Pitching fresh yeast into every batch of beer is more than just wasteful; it’s expensive and can create an inconsistent brand. To avoid this, most commercial brewers harvest their yeast, meaning they collect viable yeast from one batch to use in the next.
While some brewers crop yeast floating at the top of an open fermentor, most crop from the bottom of conical tanks, where clumps of yeast cells fall during fermentation. Read on to learn exactly how to generate three to 10 generations of usable yeast using the bottom-cropping method.
The Three Layers of the Cone—and Why They Matter
Toward the end of fermentation, yeast cells bind into larger groups and settle out from the beer. As the liquid cools, these clusters fall to the bottom of the tank and mold into the shape of the cone. But this pile of flocculated yeast doesn’t display the same characteristics the entire way through. Rather, it forms three unique horizontal layers.
The bottom layer consists of trub—dead or prematurely flocculating yeast cells, hop detritus, and denatured proteins. The thin top layer holds poorly flocculating yeast. Brewers discard the bottom and top portions and keep the middle. This is where the healthiest, densest, and most genetically beneficial yeast form the heart of the extraction.
One might compare this creamy off-white layer to vanilla ice cream. Though dark beers make it harder to suss out the boundaries, in a paler beer, you can literally see, smell, and taste the difference.
“The layer to harvest reminds me of a milkshake,” says Matt Cole, who owns and oversees an annual production of 47,000 barrels of beer at Fat Head’s Brewery, in Ohio.
“Or soft-serve,” adds his QC and cellar manager, Tynan Smith.
Cole smells and sometimes tastes the slurry that comes off all 32 conical fermentors at Fat Head’s to monitor for off-flavors. He says a thriving middle layer is “very bready and earthy, and there’s an esterey characteristic to it. It smells like fresh-baked bread or when I make pizza dough.”
Equipment Needed to Harvest Yeast
The most basic harvesting setup calls for a hose, two clamps, and a valve to connect the donor and receiver tanks and regulate the flow of the slurry. But commercial brewers usually also rely on a couple more pieces of equipment than that. A sight glass lets them see when one layer gives way to the next, and a storage vessel is fundamental for brewers who don’t brew similar styles frequently enough to repitch right away.
Meet the brink, a vessel that can take the form of everything from a plastic bucket (not recommended) to custom models often outfitted with a hefty price tag. Breweries with three- to 15-barrel systems usually take the middle path by repurposing a stainless-steel keg. Both Cornelius (Corny) and Sanke kegs make for relatively affordable containers that can be pressurized, easily sanitized, and adapted to perform perfectly as brinks.
Breweries usually discover the sweet spot in an unmodified Corny keg or a Sanke keg retrofitted with a 4-inch cap and a 1.5-inch tri-clamp port down below. Both types require some pressure-relief capability such as an air lock, blow-off hose, check valve, or sterile filter.
If you’re repurposing a Sanke, resist the temptation to grab any old, used half-barrel. Unmodified Sankes can create stress for both brewer and yeast with their easily clogging ball-lock valves, as well as internal parts, grooves, and threads that can harbor bacteria. Many tank suppliers sell modified kegs along with new brinks designed for holding yeast.
The tank hookup remains the same whether you’re harvesting directly into a waiting fermentor full of wort or you’re siphoning off slurry to save in a brink. A sanitized hose attaches to the original tank via a tri-clamp, while a butterfly, ball valve, or block-and-bleed setup latches on to regulate or direct the flow.
The all-important sight glass allows a brewer to gauge the moment it’s time to start and stop gathering by watching as one layer gives way to another.
Sanitation Protocol
It goes without saying that careful brewers thoroughly clean and sanitize every piece of equipment that will touch the slurry. And it’s not enough to spray the brink with a keg washer or even go through the normal CIP/SIP process.
In its e-book, Yeast Management & Repitching, Escarpment Laboratories recommends manually cleaning “all parts of debris, scale, soils, etc., before sanitizing.” It identifies the biggest cleaning challenge as the leftover yeast that cakes the inside of the brink, especially the chimes. The lab advises scrubbing out the particles then filling the brink with hot caustic. Seal and soak from a few hours to overnight then rinse well.
Next, fill and seal the brink with a sanitizer like peracetic acid, iodophor, or even Star San. Don’t overlook wiping the ports. Meanwhile, soak the fittings in caustic and follow up by spraying the connection points with isopropyl alcohol. For an optional extra step, sterilize the fittings with a safe source of flame that’s long enough so you’re not close to where the isopropyl alcohol is sprayed.
Because sanitizers tend to lose their effectiveness within a few days, re-rinse, and refill the brink with fresh sanitizer right before you put it to work. Then purge with CO2.
“It gives enough counter-pressure so that you’re not rushing the yeast in there and channeling in beer. You’re at a way more controlled rate,” says Pete Barraud, owner and head brewer at North Fork Brewing on Long Island, in New York. He pressurizes at 15 psi.
Timing Matters When Cropping
Lower temperatures coax the yeast into temporary dormancy and cause them to floc out of suspension; it follows that breweries harvest after they lower the temperature near or at the end of fermentation. To get the thick, clean, silky slurry they seek, harvesting happens after they’ve brought down the temperature but before the slurry thickens into paste.
Unless they’re dry hopping or dosing with other late additions, brewers harvest just before the diacetyl rest—either once fermentation finishes or around 1°P short of terminal gravity. At that point, they cold crash the beer to 34–40°F (1–4°C) then leave it for 24–72 hours while the yeast compacts at the bottom.
To avert particulate matter and decreased viability, cropping occurs before dry-hopping or other factors that complicate fermentation. In these cases, brewers generally soft crash by lowering the temperature to slightly below primary fermentation levels. The exact numbers vary by strain, and you should explore and experiment with temperature ranges on your own.
When Barraud reaches 75 percent attenuation on an ale he plans to dry hop, he drops his tank to 58°F (14°C), then harvests after 24 hours. That’s when he cranks the heat back up to 66–68°F (19–20°C) and adds the hops.
“You still continue to attenuate a little bit, so you still can do the final five to ten percent,” he says.
Trub Drop—The Step Most Brewers Skip
Alas, one more step still stands between you and the actual accumulation: the trub drop. By removing and discarding the trub layer underneath the yeast cake, you eliminate dead cells, hop debris, and early flocculating cells that Escarpment says can contribute “rancid, rubbery, or sulfur-like off-flavors in the beer.”
But first, don’t open a single valve without applying 1–5 psi of CO2 or N2 to the tank. This prevents oxygen from entering the port as the cropping procedure displaces the trub and yeast. Be careful, though, because too much pressure can force beer to tunnel through the cone, leaving the yeast cake behind.
To release the trub into a drain or dump bucket, slowly feather open the valve. Though you’ll want to feather the valve during the entire harvesting process to keep beer from tunneling, feathering often serves a second purpose during trub drop—to unstick the yeast plug that may be blocking the exit.
“Sometimes that bottom chunk doesn’t come out with ease,” Fat Head’s Cole says. “So you have to gently open and close the valve to let that out without it blowing out too quickly.”
If you notice your beer start to channel, turn off the flow and let things settle down.
After you’ve dispensed approximately 1–3 gallons (4–11 liters) of trub, monitor the sight glass to spot the middle layer as it starts moving through. Before you begin capture, discharge a little more slurry to steer completely clear of any remaining dead or early flocculating yeast.
The Harvest Procedure
Now, it’s finally time to start collecting.
Close the dump valve and attach the hose or open the pathway into the brink or receiving tank (depending on your setup). Don’t open the valve all the way because, as with the trub drop, too fast of a flow can bring about channeling.
“If you pull too quickly, you’re going to pull a hole through the center of that cone where your beer can come through. If you pull nice and slowly, you’re going to end up getting a consistent pull of the section that you want,” Fat Head’s Smith says.
Stop harvesting once the visibly thinner and less-fluffy top layer appears. Alternately, stop when you’ve got one-quarter to one-third of headspace left in the brink—CO2-rich slurry needs room to expand as it warms.
Before you put the filled brink into cold storage, measure the gross and tare weights to calculate the net slurry weight. (You’ll want to know the weight of the empty brink ahead of time.) Label with the strain, generation number, harvest date, batch name, and tank number to record performance and troubleshoot any problems.
Brink Storage
Harvested yeast doesn’t respond to stress any better after fermentation than before. It degrades quickly in storage, where it demands an environment free from oxygen, light, or agitation. It also begins to weaken after a few days to two weeks, depending on strain and conditions.
Harvested yeast also insists on cold temperatures—generally accepted to be between 34 and 40°F (1–4°C). Though some brewers narrow or extend the window, research published in the Journal of the American Society of Brewing Chemists found no difference in viability or glycogen and trehalose intracellular levels in lager yeast stored within this range.
No matter how cold or coddled, these living cells do continue to produce CO2 inside the brink. That poses risk of explosion and poisons the yeast. For these reasons, brewers keep the head pressure at less than 2–5 psi and burp the brink at least once a day. Busy brewers can find peace of mind by adding a mini blow-off valve, hose, and bucket to purge automatically.
Barraud stores his harvested yeast at 38°F (3°C) and vents daily.
“Even at that cold temperature where everything’s sluggish, it will still build up pressure,” he says. “It’s imperative to bleed the pressure—one, so you don’t have a ticking time bomb, and two, so you’re not bursting your yeast cells and really lowering your viability.”