Modern breweries make good use of literally centuries of scientific study in heat transfer and exchange. In our modern breweries, we have highly efficient means of heating and cooling our wort but in in the 18th Century, all of this was much more primitive. Those of you who have cooked over an open hearth or attended one of my brewing demonstrations understand the complexities of heating wort over a wood fire. Its constant work because the fire is either too hot and the kettle boils over (generally extinguishing the fire!) of its not hot enough and it takes forever to bring the kettle to a boil. Of course, what goes up must eventually come down so as if that weren’t enough aggravation to heat the wort, once you have boiled it, you must be cool it down enough so that it doesn’t kill the yeast when you try to start the fermentation.
When I do demonstrations, I generally only show the “hot side” of brewing. Once the wort is made, I package it up and take it off site to ferment. This allows lots of time for the wort to radiatively cool and since I am making small batches (5 gallons) the timing all works out for me to safely get home and finish the job but that’s not what colonial tavernkeepers did. Brewing is a time-consuming task so when they brewed beer, they brewed BARRELS of beer at a time. What I do is not scalable to one barrel (the radiative cooling would take a full day) let alone 5-10 barrels a month a typical tavern in colonial Philadelphia might consume. Like their modern compatriots, colonial brewers employed cooling systems to chill their wort but not modern counterflow glycol chillers.
A koelschip is a vessel used in a brewery that serves the purpose of chilling through evaporation and convection. It works by dramatically increasing the surface area of the wort so that more heat can be liberated over time. Of course, the disadvantage of a koelschip is that if this is done in a less than ideal location, you may find that you are inoculating the wort with ambient microbes while you are trying to chill it. When those microbes are the good kind, like brewer’s yeast — Saccharomyces cerevisiae, that’s and advantage but when these microbes are of the wrong sort, like most bacteria and other fungi, it can cause real problems. I don’t bring koelschips to reenactments and field demonstrations because, unlike a fixed brewery with an enclosed space for the wort to chill, I cannot control the environment. I imagine beer inoculated with the open air of Fort Mifflin would be rather funky with a jet fuel aftertaste, not what I seek to represent.
Several of us are trying, however, to build a “brick and mortar” 18th Century Brewery near Brandywine (see We’re Creating a Brewery and You can help) and in this controlled environment, we plan to install a classic piece of 18th Century brewing technology – the koelschip.
A koelschip is a very wide and shallow basin with an open top so that the wort remains exposed to the air above it. At the end of the boil the boiling wort is transferred directly to the koelschip and it is allowed to cool through outgassing, evaporation, and occasionally by counterflow chilling that happens when a nearby stream is redirected to flow under the koelschip (but not in contact with the wort). Koelschips are traditionally made of wood but toward the end of the 17th Century, copper, tin plated iron, and other metals became more common. The high surface area-to-volume ratios has three functions: cooling, aeration of the wort, and separation of the cold trub. In some situations, a hop-filtering basket is attached to clarify the wort before fermentation.
Because the hot wort is left in the koelschip for an extended period of time, often overnight, as it slowly cools the organisms in the air, bacterial and yeast imbedded in the wood, and yeast deliberated inoculated by the brewer all end up in the fermentation. The result can be very complex and rich assortment of interesting and challenging flavors like a Belgian lambic or they can have funky e sour tastes. When done right, Koelschips create beer’s equivalent of “terroir” which is the vintner’s term for the confluence of everything local — soil, precipitation, climate, etc. – that determines the unique characteristics of a particular region’s wine.
In our post Prohibition world, almost all beer is brewed using much the same equipment, material and methods. We make a lot of noise about very small differences in craft beers but in the wider range of beers and brewing, there is so much more. The koelschip gives us some insight into what beers and brewing were like and could have evolved into. In the experimental world of “living history”, we are relearning the art and science of heating and cool wort.
Next challenge – Cold Beer during summer! Well, England never really learned that either.