Making corn mash for bourbon starts with cooking cracked or milled corn in hot water to break down its starches, then converting those starches into fermentable sugars using enzymes from malted barley. The process takes a full day of hands-on work before fermentation even begins, but the steps are straightforward once you understand the science behind each one.
Choosing Your Grain Bill
Bourbon must contain at least 51% corn by law, but most recipes push that number much higher. The remaining grains shape the flavor profile. A classic “high rye” bourbon might use 75% corn, 12.5% malted barley, and 12.5% rye, which adds spice and bite. A “wheated” bourbon, modeled after the Maker’s Mark style, runs 70% corn, 16% wheat, and 14% malted barley, producing a softer, sweeter spirit. If you’re making your first batch, either of these ratios is a proven starting point.
The malted barley isn’t optional. It provides the enzymes that convert corn starch into sugar. Without it (or a commercial enzyme substitute), your mash will stay starchy and the yeast will have nothing to eat. Two-row malted barley is the standard choice. For a 5-gallon batch, you’re typically working with around 8 to 10 pounds of total grain.
Grinding the Corn
Corn needs to be milled before cooking. The goal is to expose as much starch as possible to hot water without grinding so fine that the mash turns into an unmanageable paste. A coarse cornmeal consistency works well for home-scale batches. Some distillers aim for particles around 2 mm, while others prefer a slightly coarser crack. Finer grinds convert more efficiently but are harder to stir, strain, and work with. Cracked corn from a feed store, run through a grain mill one more time, hits the sweet spot for most people.
Water Matters More Than You Think
The mineral content of your water directly affects how well your mash converts and ferments. You want water with moderate calcium content, which stabilizes the pH of your mash in the ideal range of 5.2 to 5.6. At this pH, the enzymes from your malted barley work most efficiently at breaking starch into sugar.
Iron is the mineral to avoid. Even small amounts give the spirit a metallic, ink-like flavor, and high concentrations are toxic to yeast, leading to sluggish or incomplete fermentation. If your tap water has noticeable iron content, use filtered or spring water instead. Carbon-filtered water is fine for most batches. Kentucky’s famous limestone-filtered water is naturally high in calcium and low in iron, which is one reason the state became bourbon country in the first place.
The Cook: Breaking Down Corn Starch
This is the most physically demanding part of the process. Corn starch doesn’t dissolve in lukewarm water. It needs to be gelatinized, a process where heat forces the tightly packed starch granules to swell and burst open. For corn, this happens across a temperature range of roughly 150°F to 200°F (64°C to 95°C), with the critical action occurring in stages. The starch begins to break down around 154°F (68°F) and gelatinization accelerates through 163°F, 172°F, and 190°F (74°C, 78°C, and 88°C), each stage softening a different fraction of the starch structure.
In practice, here’s what you do: heat your water to about 200°F (just below boiling), then slowly stir in your corn. The temperature will drop as the grain absorbs heat. Maintain the mash between 190°F and 200°F for 60 to 90 minutes, stirring frequently. The mixture will thicken dramatically at first, then gradually thin out as the starches break apart. This thinning is your visual confirmation that gelatinization is working. Some people add a small amount of malted barley (about 10% of your total malt) at this stage to help thin the mash. The enzymes start chewing through starch even at these higher temperatures, making the pot easier to stir.
The Conversion: Starch to Sugar
Once the corn has cooked, you need to cool the mash before adding the bulk of your malted barley. The key enzymes in malt are destroyed above about 160°F, so you need to bring the temperature down to the 148°F to 155°F range. An immersion chiller speeds this up, but you can also just let the pot sit, stirring occasionally, until it reaches the target.
When you’re in range, stir in your rye or wheat (if using) along with your crushed malted barley. These grains gelatinize at lower temperatures than corn, so they don’t need the aggressive cook. Hold the mash at 148°F to 155°F for 60 minutes. This is the saccharification rest, where the barley’s enzymes convert all that gelatinized starch into fermentable sugars. Stir every 10 to 15 minutes.
You can test conversion with an iodine drop test. Take a small spoonful of liquid from the mash, let it cool slightly, and add a drop of iodine. If it stays amber or yellowish, the starch has been fully converted. If it turns dark blue or black, starch remains and you need more time at temperature. Most well-made mashes convert completely within that 60-minute window.
Checking Your Starting Gravity
Before pitching yeast, take a hydrometer reading of the liquid portion of your mash. A target original gravity around 1.080 is a reliable benchmark for bourbon mash. This translates to roughly 8 to 10% potential alcohol before distillation. Going much higher than 1.090 can stress the yeast and produce off-flavors. Going too low means less alcohol in your wash, which means less bourbon per batch. If your gravity reads low, you may have under-converted (not enough enzyme activity or time) or used too much water relative to grain.
Cooling and Pitching Yeast
Yeast dies in hot liquid, so cool the mash to around 75°F to 80°F before adding it. An immersion chiller or ice bath around your fermenter can bring the temperature down in 30 to 45 minutes. Without active cooling, a 5-gallon batch can take several hours to reach pitching temperature, and the longer it sits warm without yeast activity, the more vulnerable it is to wild bacteria.
For yeast selection, distiller’s yeast strains designed for grain washes are the standard choice. Bread yeast also works and has a long history in home distilling. Transfer the cooled mash into your fermenter, pitch the yeast, and seal it with an airlock.
Fermentation
Keep your fermenter between 75°F and 95°F. The lower end of that range produces a cleaner-tasting wash with fewer harsh byproducts. Higher temperatures speed up fermentation but can create more off-flavors, some of which carry through distillation. A consistent 78°F to 82°F is a good target for most home setups.
Fermentation typically takes 5 to 6 days. You’ll see vigorous bubbling through the airlock within the first 12 to 24 hours, peaking around day two or three, then gradually slowing. The mash is finished when the airlock stops bubbling and a hydrometer reading shows a final gravity near 1.000 or just below. At that point, the yeast has consumed the available sugars and your wash is ready for distillation.
Don’t rush it. Pulling the wash early leaves sugar behind, reducing your yield. And if you notice no activity within 24 hours of pitching, the yeast may have been killed by too-hot mash, or your pH may be outside the workable range. Repitch with fresh yeast after confirming the temperature is correct.
Putting It All Together
Here’s the full sequence for a 5-gallon batch using a high-rye grain bill (75% corn, 12.5% rye, 12.5% malted barley):
- Mill your corn to a coarse meal consistency.
- Heat 5 to 6 gallons of low-iron water to about 200°F.
- Stir in the corn slowly and hold at 190°F to 200°F for 60 to 90 minutes, stirring often.
- Cool to 152°F and stir in your rye and malted barley.
- Hold at 148°F to 155°F for 60 minutes, stirring every 10 to 15 minutes.
- Cool to 75°F to 80°F using a chiller or ice bath.
- Check pH (target 5.2 to 5.6) and original gravity (target around 1.080).
- Transfer to fermenter, pitch yeast, and seal with an airlock.
- Ferment at 75°F to 82°F for 5 to 6 days until the airlock goes quiet and gravity reads near 1.000.
The finished wash will smell yeasty and slightly sweet, with a noticeable corn character underneath. What happens next, the distillation, is where you separate that wash into bourbon. But the mash is where flavor begins, and getting it right is the difference between a spirit worth aging and one that isn’t.