Of all the topics discussed regarding the process of producing high-quality distilled spirits, fermentation always seems to get short shrift. People love to wax ecstatic about barrels, cut points, and grain bills, but the second you bring fermentation to the convo, shoulders shrug with the apathy of a thousand bored teenagers.
I’ve never quite grasped why this is the case. Maybe it’s because fermentation is the least hands-on portion of the process (aside from years of maturation time). Or perhaps more to the point, this is the process where distillers feel they have the least amount of control. After all, it is the yeast that produces the alcohol we’re chasing. Pitch the little guys into the fermenter and the onus of producing your distillation opus is on them at that point.
No matter the reason for our professional indifference, more distillers would benefit from paying closer attention to the nuances involved in the fermentation process. A good fermentation program sets the distiller up for success going into distillation. It also has a massive influence on the final outcome of a cask program; fermentation-derived congeners greatly impact the maturation process and bottled spirit character. Let’s look a bit deeper at fermentation, how it works, and how we can affect it.
The Basics of Fermentation
On the surface, fermentation looks deceptively simple, especially when we compare what distillers deal with versus that of brewers and winemakers. Create a fermentable substrate and add some dried yeast. Pour a beverage of your choice and sit back for the next few days while the yeast does all the work. Easy, right? Well, hold on there, buckaroo. There’s more to it than that.
That yeast you just added? It’s sitting in a weird position, somehow juxtaposed between microbial heaven and hell. Yes, you’ve supplied them with ample food, but let’s look at it from their perspective (inasmuch as yeast cells have such a thing).
The majority of craft distillers utilize dry yeast for its ease of handling and convenience. Dried yeast has undergone some fairly stressful dehydration processes to reduce its moisture level and become, well, dry. Now, all of a sudden, these yeast cells are being thrust into an aqueous environment with more sugar than they know what to do with. This is all a bit much for an individual yeast cell, so instead of consuming sugars immediately, the yeast will begin taking in non-sugar nutrients for cellular activities and building up their cell walls. This phase of seeming inactivity is what we refer to as the “lag phase.” It lasts usually less than eight hours in most distilleries, and the visuals inside the fermenter are boring to say the least. There’s nothing really happening, at least from our point of view.
Eventually, the yeast cells start to feel confident and the feast can really begin. Sugars get consumed, alcohol and CO2 are produced, and the yeast cells reproduce through budding to make even more yeast cells. This is the Exponential Growth Phase. Look inside the fermenter during this phase and things are probably wild and wooly. Like a witch’s cauldron of magical excitement and alcoholic potential, there’s bubbling, gurgling, and motion aplenty.
Finally, the yeast reaches a point where the majority of sugars have been consumed and there’s little need or point to producing new cells. This is the Stationary Phase of the fermentation process. Things calm down inside the fermenter, but beneath the surface things are still happening. The yeast are finishing off the final dregs of fermentable sugars and metabolizing whatever nutrients and workable substrates they can find. Arguably, this is where a lot of fermentation character is polished and refined prior to distillation.
In most distillery ferments, this three-phase process should take three to five days (this may be longer in some extended ferment categories such as ancestral mezcals, high-ester rums, and many brandies). But now the cells are stuck in an environment with (hopefully) tons of alcohol, and just like humans, alcohol in high concentration is toxic to yeast. As a result, some of the yeast starts to die off or otherwise drop out of solution. (See figure 1 below).
Matching the Right Yeast to the Right Ferment
Virtually all alcohol-producing yeasts will follow the above-described course. But that’s not to say they are all the same. In the majority of distillery fermentations, distillers use commercially available cultured yeast. (For my purposes here, I’m not considering the many spirit categories that make use of native and/or multi-culture yeasts and bacteria. That’s another story for another time.) That commercially purchased yeast will likely be of a single species, Saccharomyces cerevisiae.
This species of yeast has been hanging out with Homo sapiens for millennia, because where there are humans, there’s probably going to be a source of sugar somewhere. The relationship has been a mutually beneficial one. Yeast get access to sugar, and people get delicious booze; it’s a win-win situation if I’ve ever heard one. To make things even more interesting, throughout our coexistence, a number of Saccharomyces cerevisiae variants (strains) have evolved to excel at different things, meaning modern fermentation-inclined folks such as distillers have an array of yeast options to work with. And not all yeasts are equally suited to all spirits.
At one point in time, distillers had to make do with a very limited set of available yeasts. The “best” ones were at most uninspiring on their best days. For instance, the Scotch industry used to rely heavily and sometimes solely on spent brewer’s yeasts, which had already been through fermentation and were not usually the most vital. My, how the times have changed. While we still don’t have the same level of variety that brewers and winemakers have, things have gotten much better for distillers over the years. Depending on the supplier, you’ll likely have access to myriad strains that are well suited to specific whiskey styles such as rye or single malt. Want to make an Agricole-style rum? There’s now a commercial yeast for that. Agave? No problem. Need something near-neutral in character for vodka? You’re covered there, too.
Choosing the right yeast starts with having an understanding of the fermentable substrate you’re working with. For instance, whiskey worts have high levels of maltotriose (~15% of the fermentable sugars). Many standard brewer’s yeasts will not ferment maltotriose, instead leaving it and other higher-weight sugars behind. This is important for beer because these sugars provide valuable residual sweetness (to balance hop bitterness) and mouthfeel properties. However, as distillers, we don’t care about residual sweetness in the beer since it is going to be distilled. Leaving those sugars behind means we’re not getting as much alcohol out of our fermentation as we possibly can. So, whiskey distillers should opt to use yeast that can ferment maltotriose.
Another example comes in the agave world. Agave is composed of inulin, which is in some ways analogous to starch. The difference is that starch is primarily composed of glucose units whereas inulin is nearly all fructose. Most yeasts that readily ferment glucose do not particularly care for fructose. Try putting whiskey yeast into an agave fermentation and you likely won’t see anything happen, even after weeks of waiting. Agave ferments require a fructophilic yeast that will readily consume fructose without hesitation.
Beyond differences in preferred substrates, your yeast choice is largely going to be dictated by your desired spirit style. The single malt Scotch industry heavily favors the use of the M-type yeast strain. This yeast works brilliantly for single malts, fermenting well at cooler single malt fermentation temperatures and producing lightly fruity washes ready for distillation. While you could certainly use this yeast for producing a classic-style bourbon, the profile would probably not match your expectation. Similarly, if you want to produce a light rum, you may not want to use a high ester-producing yeast.
Hydration and Pitch Rate
No matter which yeast you opt for, it is crucial that you take good care of it. Dry yeast is the preferred format by many distillers as it is relatively stable and easy to work with. Many distillers simply take a brick or two of dried yeast and toss it directly into the fermenter without a second thought. But most dried yeast (there are one or two exceptions) really needs to be rehydrated prior to pitching. Remember, the yeast dehydration process puts some strain on the cells, and tossing them into a sugar rich environment without any kind of preparation doesn’t exactly start your fermentation off on the right foot. In fact, in many instances a direct pitch of dry yeast may result in up to 50% of your yeast cells dying off. Most manufacturers recommend a simple rehydration step prior to pitching. (See individual manufacturer instructions for more details.)
How much yeast should you pitch? Manufacturers have their recommendations, but there are other things that come into play before you make your final decision. Pitch rates of around 1 x 106 cells per ml of wort are common in the brewing world. Distillers typically pitch at higher rates ranging from 4 x 106 cells per ml up to 40 x 106 cells per ml. There are a few reasons for the higher pitching rates. One is that higher pitch rates usually get fermentation started and completed a bit faster, which is important in many distilleries who may be running fermentations in as little as 72-hour cycles. Another reason particularly prevalent with whiskey is that higher pitch rates allow for better competition of the yeast against native and resident bacteria which commonly come in on grains (and grapes in the brandy world). And while good yeast handling practices dictate that most (there are exceptions) dried yeast strains should be rehydrated prior to pitching, not every distiller does this. The lack of rehydration often causes a die-off of up to 50% of the cells pitched. A higher pitching rate can hedge against this issue.
But pitch rate affects the fermentation outcome in more ways than simply kicking it into high gear. With few exceptions, higher pitch rates produce lower levels of esters and reduced fruitiness. Lower pitch rates generally increase ester levels in the final wash (and subsequently the final distilled spirit). Which route is better? That all depends on the style of spirit you’re trying to make.
Temperature, Nutrients, and pH
Fermentation temperature is another variable that distillers would do well to pay more attention to. I’ve lost track of how many distilleries I’ve visited over the years where there is no real temperature monitoring during fermentation. And it must be said that monitoring is not the same as controlling. I’m not advocating for anyone to change their fermentation temperatures as long as they’re happy with the results. However, I will make the case that watching for temperature changes over the course of fermentation allows for more consistent results in the distillery. The reason is simple: things happen. The power goes out and cooling jacket controls can break. Someone forgets to set the temperature limit. The list goes on. In many distilleries, these issues may not seem apparent unless you’re paying close attention to the temperatures in the tanks. And besides, temperature can have a significant impact on distillate character in that higher fermentation temps tend to bring out higher ester levels from the yeast. In some instances, higher temperatures will also reduce diacetyl, one of the compounds responsible for buttery aromas in spirits.
Monitoring nutrients is another area worthy of your attention. Many spirit categories start with fermentables that benefit from at least a small amount of nutrient additions. Agave and rum both begin with little in the way of good yeast nutrition. Many American-style whiskeys with their heavy use of unmalted grains tend to be somewhat nutrient deficient and can benefit from a small nutrient addition.
Nutrients are measured in terms of free amino nitrogen (FAN) and yeast assimilable nitrogen (YAN). FAN is simply the total amount of amino acids and minor peptides that are available for your yeast to take up. YAN is most commonly used in the wine world and is a measurement of FAN plus ammonia and ammonium ions. A good rule of thumb is that most fermentations require a minimum FAN level of 150 ppm and do well up to 250 ppm. Levels above that may cause an increase in higher-alcohol production by some yeast strains. Frustratingly, it’s also possible for some strains to produce more higher-alcohols when the FAN level is also too low. For these reasons, it’s best not to add nutrients willy-nilly without knowing what your starting FAN level is. And for that bit of information, you’re going to need to test it. This often requires sending a sample off to a lab such as Brewing and Distilling Analytical Services (BDAS) in Lexington, Kentucky. Once you have your results, you can easily work with your nutrient supplier to determine what the best dosage rate should be for your given nutrient. Personally, I suggest a good nutrient complex with lots of amino acids and micronutrients. A preparation containing zinc, an important yeast micronutrient often lacking in many commercial nutrient preparations, is a plus.
The final thing a distiller should monitor on their fermentations is the pH level. For most fermentations the pH is kind of going to be whatever it is and you shouldn’t have to think about it too much. This is certainly the case with the majority of grain fermentations. However, it’s still worth checking, because a low pH could be an indicator of something gone awry with the fermentation such as rogue contaminative bacteria causing problems.
Low pH levels can also cause your yeast to struggle. I occasionally get asked to ferment agave syrup for the production of agave spirits. It can be tricky stuff to work with, and pH is definitely an area where agave syrup loves to throw a curveball. With little warning, I’ve seen these fermentations completely stall out because the pH has dropped below 3.0, which is not ideal. In fact, many yeasts used in distilling often struggle at pH levels 3.5 and under. Having a simple $75 pH meter around will keep you in the know and help you take decisive action when pH issues arise during fermentation.
We could spend a book’s worth of pages talking about distillery fermentation. Unfortunately, the only books written on the subject are aimed more at brewers and winemakers rather than distillers. Nonetheless, there is a wealth of information out there, and most quality yeast suppliers will gladly help you troubleshoot and design fermentation protocols to suit just about any fermentation you can imagine. All you have to do is ask.
