Temperature, pH, and Fermentation Direction

Learn how temperature increases to faster reactions (roughly doubling per +10°C) and how the same formula flips your strategy for vodka versus whiskey.

Quick summary

Core principle

This one, time and temperature, or temperature and time, For example, we start with temperature. In general, chemical reactions speed up at higher temperatures.

As a rule of thumb, every 10 degrees Celsius, higher temperature, the amount of chemical reactions that take place basically double. So if we apply that to this formula, simply by looking at the T for temperature, this means that if we want to create more flavors, we basically need to ferment at higher temperatures.

If you want less flavors, go for lower temperatures. So for example 20 degrees Celsius room temperature is an okay fermentation temperature.

Be careful because bigger ferments warm up very quickly by themselves, so you may need to check that, to cool that, to make sure it stays at 20 degrees. Depending on the yeast strain you use, you create a pretty neutral flavor profile as far as temperature is concerned, and maybe as far as pH is concerned if you keep it close to 6, close to 7.

If we're the same substrate, for example grain, if we're the same yeast strain, we use higher temperatures than 20 degrees Celsius, so all the rest stays the same. Higher temperatures, For example, we ferment at 30 degrees Celsius, that's like what it is right now here in the midst of summer, pretty warm.

We'll see that more esters are formed, and basically 10 degrees Celsius more means double the amount of esters. If you want to have more flavors, you go lower in pH, and you go higher in temperature.

Now we're almost there, right? Let's look at the last T, time.

The longer you let your fermentation go, the more esters you create. It's a simple formula, it's chemistry.

If certain reactions take place, if you give them more time, more of those reactions will take place. If these reactions happen to be alcohol and organics mixing and mingling and forming taste molecules, the longer we allow a fermentation to sit, the more esters, the more taste molecules will be produced.

So for a vodka, again, you want a short ferment, neutral pH, not too warm. For a whiskey, you want to go the other way around, right?

You want to create bold flavors. So let's take that as an example.

Let's dive into whiskey making, and the same applies to brandy and rum. And vodka is the same, only the other way around.

We want a bold and flavorful whiskey.

How it works in practice

We need to have alcohol and organics present in order to create the whiskey. So one of the first questions that you need to deal with preparing how you're going to make planning, how you're going to set up your distillery for whiskey making, is like, all right, I've got a masher, I've got fermenters, and I've got one or two stills. What am I going to do with my mashing and fermenting?

Am I going to ferment on the grain, or am I going to take the grains out through a lautering process or via a lauter ton, and basically ferment and then distill a clear beer? So do I leave the grains present, or do I take the grains out? Now, using this formula, your decision becomes very easy.

If you want a bold style whiskey, you want as many organics present as you can. If you filter out your grain before fermentation, you basically ferment a clearer beer. There's not as many organics present, right?

So what happens to esterification? If you want to have a medium flavor profile whiskey, maybe take the organics out. If you want to have a bold and very, very high flavor whiskey, like on that taste grid that we spoke about before, on a level three instead of a level two, you leave the organics in during fermentation and during distillation.

The longer you leave the organics in, the more esterification will take place. I hope it now makes perfect sense to you. We already touched upon it, right?

The more sour you go, the more flavors you create. But there is a distinction between more flavors and better flavors. We have to imagine that yeast is doing the work for us.

They're converting those carbohydrates, sugars, fermentable sugars, into alcohols and doing a lot of the flavor composition for us. Now yeast is a living organism. So if we put it in a very sour environment, if we put it in a very, very hot environment, it's going to become stressed.

The more we stress it, the more of certain flavors we get. The other video will dive into that. But for now, I want you to understand that if we push pH too low, we create such a stressful situation for the yeast that it can start to produce a lot of flavors, but very, very much off flavors because the yeast is dying and not performing in a healthy way.

The fermentation may stop altogether and stall because it's too sour for the yeast to actually do anything else but cling on. It can't make your spirit, your fermentation, your beer, your wine for you anymore. So even though lower is better in terms of esters, there is a threshold that I feel that you shouldn't go below.

For a whiskey, we usually advise to have a pH of around 4 to 4.8, 4 to 4.8. If you go below 4, especially with the grains present, there's going to be so much stress and pressure on those yeast cells that you're going to stress it out.

Common mistakes and decisions

If you go above 4.8, well, please know that bacteria actually can live in a fermentation that has a 4.8 or more pH. Below, they've got difficulty, except Lactobacillus, but most bacteria can't survive below 4.8. That basically means that if we stay between 4 and 4.8, we have a protection against most bacteria and we create as much flavor as we can, which is good for a whiskey.

For a rum, especially since there's no grains present and since there's some stabilizers naturally occurring in the molasses that we use as a substrate, instead of 4, you can go down as low as 3. I'm not sure you should go there because you are on the limits of the yeast surviving, but if you hit a 3.5 instead of staying at 4, like for a whiskey, totally fine. If you want to go a bit lower on your fermentation, you can check it like twice a day and take a reading and if you use the same protocols, the same yeast, the same substrate, it will give the same results time and again.

For rum, you might even want to go a little bit lower. We're talking about whiskey, right? Sorry, I hope I'm not confusing you guys.

Higher temperature, more esters. So for a whiskey, the weird thing is that most whiskeys sort of follow traditionally a beer protocol. And that's because there wasn't a whole lot of information on how to ferment whiskey beer.

So craft distillers of old used to turn to brewers and say like, so how do you actually brew your beer? And they would say, well, we brew our beer without the organics present because we want our customers to drink, not to sort of eat their beer and choke on the grains while they do so. It's not very pleasant to have a lot of grains in your beer.

It's not pleasant to have even one or two particles of grain in your beer. The beer brewer would say, pH, we do that pretty neutral. And we ferment for a long time.

Basically, we're not creating a whole lot of flavor because in beer, a lot of the flavors come from the hops. And if you use these protocols as a craft distiller, listening to a brewer, you're going to make a very one-dimensional product because no organics present, so not a whole lot of flavor, pH not too low because that's what brewers do.

Continue with Continues the control knobs and focuses on temperature as a driver. to build directly on this foundation.

Putting It Together by Spirit Goal

Set fermentation temperature and pH according to product objective. Neutral goals prioritize control and reduced stress; flavor-rich goals may use intentional stress windows when reproducibility is maintained.

Key Takeaways