Yes, you can safely reuse yeast slurry.
If you are brewing in perfect conditions and caring for your yeast correctly, you can reuse it up to 10 times.
However, I suggest you err on the side of caution and buy a fresh pack after four generations.
The benefits of reusing a yeast slurry are obvious. You save time and money. You get a better beer.
You don’t have to rehydrate your yeast or make a starter as there is ample yeast volume to smash out your fermentation.
Check for weirdness
Always taste the source batch before pitching the yeast slurry. If there are any issues, these will likely transfer to your next batch.
Don’t repitch if fermentation in the source batch was weak, had a long lag time or was otherwise odd.
Check the yeast’s color
The yeast slurry should be a light tan color. As it ages, it will darken. Once it looks like peanut butter, it is past its prime.
Cold pitching your yeast will shorten the lag time before high krausen. It’s simple to do – just chill your yeast slurry and pitch into your wort. It’ll take care of the rest.
Start with small beer
Don’t reuse yeast from a strong ale fermentation. It’s a big job, so it’s tired and run down afterward. Instead, start with a light or mid-strength beer and build up.
In fact, a perfect way to develop enough yeast for a strong ale fermentation is to brew a weaker beer and reuse the slurry.
Stick with the primary
If you rack your beer, harvest the yeast from the primary fermenter not the secondary. The yeast is healthier and more active.
How to wash and store a yeast slurry
When you pitch your yeast slurry directly from one batch to another, you are also transferring dead yeast, proteins and spent hops.
This can adversely affect beer quality.
Instead, you can easily ‘wash’ your yeast so that you pitch mostly fresh, healthy yeast.
The process from here:
Bottle or keg your beer as per normal, draining all liquid from the top of the yeast cake.
Turn the tap off.
Pour the chilled water on top of the yeast and swirl to mix.
Let sit for 10 minutes while the solids settle to the bottom.
Pour the yeast solution into cleaned and sanitized plastic bottles (25 oz or 750 mL are a great size). Never use glass – pressure can build up even when refrigerated.
To use, simply pour off the liquid and pitch the yeast slurry directly to the fermenter. You should have at least ½ cup for a standard ale. Double this for lagers and strong ales.
Pitch within a couple of weeks for the best result.
Some homebrewers also acidify their water to inhibit bacteria growth. The problem is that acidifying doesn’t completely remove bacteria, can hurt your yeast performance and doesn’t kill wild yeast.
This step isn’t necessary if you are cleaning and sanitizing properly, and not reusing your yeast more than four times.
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