Fermentation sits between agave preparation and distillation. The sugar-rich liquid prepared from agave, called mosto, goes into a tank where yeast produces alcohol and aroma compounds. The alcohol will later be concentrated in the still; some of the aromas formed in the tank contribute to the spirit’s character.
Distilleries have several choices at this stage. They can add a selected yeast culture or let fermentation develop from microorganisms already present, and ferment with agave fibers or with liquid alone. Tank conditions such as temperature and sugar concentration also influence how the yeast works and what it produces.
What goes into the tank
The mosto contains sugars released from agave's stored carbohydrates.
The fermentation tank may also contain agave fibers, called bagazo. Along with sugars, the mosto contains nutrients and other material that affect microbial growth.
For “100% agave” tequila, all fermentation sugars come from blue agave. Mexico's category labeled simply “tequila” allows other sugar sources, with agave supplying at least 51% of total reducing sugars by mass in the formulation before fermentation.
Added cultures and spontaneous fermentation
A distillery can add a grown yeast culture or let fermentation develop spontaneously. Adding a culture supplies a starting population of a chosen strain. The producer can select that strain for its ability to ferment the juice, tolerate heat and produce the desired aroma compounds. In spontaneous fermentation, microorganisms already present in the mosto carry out the work.
A native yeast can be isolated from agave-juice fermentation, grown and added as a culture. Using a local yeast therefore does not necessarily mean the fermentation is spontaneous.
Sugar and tank conditions change fermentation
The yeast must tolerate the juice's sugar concentration. Water added during extraction dilutes the sugars; nutrients such as nitrogen help the yeast grow.
Controlling temperature and acidity helps a distillery favor its chosen yeast over competing microorganisms. Excessive heat can stop fermentation, while a heat-tolerant strain can keep working in a warm tank.
Yeast also produces esters, a family of aroma compounds. Ethyl acetate can smell fruity or solvent-like; isoamyl acetate has banana or apple-like aromas. These compounds occur in much smaller amounts than ethanol but can still influence aroma. Different yeast strains produce them in different proportions, so changing the yeast can change the aroma of the fermented juice.
Fermenting with agave fibers
Bagazo can still contain fermentable material after extraction. Some distilleries keep fibers in the tank; others ferment only the liquid. Floating fibers can form a cap over the liquid. Patrón describes this layer as limiting evaporation and the liquid's exposure to air in its 2021 account of fermentation with fibers.
At Patrón, roller-mill juice and tahona juice with fibers ferment separately in pine-wood tanks for three days.
After fermentation, the liquid enters the still. Heating, vaporization and condensation separate and concentrate the alcohol along with other compounds.
Questions about tequila fermentation
Does tequila fermentation need yeast?
Yes. Yeast turns the sugars prepared for tequila into alcohol. A distillery can add a yeast culture or rely on yeast already present in the mosto, the liquid prepared for fermentation.
Does using native yeast mean tequila is fermented spontaneously?
No. Native yeast used in tequila fermentation can be grown and added as a selected culture. Its local origin does not make the fermentation spontaneous.
Is tequila fermented with agave fibers?
Some tequila is fermented with agave fibers. Patrón's tahona stream includes them, while its roller-mill stream ferments without them.
Does longer tequila fermentation always taste better?
No. Fermentation time alone does not determine tequila's flavor. The yeast, juice, temperature and other conditions affect the compounds produced; cooking, distillation and aging also contribute.
How long does tequila fermentation take?
Patrón ferments its tahona and roller-mill streams for three days. Fermentation time varies between production systems with the yeast strain, juice composition and temperature.
Sources
- Miguel Cedeño Cruz, “Tequila production from agave,” The Alcohol Textbook, fourth edition (2003), pp. 230–238.
- Inge Russell, “Understanding yeast fundamentals,” The Alcohol Textbook, fourth edition (2003), pp. 109–110.
- Academia Patrón (2021), pp. 44–47; Patrón's production guide.
- NOM-006-SCFI-2012, §§4.14, 4.17, 4.18 and 5.1.