The Magic of Mixed Fermentation
The brewer’s trade is one of those cases where the name doesn’t really ring true. Let’s be honest – a brewer just “sets the table” and then waits, hoping that after the feast the result will be as close as possible to what was intended. This is why the words “yeast herder” or “yeast farmer” are often mentioned ironically at brewers’ meetings as the true names of the trade. In recent years, the term mixed fermentation has come up quite often. These are beers in which the brewer deliberately sets the table for more than one yeast or bacterial culture.
In Search of Purity
First, a little digression – any beer before the 1880s was a mixed fermentation beer. A large proportion of small brewery beers, as well as a number of world-famous Belgian beers, are still mixed fermentation products. Any beer in which more than one yeast or bacterial strain has been involved in the fermentation is called a mixed fermentation beer. Consequently, the ability to predict the outcome is reduced due to the increase in the number of variables, while the complexity of flavours and aromas increases. It is important to mention here that the brewer should know that the beer will be mixed fermented before it reaches the final consumer. Otherwise, it is simply a poorly brewed beer.
I wonder if there is a more important time in the beer industry than the mid-19th century! At that time, European brewers had discovered the thermometer, which allowed more precise preparation of sugars for yeast. Experiments began which made it possible to cool the wort and to control the fermentation and storage temperatures of beer, regardless of the season or how much ice was left in the cellar from the last winter. It became possible to brew beer all year round, not just from October to March (we have a longer brewing season). But the most important push came from Louis Pasteur, who declared that fermentation was the responsibility of living organisms. Breweries feverishly started buying microscopes to look at their smallest employees. And in 1883, at the Carlsberg laboratory, then and still the leading scientific institution in the beer industry, Emile Christian Hansen separated a single yeast cell, fed it with sugar and managed to grow the first pure yeast culture.
Mixed Fermentation Beers
All this is unimportant, because when brewing a mixed-fermentation beer, it is rarely a task to “throw out of the brewery door” beers that are indistinguishable in taste and smell and have identical labels. Such beers are a unique fixation of the passage of time in the bottle – different micro-organisms are allowed to coexist in the same wort to produce the richest possible bouquet of flavours. It is not even a question of objectively measurable quantities such as the sugar, protein, starch, vitamin or water content, the number of different yeast cells, the amount of air in the wort, the proportions of the tank, the fermentation temperature and pressure, all of which can be reproduced, but of the interaction of the different micro-organisms, where the human ability to calculate the result is still limited.
Below is a list of the most popular “actors” that tend to appear in different proportions on the mixed fermentation beer flavour stage. Each genus is rich in tribes, each of which has its own niches that are reflected in the taste and/or aroma of the beer. Almost all micro-organisms are still found in the wild:
The Saccharomyces Genus
Mainly known for Saccharomyces cervisae , responsible for wine, cider, honey, beer and all other fermented beverages, it is the most studied micro-organism in human history. The great variety of strains gives different beer styles their characteristic notes. The lager yeast Saccharomyces pastorianus differs slightly from them, being able to perform the same task perfectly at lower temperatures , allowing the malt and/or hops to dominate the flavour of the beer. Both love sugars, produce alcohol and a strain-specific flavour nuance, as well as carbonation. It is a fast and predictable workhorse throughout the fermenting world.
The Brettanomyces Genus
A few decades ago, only Belgian brewers and winemakers from certain regions considered its representatives as welcome guests. Like Saccharomyces, it loves sugars, but does not eat starch or protein. It is considerably slower, can produce varying quantities of acetic acid alongside alcohol , and has the famous ‘horse-decoc’ aroma. It is the spicy, maturearoma and old-age flavour of ‘horse blanket’ that brewers usually seek to achieve in their mixed-fermentation beers by introducing members of this genus. At the same time, patience is required, as the desired flavours can take several years to develop.
The Lactobacillaceae (Lactic Acid Bacteria) Family
Best known for dairy and vegetable cooking. In beer, it mainly brings acidity of various kinds. From juicy citrus to slightly bitter. Usually plays on the flavour stage in pairs with the alcohol level, covering or even replacing it. Loves sugars but prefers lactic acid to alcohol. In the wild lives on malted grains, plants, fruits and vegetables. It is hops that it dislikes most, which is the main reason why hops are present in almost all beers.
The Acetobacteraceae (Acetic Acid Bacteria) Family
Most often, beer is not the primary choice. When wild yeasts are “fermented” or raw berries and fruits are used, members of the acetic acid family are often also present in the beer. These are slow but very determined ‘guest workers’ that ‘chew’ the alcohol and convert it into acetic acid. They love heat and oxygen, but dislike sulphites.
