Beer requires yeast to ferment sugars into alcohol and to form new flavours in the process. Most beers today are fermented by a single strain of yeast, usually a species of saccharomyces, or brewer's yeast. However, before the rise of monoculture in the early 20th century (which inspired the use of single strains), beer was fermented by a combination of saccharomyces and other microbes, in a natural process that we now call mixed fermentation.

These other microbes are mostly cultures of bacteria and yeast other than saccharomyces. A well-known example is a species of yeast called brettanomyces. These cultures produce uncommonly deep and complex flavours, but require more time than brewer's yeast to complete their fermentation. They need several months or even years to produce the desired flavours, and all the while it is never quite certain what the end result will be. Experience with mixed fermentation is indispensible for coaxing microbes into producing well-balanced beers.

Mixed fermentation is a delicate process that requires courage, careful study and a good deal of patience, but when done right gives access to a range of flavours that will leave you amazed.