Leah’s Beer School: Lesson 19

What is a coolship?

The term coolship, from the Flemish koelschip, is a large shallow vessel made of stainless steel or copper, used to cool wort. Coolships are often housed in large rooms, where they lie open to the elements allowing for airborne yeast to flow in and inoculate the beer. This type of fermentation is referred to as “spontaneous fermentation” because the microbes come at random, rather than using a controlled addition of cultured yeast strains. Predominantly used to create sour beers, a renewed interest in coolships for brewing has been inspired by Brasserie Cantillon, a world-renowned lambic producer based in Belgium, established in 1900.

How a coolship is used

Coolships are generally only used a couple of months of the year, as the ideal exposure season is highly dependent on ambient temperature. At very low temperatures, not enough yeasts will survive, when the weather is too warm, too many undesirable airborne bacteria will contaminate the wort. In an ideal environment, brewers let the wort (or “beer starter”) cool overnight, where the high surface area of the coolship allows the liquid to cool as quickly as possible. The uncovered vessel is able to pick up airborne yeast and microflora that will inoculate the wort and give beer its funky or sour flavor once it is fermented.

Spontaneous Fermentation

Naturally-occurring yeasts are in the air around us, but also land on places such as brewing equipment and exposed brewery rafters. In a coolship, local yeasts settle on the wort after it has cooled, devouring the sugars and diligently multiplying to make beer. These local yeasts provide terroir, a sense of place, through a mixture of acidic and funky flavors at different intensities depending on various factors like location, time in the coolship and season. As a testament to the importance of this unique environment, Cantillon brewers sprayed the walls and roof of their new brew space with Lambic beer to recreate the same microbial environment as the original building.

Rebirth of Coolships

The use of coolships was mostly abandoned after refrigeration and temperature-controlled equipment like heat exchangers and cone-shaped fermentation tanks came into play. In the past decade, coolships have experienced a small renaissance from North American craft brewers, to make spontaneously fermented sour beers. Inspired by Cantillon and other lambic producers, a handful of North American breweries have created modern versions of these coolships, most often located in an enclosed room with vents or windows that allow air (and its natural bacteria) to flow in.

Used in modern brewing, coolships bring back a tradition where the natural environment, or unique terroir, defines the flavour of the beer. Bellwoods Brewery in Toronto, and Pit Caribou in Quebec, are two breweries making some pretty incredible spontaneously fermented sours. Coolships are a labour of love, as they require a huge investment of resources, space, time and an incredible amount of patience. Spontaneously fermented sours offer much more character and depth than kettle sours, and are fairly expensive to produce. That said, a great barrel-aged sour is worth the wait and the price– it reflects a very specific time and place, unique to both the environment and brewing team.