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On The Intricacies Of Agave Spirits Fermentation

Suggested pairing with this article; Mezcal fermented with natural yeasts.

Mezcal Cowhide Fermentation, Puebla

We have a saying in Spanish here at Finca 18.

Cuando la tina no está fermentando, las bacterias están trabajando.

Simply put, when the fermentation is not working, the bacteria is.

In our Level 4 Master Class at the Agave Spirits Institute, our fermentation professor taught us that fermentation is a sanitation process that helps prevent and deter bacteria and fungus.

It wasn’t until I heard this point when the secret of fermentation became super clear to me.

In this article, we will expand on that and hopefully capture the essence of agave spirits fermentation.


Fermentation is a natural process that humans have been perfecting for thousands of years.

Fermentation has been used to preserve food, create wine, medicine, beauty products, and most recently discovered, to clean historical buildings, paintings, and other relics of the past. It also just happens to be the last part of the process before you distill an agave spirit.

The most interesting part of fermentation in relation to agave spirits is how delicate it is, how it can be manipulated to impact flavor, and how many variables are at play that can determine the complete success or utter failure of any batch.

For distillers, tequileros, mezcaleros, and raicilleros this is where many trade secrets lie.

Today we attempt to reveal the many nuances between an artesanal producer and an industrial one.

Formulation Tank at El Tequileño

Let’s Talk About Fermentation

What Happens After The The Cook

As soon as the agave oven or horno has cooled to 20 to 30 degrees Celsius, the right conditions have been created for the agave to naturally start fermenting even before it is milled.

The longer the agave waits to be milled, the more bacteria and fungi start to take over. This bacteria contributes to the flavor profile. Some mezcaleros will let the agave sit there for nearly 10 days before milling. At that point the agaves may have several different colors of mold; white, red, green, and even black.

Old Jose Cuervo Pipon at the Tequila Museum

Why Milling Is Important

The reason why we mill the agave is because we want to expose as much sugar as possible for the yeast to eat and convert into alcohol. If you put large chunks of agave into the fermentation vat, there will be a lot of residual sugar left over and you will have a much lower alcohol yield.

So it is important to shred the agave as much as possible to expose all of the fibers.


How producers treat yeast is a telltale sign if they are an artesanal producer or an industrial one.

Although there are exceptions, here are some of the main differentiators.

  1. Added / not added yeasts - Except for tequila, you will see many producers in other denominations of origin letting mother nature do her work without any intervention. What this means is that the distiller is relying on the natural yeasts to work their magic and convert those sugars into alcohol. Conversely there is added yeast. Added yeast can help jump-start the fermentation process and act as a form of control by preventing other yeast strands from starting or taking over.

  2. Natural yeast, cultivated yeast, and lab-grown yeasts - As alluded to in the above, natural yeasts require no intervention and start working on their own. The downside is that it typically only creates a three to four percent alcohol by volume musto. Smaller and larger producers can cultivate their own strain of yeast and save the strand(s) for future batches to help start and control a fermentation. Lastly there is lab-grown yeast that have certain optimizations whether it be flavor, alcohol tolerance, or fermentation performance. With a cultivated or lab-grown yeast, you can have a musto up to six to eight percent ABV assuming you have sufficient sugar in your agaves.

  3. Stopping Fermentation - A distiller has a choice where and when to stop their fermentation. More industrial producers are more focused on yield and will try to ferment as close to one percent BRIX as possible. The more interesting thing here is when distillers stop a fermentation with sugar left over giving your spirits a sweeter taste. This is what could really determine flavor profile and is one of the more guarded secrets. A smaller producer may not even measure BRIX or alcohol percentage, but will know when his fermentation is ready by smelling and tasting it.

Fermenting in Clay Pots at Finca 18

The Delicate Dance

Only in the right vessel, at the right temperature, with the right yeasts can we achieve a properly fermented musto. But sometimes things do not go as planned.

The ferment can stop early on its own with plenty of sugars left over. A stuck fermentation is the maestro's worst nightmare. The yeast could have gotten too stressed and went to sleep or died. There are a lot of different reasons why a stuck fermentation may happen.

At this point, the maestro is faced with a choice. Try to revive the fermentation or distill it.

To revive it, the distiller must first diagnose the problem. Was it a temperature issue or a yeast issue?

If it was a temperature issue, he would need to figure out a way to warm the fermentation to the proper temperature (and keep the temperature in a good range). If it was a yeast issue, the distiller would have to add some new yeast in hopes of creating a new colony.

When the fermentation is stuck or stopped is when the bacteria and fungus take over. It is critical that the maestro do something before it turns sour, gets worms, or even molds.

Fermentation Vessels

Here is a list of the most common fermentation vessels used to make agave spirits.

  1. Stainless steel tanks

  2. Plastic barrels

  3. Concrete

  4. Wooden vats

  5. Cowhides

  6. Clay Pots

  7. Carved out rocks

Really anything that is concave and can hold fluid will work. Each type of material will contribute to flavor in certain ways. However, the performance of the fermentation is dictated by the type of tank you are using.

Wooden Pine Vats in Puebla

Natural materials are more porous and do not retain heat as well. They are also not that easy to clean and can contain a lot of bacteria. In parts of Mexico where it is hot during the day and cool at night, fermenting in porous materials is a tough task. Wood is likely the best natural material to use.

Materials such as stainless steel or plastic are clean, retain heat, and provide a good performing fermentation. The downside is that it may not add as much flavor.

A lot of mezcal from Puebla is fermented in tambos de plastico and there is a certain buttery flavor and texture the plastic creates that I really enjoy. Puebla is one of the only agave spirits producing regions that I have seen that have a strong tradition of using plastic.

Made famous on the hit series Breaking Bad for getting rid of victims bodies with acid, plastic barrels are not exactly eye candy. However they are the ideal material for fermentation performance.

Key Takeaways

  • A perfect fermentation is slow, consistent, and never stops.

  • Agave spirits fermentation is very delicate and a distiller must always be on guard.

  • Yeast and how it is respected is an important factor and a direct reflection of the producer.

  • Different parts of Mexico use different fermentation vessels because of tradition and climate.


Fermentation is a wonderful hobby and you really get to learn so much about different cultures, the history of humanity, and how to make agave spirits.


Greg Rutkowski

Greg is a certified Master of Agave Spirits and a distiller at Finca 18. With a Polish-Mexican background, he moved to Puerto Vallarta, Jalisco from Chicago. Today he dedicates most of his time tending his agaves and giving tours of the distillery.

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