How to Top Crop Yeast
Brewers figured out quite quickly how to top crop yeast in early times, and it has been a tried-and-true method of harvesting and growing your own yeast for millennia.
Top cropping yeast has not changed in all that time, and it is still a valid way to continue on using the yeast you love without having to spend more money on a new pack every pitch.
The Power of Yeast
It is a great misconception that yeast wears down over time, so it must be thrown out.
In fact, fermentation only makes yeast grow stronger with each pitch.
Under proper conditions – an open vessel, plenty of oxygen, and ample sugar – yeast will reproduce and thrive, making for an even stronger crop of yeast to harvest for your next pitch.
The misconception that fermentation kills yeast likely comes from those who intentionally destroy yeast by adding alcohol as it is the higher concentrations of alcohol that typically kill yeast off, though many may still survive.
Indeed, this survival is precisely what allows us to crop the yeast after primary fermentation is complete, and how some yeast will remain even then to continue into secondary fermentation.
Yeast really is a powerful living organism that is not typically killed by the alcohol it produces, instead lying dormant and ready to be collected and put back to work.
What Is Top Cropping Yeast?
Top cropping yeast is the oldest method of collecting yeast that has undergone fermentation.
Top cropping refers to the collection of yeast from a fermentation vessel that is open to the air, the types of vessels that have been traditionally used by brewers throughout history.
From the beginning, brewers brewed their batches in large vats or barrels with only a light covering to keep out large debris.
Today, the thinking seems to be that an open-air vessel will be rife with contaminants, and while it may be true that a wild yeast or bacteria can enter the brew from the open air, it is also true that the yeast and good bacteria in the brew will most likely kill off any bad bacteria or harmful contaminants.
And what the yeast and good bacteria do not catch, the alcohol most certainly will.
Think about it, beer and wine have been produced since virtually the beginning of mankind, and no major sanitizers and hyper cleaned areas were necessary. Likewise, we do not hear about major outbreaks of illness due to contaminated beer and wine.
On the contrary, beer and wine have been used as the primary drink of choice to go with meals for entire families precisely because it was not only nutritious but also clean, cleansing contaminants that would otherwise be in running water from a nearby source.
Thus, even in modern times, many brewers top crop their yeast from open air vessels.
Also see: How to reuse beer yeast: 3 steps to repitching yeast slurry
Benefits
The primary benefit of top cropping yeast is that you can catch it in real time, skimming off the healthy yeast as it develops at the top of your brew, without halting or interfering with the fermentation process while it is in action.
This way, you know you are getting the healthiest possible crop from your batch.
Unfortunately, you can only top crop with ales, which use the yeast Saccharomyces cerevisiae, which flocculates on top of the brew, as opposed to lager yeast, Saccharomyces pastorianus, which flocculates on the bottom of the vessel and must be cropped using a conical fermentation vessel.
How to Top Crop Yeast
To top crop your ale yeast, have a mason jar, or several, depending on how much beer you are making and how much yeast you expect, on hand.
Once you have pitched your yeast, wait 24 to 48 hours and you will see your first, “dirty” foam rising and bubbling to the surface of your brew.
Skim all this foam off and discard it. This yeast is the early flocculating yeast that is quickly killed off as the alcohol levels rise.
Next, you will see your milky white foam rise after about another day of fermentation. This is the yeast you want. It is actively krausening and thriving. You want to catch it while it is at its peak fermentation.
Skim this foam off and place it in your mason jar, pouring clean, sanitized water on top of the foam and covering the jar with a lid. Then place the jar, or jars, in the refrigerator for one day.
The next day, pull your jars out of the refrigerator, and you will notice layers of trub, yeast, and slurry have settled in.
The trub is dark brown and is on the bottom and can be discarded. Your milky white yeast is in the middle and can be accessed by pouring off the water and slurry on top of it.
You can then rinse your yeast again in a newly cleaned mason jar with clean water until all you have is thick white yeast ready to pitch.
After another few days, you will notice another dirty foam rising and falling on your brew. This is more dirty, dead yeast that can be discarded.
Consistency Is Key
With yeast cropping, consistency is the biggest factor to a successful crop. Always skim your yeast on the same day during fermentation, so you will know you are pulling a similar culture to the ones you skimmed before. These are the most successful and most likely to reproduce yeast cells that you can typically pitch for several generations before they finally give up and provide no more milky white yeast to crop.
You also want to be sure to use sanitized equipment here so as to avoid as much as possible wild yeast and bacteria from entering your yeast culture once it has been cropped.
This step is not out of fear of contamination but more in favor of a consistent flavor and aroma profile as you pitch and repitch the same culture of yeast.
Cheers!
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