The most microbiologically sensitive part of the brewing process is between the heat exchanger, where we cool the boiled wort, and the completion of primary fermentation. Wort in the kettle is hot enough to prevent microorganisms from establishing residence, but once it’s cooled it’s basically bacteria and fungus heaven—lots of sugar, full of nutrients, and moderate in temperature. The way it’s supposed to go is we pitch a carefully selected and cared-for yeast strain into the wort, which ferments wort into beer. When fermentation is complete, the depleted reserves of sugar, low pH, and alcohol content mitigate the chance and severity of infection. Knockout—pumping the wort from the kettle through the heat exchanger to the fermenter—is, therefore, a critical quality checkpoint to make sure that the yeast strain we select takes hold and prevent foreign organisms from doing the same. Any process equipment that touches cooled wort has to be thoroughly cleaned and sanitized, so when preparing to harvest yeast, we start by soaking all our yeast brink parts in caustic, rinsing them, and finally soaking them in a no-rinse sanitizing solution.
A yeast brink is a modified stainless steel keg that we use to collect yeast from a fermenter and re-pitch into a new batch of beer. It’s been modified with a four-inch port on the top instead of the standard ball-and-gasket seal you may be used to seeing on standard kegs. It also has a one-and-a-half-inch port near the bottom of the sidewall.
We use a specific amount of yeast, measured by weight, per degree Plato per barrel of beer.When a batch of beer is done fermenting, the yeast drops out of solution, or flocculates, to the bottom of the fermenter cone. We take advantage of this by collecting yeast from the cone while it is still healthy. We don’t recollect yeast from beers that have been dry-hopped, high gravity beers, beers that showed an irregular fermentation curve, or beers whose fermentation character isn’t smelling or tasting standard. We only re-pitch our yeast up to five generations from lab-propagated cultures--less for Belgian strains. Any longer and we’re wary of selective mutations changing the desired fermentation characteristics of the yeast strain.
When it’s time to collect yeast, first, we run off about ten gallons of yeast slurry from the very bottom of the cone and dump it down the drain. This yeast was the first to flocculate and therefore the least vital. Collecting this yeast and re-pitching it would likely lead to off-flavors and under-attenuation in subsequent batches. It looks clumpier and darker than healthy yeast and tastes harsh, bitter, or meaty. We can see and taste the change as we approach that ten-gallon mark—lighter, fluffier, fruitier, floral. At that point, we start collecting yeast in our cleaned and sanitized yeast brink.
We use a specific amount of yeast, measured by weight, per degree Plato per barrel of beer. What that means is the stronger the beer we’re making—the more sugar the yeast has to consume—the more yeast we start with. We dial back that amount for Belgian beers because we’re looking for more of a pronounced ester profile in the finished beer and under-pitching can help promote ester production. Once we hit our target weight, we cut off the flow of yeast, cap our yeast brink, and set it aside.
We only re-pitch our yeast up to five generations from lab-propagated cultures--less for Belgian strains.When it’s time to knock out we get our cleaned and sanitized yeast hose ready alongside the yeast brink. When we’re dialed in to our knock-out temperature—usually around sixty-eight or seventy degrees for ales—we hook our brink up to the block-and-bleed assembly at the fermenter inlet. This includes an aeration stone that we use to inject oxygen into the wort at a set flow rate--a vital component of yeast's reproductive process that allows the colony grow to the needed size for fermentation. We divert wort into the brink to rouse and mix the yeast, then use carbon dioxide to push the mixture into the fermenter. We repeat this process a few times to ensure that we capture all the yeast we pulled, lifting and shaking the brink in the process, and verify with a flashlight after pitching that the brink is empty.
There are more and less intensive yeast harvesting and pitching methods. As a new-ish brewery, we don’t have the laboratory equipment and procedures we need to check cell counts and viability. That’s all in our plans for quality control this year and will give us a greater degree of knowledge of and control over our yeast management. The process we use is typical of brewpubs and small production breweries.
So the next time you are hanging in the taproom and peeking over the wall, realize that everything you see serves a purpose. We're too small and running too tight to waste space on unnecessary pieces. Even the 'No Sublime' poster has it's role in the beer-making process. Actually, especially the 'No Sublime' poster.
This is the six post in my “What’s That Thing Over There?” series in which I reveal the extraordinary and mundane of brewery miscellania. In my previous posts, we talked about how beer is like sausage and legislation, got the skinny on the troll that lives under the brewdeck, proved how reading CO2 levels can make or break you, why chemical burns are no fucking joke, and how our buddy Grant helps along in the brewing process. Next time, who knows? The Thing you’re wondering about may be on deck.