wheat produces a softer, sweeter profile, which is the approach behind wheated bourbons.
Malted barley serves a dual purpose. It adds flavor, but more importantly, it contains enzymes that convert raw corn starch into sugars yeast can eat. The measure of this enzymatic power is called diastatic power (DP). Standard two-row malted barley has a DP around 140, which is more than enough. You need a minimum DP of about 30 per pound of grain in the total mash to get full conversion, so most home distillers aim for at least 35 per pound as a safety margin. If your malt percentage is too low, the enzymes won’t fully break down all the corn starch, and you’ll leave sugar on the table.
Milling the Corn
Grind your corn to a coarse meal, not a fine flour. A particle size around 2 mm works well for most setups. Finer grinds expose more starch surface area, which speeds conversion, but they also create a thick, sticky mash that’s hard to stir and prone to scorching on the bottom of your pot. Coarser cracks take longer to gelatinize but are much easier to work with. A standard grain mill set to a medium-coarse crack, roughly the texture of coarse cornmeal, hits the sweet spot for home-scale batches.
Water Quality Matters
Kentucky’s reputation for great bourbon water comes from its limestone bedrock, which filters water to be naturally high in calcium and low in iron. You want to mimic that profile. Calcium stabilizes pH during mashing and supports healthy yeast activity during fermentation. Iron does the opposite: even small amounts cause metallic off-flavors, inhibit yeast, and create stability problems in the finished spirit.
If you’re on municipal water, carbon-filtering it to remove chlorine is a good first step. Your target mash pH is 5.2 to 5.6, which is the range where starch-converting enzymes work best. Most corn mashes land naturally in this zone, but if yours runs high, a small addition of citric acid or the backset technique described below will bring it down.
Cooking the Corn
Corn starch is locked inside tough granules that won’t dissolve in lukewarm water. You need heat to burst those granules open, a process called gelatinization. For dent corn (the standard variety used in bourbon), this requires bringing your water and corn mixture up to at least 190°F, and many distillers push to a rolling boil.
Start by heating your water to around 200°F, then slowly stir in the cracked corn. The mixture will thicken significantly as the starch granules absorb water and swell. Keep stirring to prevent scorching. Hold this temperature for 30 to 60 minutes, depending on how coarse your grind is. A finer grind cooks faster. You’ll know the corn is fully gelatinized when the mash has thickened and the individual grain particles feel soft, with no hard, gritty centers when you rub them between your fingers.
Do not add your malted barley at this stage. The high temperatures would destroy the very enzymes you need. The corn cooks alone (or with any unmalted rye or wheat, which gelatinize at lower temperatures and can be added partway through the cook).
Converting Starch to Sugar
Once the corn is fully cooked, you need to cool the mash before adding malted barley. The target temperature depends on which enzymes you want to activate, and there are two main ones at work in malted barley.
The first, alpha-amylase, chops long starch chains into shorter sugar fragments. It works best between 150°F and 160°F, with peak activity around 158°F, and prefers a pH of 5.3 to 5.7. The second, beta-amylase, finishes the job by breaking those fragments into simple sugars that yeast can ferment. It peaks around 147°F but is more fragile and gets destroyed above 160°F.
A practical approach is to cool your mash to about 152°F to 155°F, then stir in your crushed malted barley. This temperature range keeps both enzymes active. Hold it there for 60 to 90 minutes, stirring occasionally. You’ll notice the mash thinning out as the thick starch gel breaks down into a sugary liquid. Some distillers also add a commercial glucoamylase enzyme at temperatures below 149°F to squeeze out every last bit of fermentable sugar, though this is optional if your malt percentage is adequate.
After the conversion rest, you can check your work with an iodine test. A drop of iodine in a small sample of the liquid should stay amber or yellowish. If it turns dark blue or black, unconverted starch remains and the mash needs more time.
The Sour Mash Technique
Nearly every commercial bourbon uses a sour mash process, and it’s worth adopting at home once you’ve done your first batch. The technique involves adding “backset,” which is the thin, acidic liquid left over in the still after a previous distillation run. You replace 20% to 30% of your total water volume with this backset.
The backset drops the mash pH into the ideal range for enzyme activity and, more importantly, creates an acidic environment that discourages bacteria. As former MGP master distiller Greg Metze has explained, the main purpose of sour mash is preventing bacterial contamination. The acid essentially gives your yeast a head start over any wild microbes competing for the same sugars. On your very first batch you won’t have backset, so the “sweet mash” method (no backset) works fine. Save the liquid from your first distillation run for subsequent batches.
Cooling and Pitching Yeast
After conversion, cool the mash to your yeast’s preferred fermentation range before pitching. For bourbon, that’s typically between 72°F and 82°F depending on the strain. A few proven options for bourbon include White Labs’ Kentucky Bourbon Yeast (WLP070), which ferments best at 72°F to 77°F and produces malty caramel notes. Their Tennessee Whiskey strain (WLP050) runs a touch warmer at 75°F to 79°F and delivers a cleaner, drier profile. Their American Whiskey strain (WLP065) handles the widest range at 75°F to 82°F with moderate flavor compounds.
Your target gravity before pitching should be around 1.060 to 1.080, which translates to roughly 7% to 9% potential alcohol. Going higher than that tends to stress the yeast and produce harsh flavors that carry through distillation. If your gravity reads above 1.080, dilute with water to bring it down. If it’s too low, your conversion step may not have finished completely.
Fermentation
Transfer the cooled mash to your fermenter, pitch the yeast, and seal it with an airlock. Fermentation typically kicks off within 12 to 24 hours. You’ll see the airlock bubbling steadily and a foamy cap forming on the surface of the mash.
Keep the fermenter in a spot where the temperature stays within your yeast strain’s optimal range. Temperature swings cause yeast to produce excess fusel alcohols and harsh esters that won’t age out gracefully, even in a charred barrel. Most bourbon fermentations run for 3 to 5 days. Some distillers let it go a full week to ensure the yeast has consumed all available sugars. Fermentation is complete when airlock activity stops and your gravity reading stabilizes at or near 1.000 over two consecutive days.
At this point, you have a “distiller’s beer,” essentially a grainy, low-alcohol wash sitting around 7% to 9% ABV. This is what goes into the still. The corn solids can either be strained out or left in, depending on your still setup. Pot stills with wide openings can handle some grain, but thin, scorchable layers on electric heating elements are a recipe for burnt flavors. Strain if you’re unsure.
Grain-to-Fermenter Ratios
For a 5-gallon batch, a typical charge is about 8 to 9 pounds of total grain. Using a 70/15/15 corn-rye-malt bill, that works out to roughly 6 pounds of cracked corn, 1.25 pounds of rye, and 1.25 pounds of malted barley. You’ll use about 4.5 to 5 gallons of water total (including any backset). Adjust proportions up or down to hit your gravity target, keeping in mind that more grain per gallon means higher gravity and a thicker mash that’s harder to work with.