Author: Marshall Schott

I personally think it (harvesting yeast from starters) is much easier and far less time consuming [than rinsing yeast]. Additionally, the yeast fermented a wort of ~1.040 OG that had no hops in it, so it is arguably less stressed and ultimately cleaner.

~ From the F.A.Q. section of my Yeast Harvesting page ~

I’ve tried my hand at yeast rinsing and, besides experiencing it as too time-consuming of a process, all the transfers and water additions seemed to increase the chances of picking up an unwanted infection. I wanted a way to be cheap that required less time, effort, and risk. Overbuilding and harvesting yeast from starters was my solution. It just made sense to me that this method would produce a superior pitch of yeast that was less likely to experience generational drift compared to yeast going through multiple fermentations and rinses. I reckon many would agree that it has worked great! But, it’s not the only way.

Soon after I first published the aforementioned article, I began fielding questions about other methods for harvesting and reusing yeast. One approach that regularly popped up involved racking chilled wort directly onto a yeast cake that had recently fermented another batch of beer, usually into the same fermentor, and there seemed to be quite a few homebrewers achieving great results doing this. One of the more commonly cited concerns with this method has to do with the potential for over-pitching, something I question the validity of, at least on the homebrew scale. I have tried fermenting on a used yeast cake, it worked fine, it just lacked the level of precision my neurotic mind desires. Another fairly similar option I’d heard about involved pulling off a sample of a freshly used yeast slurry and pitching that into a new batch of wort, forgoing the whole rinsing procedure. Still somewhat concerned about the impact the other junk suspended in the slurry might have on my finished beer, I was at least intrigued enough by the simplicity of this method to give it a shot.

| PURPOSE |

To investigate the differences between 2 beers fermented with the same strain of yeast, one originating from the slurry of a prior batch and the other propagated in a starter from “clean” yeast.

| METHOD |

I decided to perform this xBmt a few days before kegging the Schwarzbier batches from the fermentation temperature xBmt, as this would allow me to reuse that WLP029 yeast, which I’ve had great experience with in lager styles. I also like that the yeast was coming from a very dark beer, something I’ve heard others pooh-pooh in the past, and that I’d be pitching the slurry into fresh wort just hours after harvesting. I figured a simple German Pils would be a good beer for this xBmt, reasoning any differences would be much easier to recognize in such a light style.

German Pils Batch Size Boil Time IBU SRM OG FG ABV 11 gal 90 min 36 3.4 1.050 SG 1.012 SG 5.0 %

Fermentables

Name Amount % Belgian Pils Malt 17 lbs 94.4 Gambrinus Vienna Malt 1 lbs 5.6

Hops Name Amt/IBU Time Use Form Alpha % German Perle ~26 IBU 60 min Boil Pellet 8.6 Hallertauer Mittelfrüh 45 g/5.7 IBU 15 min Boil Pellet 6.3 Tettnanger 60 g/4.4 IBU 2 min Boil Pellet 8.9

Yeast Name Lab Attenuation Ferm Temp WLP029 German Ale/Kölsch White Labs 76% 58°F

Using a trusted yeast pitch calculator, I determined a 1L starter would lead to approximately 280 billion cells, slightly more than recommended due to the cooler fermentation temperature I prefer when using WLP029 for lager styles.

Calculating the amount of slurry required to come close to a similar cell count wasn’t terribly difficult. Assuming dense yeast slurry contains roughly 1.2 billion cells per ml (1, 2) and aiming for a total cell count of 280bil to match my starter, I ended up collecting a full pint (~473 ml) of loose slurry from the bottom of the cool fermented Schwarzbier carboy, understanding the actual yeast would ultimately compact to about half of this volume. By my calculations, this would leave me with approximately 284bil cells: (473 x 0.5)1.2 = 283.8. Perfect!

Well, maybe not perfect. Harvesting yeast is a fairly inexact science, one I wish was a bit more precise. Good enough would have to do! It was time to make some wort, which was pale as ever due to the simple grain bill of only Belgian Pils and Vienna malts.

I boiled for 90 minutes because that’s what I’ve heard I’m supposed to do to reduce the chances of DMS off-flavors when the grist consists of mostly Pilsner malt, an admitted appeal to authority (xBmt planned). I then chilled the wort to my target fermentation temp of 58°F in less than 10 minutes.

Two 6 gallon PET carboys were filled, placed in a 58°F fermentation chamber, and the yeast was pitched– decanted starter in one, slurry in the other. Visible activity was observed a mere 6 hours later.

The following morning, I noticed the krausen on the slurry batch had some darker brown spotting, something I’m guessing was caused by the fact the yeast was previously used to ferment a Schwarzbier. Would this have a noticeable impact on the finished beer?

Fermentation proceeded at about the same pace for both beers and they began to look much more similar a few days in.

By day 8, the batch pitched with a starter still had some krausen resting atop the beer, while the krausen on the slurry batch had all but disappeared.

With neither beer showing visible signs of fermentation at this point, I took a hydrometer sample and found both had attenuated to my target FG.

I let the beers sit another day before taking another FG measurement (no change), cold crashing, fining with gelatin, and kegging.

The beers were crystal clear, carbonated, and ready for evaluation just a few days later.

| RESULTS |

Similar to the recent Cal Common yeast xBmt, most of the data for this one was collected while I was in the Bay Area following my appearance on The Brewing Network’s Dr. Homebrew show. As I’ve become accustomed to doing, I sought participants at nearly every beer-related establishment my buddies and I visited including the Concord MoreBeer location and Heretic Brewing Company in Fairfield. I can’t thank the cool folks that run these places enough for allowing me access to their customers for my silly xBmts. A special shout-out to the wonderful Liz Zainasheff, wife of Jamil and co-owner of Heretic Brewing, who I’m pretty sure had to let someone from a neighboring business know that I wasn’t hawking alcohol from the trunk of my Camry… embarrassing barely begins to describe how I felt. I apologized profusely to Liz, she shrugged it off with a smile, pointed me to a more concealed area of the brewery, and encouraged me to proceed. What a rad place!

A total of 13 people participated in this xBmt, 8 of which would have to accurately select the different beer to imply statistical significance (p<0.05). Each taster was blindly provided 2 samples of the beer feremented with a yeast starter and 1 sample of the beer fermented with May The Schwarzbier With You slurry. Five participants accurately chose the slurry fermented beer as being different while the majority (8) chose either of the other 2 beers. Statistically, the null hypothesis was supported, meaning the difference between low-moderate OG beers fermented with WLP029 propagated in a starter versus yeast stolen from a used cake is not discernible by the general population.

My Impressions: I’m often swapping beer lines in my keezer since it holds 7 kegs but I’ve only 5 faucets. When I do these swaps, I rarely change the name associated with each faucet/beer on the chalkboard lid, as I know I’ll eventually re-connect that keg later on; every keg is marked, so it’s not like I lose track of what xBmt beers are in which keg, something I triple-check prior to administering the actual test. Anyway, a couple days after I’d completed data collection, I did some keg swapping, as I wanted to try a freshly kegged beer. At some point, I moved the disconnect from the yeast starter keg to the slurry keg, then moved the disconnect that was on the new keg (originally on the slurry keg) to the yeast starter keg. Follow? Basically, the faucet I thought was serving the yeast starter beer was actually serving the slurry beer, and vice versa. Okay…

I began drinking these beers about a week before data collection began, I couldn’t get enough, I thought they were so good. While not tested blindly, I did compare the beers next to each other on multiple occasions and quickly developed a preference for the one fermented with the yeast starter, I thought it tasted cleaner, crisper, and generally better. This was about a week before data collection began and any line swapping occurred (again, this only happened after the last participant completed the survey). Given my preference for the yeast starter beer, I kept pouring from that tap until the keg finally kicked. It was only when removing the empty keg from my keezer that I realized I had in fact been drinking, and preferring, the slurry fermented beer.

Bias in action.

I couldn’t help but laugh at my own self-delusion as I recalled haphazardly swapping the lines a few days prior. Duh. I’d actually convinced myself that I preferred each beer over the other at different points, assuming the preferred beer had remained the same the entire time. It stung a bit to admit to myself that I likely wanted the yeast starter beer to be better because of the energy I’ve invested in advocating for a particular yeast harvesting method. It wasn’t conscious, if so I likely would have avoided this xBmt altogether; rather, my biased preference appears to have been driven by something deeper, some motive out of my awareness. The beers, for all intents and purposes, were the same. I obviously couldn’t tell them apart.

| DISCUSSION |

While I’m not quite prepared to replace harvesting from starters with pitching “sloppy” slurry from a prior batch, it would appear the practice can in fact produce great beer, even when the slurry previously fermented was very dark beer. Of course, this is only a single data point and these results may not necessarily carryover into other situations. I’m sure there will be at least a few who, after reading these results, will suggest that I should have pitched slurry that fermented a higher OG wort or compared starter yeast to washed/rinsed yeast. Good ideas, indeed, both of which I promise to eventually get to, but that wasn’t what this xBmt was about. Rather, I was interested in comparing a sloppier approach to harvesting versus a more controlled one, and to that end, I think we done good.

If you’ve any thoughts you’d like to share about yeast harvesting methods, please share them in the comments section below. Cheers!

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List of completed exBEERiments

How-to: Harvest yeast from starters

How-to: Make a lager in less than a month

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