Flavors from yeast starter

I have a routine when preparing to answer questions for my column, and this routine begins with editing questions that come into BYO magazine from email, social media, and snail mail. It is tempting to edit your question because your use of the word “trub” can be interpreted two ways. In the context of your question, I read “trub” to mean yeast solids that fall to the bottom of the fermenter after fermentation. But trub specifically refers to the protein-polyphenol-hop acid complexes that precipitate from wort during wort boiling and wort cooling. Although there is some trub in yeast solids, most brewers do not refer to harvested yeast as “trub.” I am not taking this digression into semantics to nitpick, rather I want to draw attention to a term that is often misused. And this misuse can lead to confusion about brewing advice that pertains to wort boiling, trub, yeast solids, and the meaning of life. So, without further ado, onto your question!

To paraphrase, you are harvesting yeast at the end of fermentation, storing it for an unspecified time period, and growing it up in a starter before re-pitching into a fresh batch of wort. Harvesting, also known as cropping, and reusing yeast is quite common, but, as you suggest, can be problematic. Harvested yeast with low viability, with excessive trub, and dilute crops can lead to issues when re-pitched. The good news is that these challenges are fairly easy to overcome.

Towards the end of fermentation, yeast cells, at least those with good flocculation properties, begin to stick together to form large flocs of cells. So-called top fermenting yeast may form thick yeast layers on the top of the beer, and so-called bottom fermenting yeast flocculate and sink to the bottom. Depending on the yeast strain and the type of fermenter used, many top-fermenting strains will end up on the bottom of the fermenter. Whether harvesting yeast from the bottom of the fermenter (most common method) or the skimming from the top, it is important to harvest the yeast shortly after fermentation has ended and the yeast has risen to the top or settled to the bottom. Prolonged storage in beer, especially warm beer, reduces viability. Chilling beer after fermentation often precedes yeast harvesting and some breweries store yeast for a few days at the bottom of the fermenter as yeast flocculates. The rule of thumb is to harvest the yeast as soon as possible following sedimentation.

Yeast is not the only thing that settles from beer in a fermenter. Trub also settles, and trub is not the tastiest stuff in the brewery! Cone bottom fermenters are very handy when it comes to dealing with trub because they allow for trub removal before most of the yeast cells settle. It is common practice in commercial breweries using conical fermenters to blow the cone several times during fermentation; this helps remove trub, early flocculating yeast, and dead cells from the beer before the yeast cells doing the heavy lifting flocculate and settle. If you ferment in a carboy or bucket, this practice is not possible because there is no cone on these flat-bottom vessels. One relatively uncommon practice to address this limitation is racking beer about 8 hours after pitching, and before active fermentation begins. This allows for trub to settle before the stirring action of fermentation mixes the trub up and into the fermenting beer. Trub can also be skimmed from the top of the fermenter by encouraging blow-off through high fermenter fills, or by actively skimming the fermentation.

The bottom line is that cropped yeast should be fresh and trub free. But some yeast strains have poor flocculation properties. These strains may not be sufficiently dense to re-pitch without adding lots of beer with the yeast into the next brew, and one very useful solution to this problem is to prepare a starter with the yeast available. This is the method you describe. Your DME weight and starter volume produces wort at about SG of 1.036 or 9 ˚Plato, and the 2 liter (2.1 qts.) volume is perfect to pitch into a 20-liter (5.25-gallon)batch of wort. No problems at all with this method as long as you are using yeast from a good brew with reasonable viability and minimal trub. Some brewers use this method to revitalize yeast that may have been stored for too long since the propagation step yields healthy, cells with lots of vigor.

But I do question the necessity of a propagation step when using yeast strains that are very well behaved, such as Wyeast 1056 (American Ale) or White Labs WLP001 (California Ale). This strain is known for clean and rapid fermentations, and good flocculation. You should have no problem harvesting enough yeast slurry from normal gravity brews (I don’t suggest re-pitching yeast from brews with OGs higher than 1.064/16 ˚Plato without a propagation step) to pitch into another batch. A good rule of thumb: Pitching volume is 10-15 mL of thick slurry per liter of wort.

Response by Ashton Lewis.