Mixed Fermentation and Sour

A popular question on our brewery tours and at Meet The Brewer events is ‘what do you mean by mixed fermentation?’. A good question, and one that inevitably leads to animated discussion and lots more questions! So in this blog I attempt to set down some of our thoughts on mixed fermentation and sour brewing.

Mixed Fermentation refers to the practice of using multiple different strains of yeast and bacteria when fermenting a beer. These fermentations can result in vastly different aroma and flavour profiles when compared to beers fermented with a standard ‘brewers’ yeast’ (Saccharomyces Cerevisiae). Yeast strains and bacteria typically used in mixed-fermentation brewing include brettanomyceslactobacillus and pediococcus.

So is a mixed-fermentation beer the same as a sour beer? This is a more thorny question, with different terminologies or interpretations. ‘Sour beer’ has become a bit of a catch-all term for many different styles of beer, many of which we make at Pastore. However mixed-ferm does not necessarily equate to sour, or at least does not necessarily result in a sour (low pH, acidic) beer. For example a beer fermented with brettanomyces but neither lactobacillus nor pediococcus will exhibit interesting and funky aromas and flavours but may not be particularly sour.

For that matter, what do we mean by ‘funky’ aromas and flavours? Well, if you know you know. Taste a Lambic or a wild ale, and that’s what you’ll get. These come from phenols produced by wild fermentations, with profiles often described as farmyard, barn, goat or horse-blanket! Wild fermentations also produce incredible esters that you don’t get from a standard fermentation, imparting beautiful fruit notes to the beer, anything from cherry pie to pineapple. The only catch is that to get these incredible flavour profiles usually requires a long slow fermentation, anything from months to years.


I’ve mentioned wild ale and wild fermentation a couple of times now. These are basically fermentations using wild yeasts, which are generally brettanomyces rather than saccharomyces strains. This can be done through ‘spontaneous fermentation’, whereby fresh wort is left open to the elements (typically in a so-called ‘coolship’), where it is inoculated by whatever wild yeasts are floating around. Pastore’s wild ‘mother culture’ was initially captured through a spontaneous fermentation in our back garden in a Cambridgeshire village. Our mother culture is a living thing, think of it like a sourdough bread starter, which we use to blend back into and innoculate all of our wild ales, giving them a great start in life.

Finally, a note on barrel ageing. Not all of our mixed fermentation and wild ales are barrel-aged. For example our table beer, A Tavola, is brewed to a low OG Saison recipe (malted barley, wheat and oats), inoculated with our mother culture and aged in stainless steel. This results in a beer with many of the notes associated with wild fermentations and yet still relatively ‘clean’ tasting. By contrast, a Pastore beer that has been inoculated with our mother culture and then aged in oak will pick up many additional layers of complexity – from the oak itself, perhaps from whatever crazy wild yeasts are leftover in the barrel from previous wine or sherry fermentations, or just from production of small amounts of acetic acid caused by ingress of oxygen.

So let’s end with one final question. Are all Pastore beers mixed-fermentation? Yes, they are! But are they all wild? No! We make wild ales fermented in both stainless steel and oak barrels, and packaged in bottles. But our mainstay is our range of fresh sours packaged in can and keg. These undergo a relatively short mixed-fermentation with a strain of yeast known as Kweik Hornindal along with Lactobacillus Plantarum. This produces our trademark soft lactic acidity, and we call these beers ‘fresh’ because they turnaround pretty fast by sour-brewing standard – from brew-day to release is around one month.


I could go on and on, there really is so much to say! If you visit the taproom, book onto a brewery tour or come to a Pastore Meet the Brewer event at a bar or bottleshop near you, then we can dive deeper into this endlessly fascinating subject!