Cracking into On-Grain Versus Off-Grain Production

It’s a question that’s been coming up more often in distilling circles: on-grain or off-grain distillation? Go to any distillers’ meet-up these days, and there’s bound to be at least one group having a (hopefully friendly) discussion about which is better for single-malt whiskey.

At first glance, those terms may seem simple and self-explanatory. However—like all things in the spirits industry—it gets more complicated once you dig in. So, let’s zoom in on those details and get a better feel for the pros and cons of each method when producing grain-based spirits.

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On-Grain Spirits

As most distillers would define it, “on-grain” production occurs when the excess solid grain matter in the mash—primarily husk and endosperm—remains in contact with the wash from fermentation through distillation. You don’t filter these ferments, and everything that enters the mash tun and/or fermentor also enters the still (minus the CO₂ that escapes during fermentation).

Distillers typically begin on-grain production by grinding the grains very finely through a hammer mill (or similar) and mixing them with a set amount of heated water until their starches gelatinize. Enzymes, whether endogenous or exogenous—that is, derived from the grain or added by the distiller—break down the starches to sugars. The distiller (or brewer) then cools the solution and adds yeast to kick off fermentation.

Once fermentation is complete, the whole mass of it goes to the still—lock, stock, and barrel.

The Pros of On-Grain Distillation

There are several benefits associated with producing spirits on-grain.

First, because on-grain fermentations require no filtering, they generally require less specialized equipment and can be used with almost any grain. That makes the process attractive if you’re dealing with heritage grains or new varietals whose processability may be unknown.

Second, because of the small granule size from the finer milling process, on-grain fermentations have greater access to starch and generally get higher yields. That can make the method more cost-effective, even if the availability of cheap commercial enzymes has somewhat lessened that advantage.

Finally, and perhaps most importantly, on-grain’s major benefit is that it can create a stronger, more grain-forward flavor in the finished distillate compared to other methods.

We can attribute that stronger flavor to the presence of grain solids in the still during distillation, and many consider that flavor essential to some spirits. Indeed, enthusiasts could consider a bourbon or corn whiskey that lacks that strong grain-forward aroma to be faulty—though the degree to which it should be present is a matter of personal taste.

The Cons of On-Grain Distillation

On-grain production does come with some drawbacks.

The first and foremost is the increased risk of scorching, which occurs when the solid matter burns onto the sides of the still. If that happens to enough of the solid matter, the distillate gets an extremely unpleasant acrid, burnt taste. Because on-grain ferments have so many solids in them, the risk of scorching is high, and you must pay special attention to avoid it.

A second drawback is the risk of fouling the still because of excessive foaming. Many of the components of grain solids—especially the husks of malted barley—contain long-chain proteins and polysaccharides that promote the formation of foam during distillation. If they produce too much foam too quickly, it overwhelms the still and may even blow out the condenser. If that happens, it necessitates a full cleaning of the still before it can be used again. That’s why it’s important if you’re producing on-grain spirits to constantly monitor your distillations for the slightest hint of fouling.

Finally, another problem with on-grain production is that it tends to be less environmentally friendly than off-grain production.

After distillation, what’s left is a mixture of highly acidic spent wash and cooked-grain solids—sometimes referred to as “bottoms.” Most municipal water systems can’t handle those solids in their wastewater streams, so it’s rarely safe to dump them down the drain. Thus, you often need to remove the solids and find another way to dispose of them, which can be both expensive and energy-intensive, increasing your distillery’s costs and/or ecological footprint.

Off-Grain Spirits

It’s more difficult to define “off-grain” production. Using the on-grain definition above, we could say that off-grain production occurs when the distiller removes the excess solid grain matter before distillation.

That sounds straightforward, but it doesn’t specify when to remove the grain matter, nor does it say how much of it should be removed for the process to be considered off-grain. Thanks to that ambiguity—and the fact that there is no legal definition for either method—several different camps have emerged on what exactly defines off-grain production.

Here, however, we’ll focus on the two main types.

Traditional Off-Grain

Sometimes referred to as “brewers’ style” or “Scotch style,” traditional off-grain spirits production involves using either a lauter tun or mash filter to remove excess particulate before the sugary liquid goes to fermentation.

During or after that filtration of sugary liquid—often known as wort—the distiller (or brewer) then adds more hot water to “wash” or “sparge” the grains to release additional sugars. From there, the distiller can further process the wort by boiling, additional filtering, or simply cooling it down and sending it to fermentation.

The first advantage of traditional off-grain production is that the liquid is easy to handle. Without grain, the wash is much easier to both pump and cool, making it both time- and energy-efficient. The leftover spent grain is also relatively easy to handle because it doesn’t go through fermentation, and you can often sell it as-is to farmers without risk of affecting their livestock.

This method also reduces the risk of infection by outside microbes, which are often found on the outer husks of leftover grains. That reduces osmotic stress on the yeast, allowing for more stable, predictable fermentations. These ferments then produce relatively clean and delicate distillates that lack grainy tannin notes or other faults common to stressed fermentations. That’s important for products such as Scotch, known for its flavorful yet often reserved palate.

The greatest drawback of traditional off-grain production is that it can be inefficient and labor-intensive. It requires additional equipment and work to remove the grain matter before fermentation, and the relatively lower yield from not fully converting starches during the mash can make it less cost-effective.

Others argue that spirits produced by this method can be too delicate and easily overwhelmed by other factors, such as barrels or local environment. However, as with bourbon, this can be very much a matter of personal taste.

Hybrid Off-Grain

Sometimes referred to as on-grain/off-grain, hybrid off-grain production has become increasingly popular with distillers, especially in the United States.

The method involves fermenting the mash on-grain, then using a type of mechanical separator, such as a rotary screw press, to remove the grain before the fermented wash enters the still.

While at first glance this method may seem like a perfect mix of both on-grain’s and off-grain’s advantages, it comes with some large caveats:

• First, the presence of excess grain matter during fermentation still influences yeast health and fermentation kinetics. That means that the fermentation flavor profile of the hybrid method won’t be identical to that of a traditional off-grain production method.
• Second, these systems are often expensive, difficult to operate, and require large amounts of electricity, which makes them infeasible for many small distilleries.
• Finally, and perhaps most importantly: During fermentation, alcohol soaks into the grain material present in the wash. Most mechanical separators can’t squeeze the grain material hard enough to remove that alcohol. That means these systems often lead to a net loss in distillate yield, which no distiller wants.

Ultimately, for now, the only real advantage of the hybrid method is that it reduces the chances of scorching during distillation and that it may help you decrease unwanted grain flavors in the final distillate. In the future, however, the technology could advance enough to become a useful tool for distillers.

Not So Simple

Clearly, there’s a lot that goes into the “on-grain” versus “off-grain” discussion. It even goes beyond process and flavor to affect the costs of equipment, labor, and time, and the impact on the environment.

Thus, it’s important that you be aware of the advantages and disadvantages of all these methods—not just the ones you use right now—so that you can decide what works best for you and your spirit.