---
title: "What Is Mixed Fermentation? Taxonomy, Strains and Microbial Interaction"
description: "Mixed fermentation brings together organisms from different genera, such as Saccharomyces with Brettanomyces or lactic acid bacteria. What defines it and why it takes months."
url: https://www.thebeertimes.com/en/what-is-mixed-fermentation-beer/
date: 2024-04-05
modified: 2026-11-07
author: "Carlos Uhart M."
image: https://www.thebeertimes.com/wp-content/uploads/2022/05/Fermentacion-mixta.jpg
categories: ["Culture"]
tags: ["Bacteria", "Brettanomyces", "Culture", "Fermentation", "Saccharomyces", "Yeasts"]
type: post
lang: en
---

# What Is Mixed Fermentation? Taxonomy, Strains and Microbial Interaction

By **[Stuart Winstone](https://brewyonder.co.uk/blogs/news/mixed-fermentation)**

What is mixed fermentation? It is a very common question in the beer world, yet one that does not always get a clear answer, so let’s dig into it a little.

![Mixed fermentation](https://www.thebeertimes.com/wp-content/uploads/2022/05/Fermentacion-mixta.jpg)*What is mixed fermentation?*

Because the term is so loose, mixed fermentation is open to interpretation, but in its most basic definition it is a fermentation in which organisms from different genera work together, such as Saccharomyces yeast with Brettanomyces or with lactic acid bacteria like Lactobacillus and Pediococcus.

Contenido

- [What Are Yeasts?](#What_Are_Yeasts)
- [Saccharomyces cerevisiae](#Saccharomyces_cerevisiae)
- [Brettanomyces vs. Saccharomyces](#Brettanomyces_vs_Saccharomyces)
- [What Is Mixed Fermentation?](#What_Is_Mixed_Fermentation)
- [Lactobacillus and Pediococcus](#Lactobacillus_and_Pediococcus)
- [The Struggle to Feed and Reproduce](#The_Struggle_to_Feed_and_Reproduce)
- [Patience Is Key](#Patience_Is_Key)
- [Frequently Asked Questions (FAQ)](#Frequently_Asked_Questions_FAQ)
  - [1. Which classic beer styles are made with mixed fermentation?](#1_Which_classic_beer_styles_are_made_with_mixed_fermentation)
  - [2. How does mixed fermentation affect the final flavor beyond simple acidity?](#2_How_does_mixed_fermentation_affect_the_final_flavor_beyond_simple_acidity)
  - [3. Does mixed fermentation always involve bacteria, or can it be just a blend of yeasts?](#3_Does_mixed_fermentation_always_involve_bacteria_or_can_it_be_just_a_blend_of_yeasts)
  - [4. Is there a risk of contaminating other brewery equipment when working with mixed fermentation cultures?](#4_Is_there_a_risk_of_contaminating_other_brewery_equipment_when_working_with_mixed_fermentation_cultures)
  - [5. What is the difference between kettle souring and mixed fermentation?](#5_What_is_the_difference_between_kettle_souring_and_mixed_fermentation)
- [Recommended](#Recommended)

## What Are Yeasts?

Yeasts are fungi, tiny, microscopic, single-celled fungi. [Fungi are a kingdom](https://en.wikipedia.org/wiki/Fungus). They are not a species or a genus. They are neither plants nor animals, and we did not even know they existed until relatively recently.

![Saccharomyces cerevisiae](https://www.thebeertimes.com/wp-content/uploads/2020/07/La-levadura-y-la-fermentación-1.jpg)*Saccharomyces cerevisiae*

It was not until the 19th century that Louis Pasteur discovered and began to understand the process of fermentation, which moved beer production from the realm of religion and superstition into the world of science.

Since then, brewers have set out to control and manipulate these tiny organisms to get the best results in alcohol, aromas, flavors and, of course, consistency.

Modern [lager beers](https://www.thebeertimes.com/en/what-is-the-real-difference-between-ale-and-lager-beer/) and their yeast, [Saccharomyces pastorianus](https://www.thebeertimes.com/en/mother-of-all-lager-yeasts-forests-chilean-patagonia/), are living proof of the innovations in brewing science, but that is a story for another article.

## Saccharomyces cerevisiae

When people talk about yeast, the word is usually used as a generic term for one particular species called [Saccharomyces cerevisiae](https://www.thebeertimes.com/en/the-use-of-saccharomyces-yeasts-in-cancer-research/).

This is the yeast species that humankind has cultivated for thousands of years to bake bread and brew beer.

![Wine, bread and beer](https://www.thebeertimes.com/wp-content/uploads/2016/09/cerveza_vino_pan.jpg)

Think of this yeast species as a domesticated dog. Consider how many variations of domesticated dogs exist, each with its own unique traits.

A [Jack Russell Terrier](https://en.wikipedia.org/wiki/Jack_Russell_Terrier), for example, was bred to squeeze into small spaces and catch rodents. A [Siberian Husky](https://en.wikipedia.org/wiki/Siberian_Husky), on the other hand, was bred to pull sleds through the snow. A Chihuahua was bred to… ride in a handbag? You get the idea.

However different they are, these dogs are still one species called [Canis lupus familiaris](https://en.wikipedia.org/wiki/Dog), and they all descend from the gray wolf.

Exactly the same thing happens with Saccharomyces cerevisiae yeast. Within a single species there are many variations, which in yeast we call strains.

There are wine and cider yeasts, selected to enhance fruit character, as well as baker’s yeast, which is much more focused on producing CO2 to raise dough.

However, standard brewer’s yeast is not the only one capable of producing alcohol and CO2. There are many other genera of yeast that consume sugar and multiply.

To keep things simple, let’s focus on one of these genera, Brettanomyces yeasts.

## Brettanomyces vs. Saccharomyces

Brettanomyces, or “Brett” as it is often called in the beer world, is frequently known as a wild yeast. Although its roots are clearly linked to spontaneous fermentation, many of today’s Brett beers use strains that have been isolated and grown in a laboratory, just like Saccharomyces yeast.

In the wine world and in most beer styles, Brett is still a dirty word associated with “infections” that can seriously damage the product because of the “unpleasant” aromas and flavors it can create during fermentation.

![Brettanomyces](https://www.thebeertimes.com/wp-content/uploads/2016/12/Brettanomyces-en-la-cerveza.jpg)*Brettanomyces*

Some of these flavors could be described as medicinal or barnyard. That is why the focus in production is on avoiding the risk of cross-contamination.

But it has not always been this way. “Natural wines,” where the winemaker intervenes as little as possible in the process, are a good illustration of what wine probably used to be like.

They are like a delicious history lesson in a glass, and fortunately they are becoming very popular. Orval, for example, the classic Belgian beer famous for its dry finish, originally contained Brettanomyces naturally, almost by accident.

![Guía Práctica Catar Cerveza Amazon](https://www.thebeertimes.com/wp-content/uploads/2025/05/Catar-cerveza-300x300.jpg)

[Guía práctica para catar cerveza: Cómo apreciar correctamente todas las cervezas del mundo](https://amzn.to/3LddZmQ)

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The story goes that, during an upgrade of the brewery, the equipment was replaced and the cleaning processes were improved, but the resulting beer simply lacked depth. It had lost its soul. People noticed the difference and no longer liked it.

The brewers had to give people back what they wanted, so they rethought the process and established mixed fermentation as a set procedure, using standard brewer’s yeast (Saccharomyces) to which Brettanomyces was then added.

## What Is Mixed Fermentation?

Brettanomyces and Saccharomyces are not different strains but distinct genera that belong to different families. It is like a shepherd and a sheepdog working together to herd sheep, two very different organisms with the same goal.

That is why combining two strains of the same species is not enough. A wine yeast and a beer yeast are both still Saccharomyces cerevisiae, and combining them is a mixed culture of strains, not a mixed fermentation in the brewing sense.

## Lactobacillus and Pediococcus

We won’t go too deep into bacterial taxonomy. However, it is worth saying that when it comes to mixed fermentation beers, bacteria are often the key.

Specifically, we are talking about the lactic acid bacteria Lactobacillus (also used to make yogurt) and Pediococcus (also used in fermented foods such as cured meats).

![Pediococcus](https://www.thebeertimes.com/wp-content/uploads/2016/10/Pediococcus.jpg)*Pediococcus*

Put simply, yeasts ferment the sugars in the wort and turn them into alcohol and CO2, while bacteria create the acidity. In reality, though, much more is happening beneath the surface.

For example, some yeast strains produce acids under certain conditions, and some types of bacteria are inhibited by alcohol or hops.

In the cultures at Yonder, the author’s brewery, Brettanomyces is also the only organism able to break down the viscous substance produced by Pediococcus.

## The Struggle to Feed and Reproduce

Every single cell in the beer is fighting for something, mainly to feed and reproduce, but in doing so they unexpectedly end up helping one another.

The interplay of all these processes ultimately produces something unique, a fortuitous balance. Take away just one of these components and you are left with nothing.

If the fermentation had only Brettanomyces, the beer would probably end up thin and very dry, but if you only had Pediococcus, the beer would become thick and undrinkable.

With all of this happening during fermentation, it takes much longer to go from grain to glass than it would with a single isolated yeast strain.

## Patience Is Key

You have to wait months for fermentation to finish completely, sometimes years. Then the beer has to be refermented in bottles and conditioned for several more months before it is ready to reach the consumer.

When you explain mixed fermentation like this, it is hard to see the benefits of the process. We could brew single-strain beers, we could sour quickly with lacto cultures and we could force carbonate, but all of that would result in beers with less complexity, less variety and, ultimately, less innovation.

## Frequently Asked Questions (FAQ)

### 1. Which classic beer styles are made with mixed fermentation?

Mixed fermentation is not exclusive to modern craft beers. It has deep roots. Classic Belgian styles such as Lambic (spontaneous fermentation, the purest form of mixed fermentation), Gueuze, Flanders Red Ale or the iconic Orval (because of its added Brettanomyces) are the best-known examples. It is also common in American sour styles and in historical sour beers.

### 2. How does mixed fermentation affect the final flavor beyond simple acidity?

The impact is multifaceted and not limited to the acidity created by Lactobacillus or Pediococcus. Brettanomyces yeast contributes compounds such as 4-ethylphenol (barnyard or earthy notes) and 4-ethylguaiacol (smoky or spicy notes). The interaction between the different organisms can produce more complex esters and phenols, resulting in stone fruit, leather and wet hay flavors and an exceptionally dry character.

### 3. Does mixed fermentation always involve bacteria, or can it be just a blend of yeasts?

It can be just a blend of yeasts, as long as they belong to different genera. Combining Saccharomyces cerevisiae and Saccharomyces pastorianus would not be mixed fermentation, because both belong to the same genus. A blend of Saccharomyces and Brettanomyces, on the other hand, is considered mixed fermentation because they are different genera.

### 4. Is there a risk of contaminating other brewery equipment when working with mixed fermentation cultures?

The risk is very high. Mixed fermentation organisms, especially Brettanomyces, Lactobacillus and Pediococcus, are known to be hard to eradicate from stainless steel, silicone and plastic. That is why many breweries keep the equipment for conventional beers completely separate from the equipment for mixed or sour fermentation, with dedicated hoses, valves and kegs to avoid cross-contamination.

### 5. What is the difference between kettle souring and mixed fermentation?

Kettle souring is a fast production method in which Lactobacillus is pitched into the wort before the boil. Once the desired acidity is reached (usually within 24 to 48 hours), the wort is boiled, which kills the bacteria, stops the souring and leaves the final fermentation to Saccharomyces alone. The slow process of mixed fermentation, by contrast, has yeast and bacteria working and evolving together in the same vessel for months or years, building layers of flavor and complexity that quick souring cannot replicate.

## Recommended

- [Flanders Red Ale Becomes a Protected Geographical Indication (PGI) in the European Union](https://www.thebeertimes.com/en/flanders-red-ale-protected-geographical-indication-european-union/)
- [Duchesse de Bourgogne: Tasting Notes and Recommended Pairings by a Beer Sommelier](https://www.thebeertimes.com/en/duchesse-de-bourgogne-tasting-notes-and-recommended-pairings-by-a-beer-sommelier/)