The method of fermentation described above works with fermenters over a fairly wide range of sizes, mainly depending on the shape of the fermenter. The dissipation of heat from the fermenter into the air of the fermentation cellar (commercial lingo for a special room for fermentation and aging) is a function of surface area, temperature differential between the beer and the surrounding air, and the heat transfer coefficient of the fermenter walls.
A fairly extreme example of large, non-jacketed fermenters are the shallow, 100-BBL, open fermentation vessels used by Anchor Brewing for their Steam Beer fermentation. The wort for this beer is cooled below the maximum fermentation temperature and the fermentation allowed to rise in temperature over time. The peak fermentation temperature for beer fermented in non-jacketed vessels is a function of initial wort temperature, vessel shape, cellar air temperature, wort original gravity, and yeast strain. Although this method takes some time to dial in the process, reproducibility among successive fermentations can be achieved once a procedure is established.
There are a few key things to consider when using non-jacketed fermenters because you are giving up individual control over your fermentations. Here are a few pointers that I hope will be helpful for your nanobrewery:
Tip #1: Set your cellar temperature around the yeast you are using for the beer(s) currently being fermented. For example, if the goal is to keep the peak temperature less than 75 °F/24 °C, your cellar temperature should be set to a cooler temperature, probably around 68 °F/20 °C, and the wort should be cooled to about 61 °F/16 °C to achieve the fermentation temperature curve that works for the style(s) of beer being brewed. Beer styles like saisons benefit from warmer peak fermentation temperatures than do Scottish ales, so plan your brews accordingly and change the cellar temperature as required.
Tip #2: Buy a hygienic temperature probe and measure the temperature at different points within your fermenter. Uni-tank fermenters have short thermowells with an insertion length into the tank ranging from 6-12 inches, so most brewers really have no idea how warm their fermentations become during peak activity. This data gap and general lack of appreciation about the limitations of short thermowells, leads many brewers who use uni-tanks to believe that they have very consistent temperatures during fermentation; the fact is that there are temperature gradients within tanks from top to bottom and from the perimeter to the center. This means that using a probe to get a feel for what is happening within your tanks may lead you to believe your fermentation temperatures are too variable because much of the data about fermentation temperature is limited to where probes in uni-tanks are located. Judge the outcome of your ferments by time, attenuation, and beer flavor before getting too caught up in comparing your brews to textbook graphs.
Tip #3: Yeast management is important in all types of fermenters, but is especially important when fermenting in non-jacketed vessels. Minimizing flavors associated with yeast autolysis by periodically removing settled yeast from cone bottoms is a technique that definitely should be considered when using tanks without cooling. Yeast skimming is another method that could benefit these fermentations, especially when using top-cropping ale strains in open fermenters.
Tip #4: Don’t force this technique on all beer styles. Beers fermented in non-jacketed vessels are likely to have different ester profiles than beers fermented in temperature-controlled tanks. So enjoy the cost savings of this method by knowing when to back off of certain styles. And you may find that some styles simply don’t work well with this method.
Tip #5: Design beers around your brewery and let people know what you are doing. There is a marketing story about this method that may resonate with your pro-organic consumers; all beer was fermented in non-jacketed fermenters prior to modern times and this method has a throw-back feel to it. In an increasingly crowded market space, this sort of differentiation is terrific when the method adds its own notes to the beer. Own it and market it!