How to Home Brew Beers: How to re-use yeast from the trub

Yeast Trub & How to Re-Use It Advanced Harvesting Techniques for the Frugal Brewer

We've talked a bit about how vital yeast is to the beer brewing process. It is the only ingredient that is alive, and consequently, it is often the most expensive component of brew day.

This cost drives many brewers to a simple realization: the "waste" at the bottom of your fermenter isn't waste at all. It is a dormant army of viable cells waiting to feast again. While commercial breweries re-pitch yeast as standard practice to ensure consistency and lower costs, homebrewers often pour this liquid gold down the drain.

Here is how to stop wasting money and start harvesting your yeast cake.

The creamy layer of yeast is distinct from the darker trub layers.

Concept

Trub Anatomy

What is in the cake?

You might think that stuff at the bottom of the fermenter is just "gunk." In brewing terms, we call this Trub. It consists of three main things:

Heavy Debris: Fats, proteins, and hop particles (usually at the very bottom).
Live Yeast: A creamy, white/tan layer of dormant cells (the gold).
Dead Yeast: Darker, brown layers of cells that succumbed to stress.

There is usually plenty of viable yeast left in this mix. In fact, a fresh yeast cake from a 5-gallon batch can contain 300-500 billion cells—far more than the 100 billion you get in a fresh store-bought packet.

Method A

Yeast "Washing"

Separation via Density

Washing your leftover yeast is a great skill. You aren't "cleaning" it with soap; you are rinsing it to separate the live yeast from the hops and dead cells. This works on the principle of Stokes' Law: heavy particles (hops) settle faster than light particles (yeast).

Boil & Cool: Boil 1.5 Liters of water to sterilize it, then cool it to room temperature.
Mix: Pour this water into your fermenter's trub cake. Swirl it vigorously to create a slurry.
Settle: Pour this mix into a large conical flask or sanitized jar. Let it sit for 15-20 minutes.
Separation: You will see dark heavy matter fall to the bottom quickly. The liquid above will stay milky/creamy—that's your yeast!
Decant: Carefully pour that milky liquid into smaller, sanitized mason jars, leaving the dark sludge behind.

Store these jars in the fridge. Over a few days, the yeast will settle into a nice white layer at the bottom of the jar. When you are ready to brew, decant the clear liquid off the top and pitch the white cream.

Method B

Simple Slurry Harvesting

The Quick Method

If washing sounds like too much work, you can do the "Lazy Man's Harvest." This involves simply scooping up the slurry without separating the hops.

Once the beer has been racked for kegging or bottling, leave a small amount of beer behind to swirl up the trub. Pour this thick sludge directly into a sanitized jar. Store it in the fridge.

Timing is Key:

< 3 Weeks: You can pitch this directly into a new batch. Warm it to room temp first!
> 3 Weeks: The viability drops. You should make a yeast starter (add a liter of fresh wort) to "wake it up" and prove it's still active before brewing.

Method C

Pitching on the Cake

Risks of Over-Pitching

Can I just pour fresh wort directly onto the old yeast cake in the fermenter?

You sure can, but be careful. A full yeast cake contains massive amounts of yeast—often too much for a standard gravity beer. While this ensures a lightning-fast fermentation, over-pitching can lead to:

Thin Body: The yeast strips the beer of character too fast.
Poor Head Retention: Old proteins interfere with foam stability.
Autolysis: As layers of dead yeast build up, they break down (explode), releasing meaty, rubbery, or soy-sauce flavors into your beer.

Recommendation: Only pitch onto a cake for high-gravity beers (Imperial Stouts, Barleywines) where you need that massive cell count.

Science

Genetic Drift & Generations

When to Stop

Commercial brewers reuse yeast for many cycles ("Generations"), sometimes up to 40 or 50 times. However, homebrewers usually stop after 5 to 10 generations.

Why? Genetic Drift. Yeast mutates rapidly. After several batches, an English Ale yeast might lose its ability to "flocculate" (clump and fall out), leaving you with cloudy beer. Or it might lose its ability to create those signature fruity esters. Furthermore, every harvest carries a tiny amount of bacteria. By generation 10, that tiny infection has been amplified enough to potentially sour your batch.

Using Conical Fermenters makes this process safer and cleaner, as you can dump the trub via the bottom valve without opening the lid and exposing the beer to air.