Author: Phil Rusher

Quality in, quality out. A common trope that applies as much to beer production as anything else. Over the years, brewers have learned that the best beer is made with high quality ingredients, and when it comes to yeast, quality is largely a function of yeast viability, which is the amount of living cells present in a given population. A method used by many brewers to improve viability involves propagation of yeast in a starter to increase cell counts, allowing for specific pitch rates based on the beer being brewed.

To be sure, yeast starters are miniature batches of beer, and there’s plenty of anecdote and even some evidence to suggest that reusing yeast from an actual batch works well. However, whereas brewers typically use just a portion of the remnant yeast cake to ferment a subsequent batch, a fairly commonly discussed approach among homebrewers involves racking fresh wort directly onto the entire yeast cake from a prior batch.

Assuming the yeast is fresh and not contaminated, reusing the yeast cake all but guarantees high viability, to the point some might be concerned about overpitching, which some believe can lead to a watery mouthfeel with lacking flavor. Furthermore, it’s not just yeast at the bottom of a fermentor post-fermentation, but all the other stuff that makes up trub from the previously fermented batch, all of which might have an impact on flavor. Still, the benefits of high viability, reduced lag, and saving some coin by not using a new pack of yeast are enough to outweigh risks for some homebrewers, many of whom report positive results.

I’ve tried my hand at reusing yeast cakes with varying degrees of success, though I’ve never done an actual comparison. Curious to learn more about this frugal method, I decided to put it to the test!

| PURPOSE |

To evaluate the differences between a beer fermented with the entire yeast cake of a previous batch and one fermented with a properly sized yeast starter.

| METHODS |

With the hope of accentuating any potential differences caused by the variable, I designed a very simple pale lager for this xBmt.

Perilous

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5.5 gal 60 min 25.6 IBUs 4.4 SRM 1.050 1.010 5.3 % Actuals 1.05 1.007 5.7 % Fermentables Name Amount % Franco-Belges Pilsen 10.25 lbs 99.79 Midnight Wheat 0.35 oz 0.21 Hops Name Amount Time Use Form Alpha % Hallertau Magnum 18 g 60 min Boil Pellet 10 Hallertauer Mittelfrueh 15 g 10 min Boil Pellet 3 Saaz 15 g 10 min Boil Pellet 3 Yeast Name Lab Attenuation Temperature Urkel (L28) Imperial Yeast 73% 52°F - 58°F Notes Water Profile: Ca 64 | Mg 0 | Na 8 | SO4 76 | Cl 60 Download Download this recipe's BeerXML file

The actual start of this xBmt occurred 2 weeks beforehand when I brewed a German Leichtbier that I used to build up a yeast cake. Then, using a pouch of Imperial Yeast L28 Urkel from the same lot as the yeast used for the Leichtbier, I made a yeast starter a couple days beforehand.

On the morning of this xBmt brew day, I started the slow process of collecting RO water prior to leaving for work.

As soon as I got home, I hit the flame on my burner and adjusted the water to my desired profile. While the water was heating, I weighed out and milled the grain directly into a BIAB grain bag.

With strike temperature reached, I mashed in and gave the grains a gentle stir before checking the ensure I’d hit my target mash temperature.

At the end of the 60 minute mash rest, I removed the grain bag and allowed the sweet wort to drip into the kettle while I weighed out the kettle hop additions.

The wort was brought to a boil and hops were added per the recipe.

With the 60 minute boil complete, I chilled the wort with my homemade immersion chiller before taking a hydrometer measurment showing it was at the planned OG.

Due to my warm groundwater this time of year, the wort was still a bit warm, so I placed the entire kettle in my chamber to finish cooling to my target pitching temperature of 50°F/10°C. It was during this time I kegged the German Leichtbier, making sure to rid the fermentor of as much beer as possible while leaving behind a think layer of yeasty trub.

With the wort adequately chilled a couple hours later, I split it equally between the fermentor containing the yeast cake and an identical empty fermentor, that latter of which the yeast starter was pitched into. Things were looking quite different right off the bat.

The beers were then left to ferment at 53°F/12°C and, predictably, the yeast cake beer began showing signs of activity before the one pitched with a yeast starter.

At just 12 hours post-pitch, the yeast starter beer had developed a healthy kräusen.

After 2 full days, the beers were beginning to look more similar than different.

Both beers were showing signs of reduced activity on day 6 of fermentation.

I left the beers alone for 5 days before ramping the temperature up to 63˚F/17˚C for a diacetyl rest. After a few more days, I took hydrometer measurements showing the yeast cake beer had a slightly lower FG than the beer pitched with a yeast starter.

I dropped the temperature for cold crashing and fined both beers with gelatin, returning 2 days later to package.

The beers were placed in my cool keezer and burst carbonated over 48 hours before I reduced the gas to serving pressure. After a 4 week lager period, the beers were ready to serve.

| RESULTS |

A total of 25 people of varying levels of experience participated in this xBmt. Each participant was served 2 samples of the beer fermented with the yeast cake from a prior batch and 1 sample of the beer fermented with an adequately sized yeast starter in different colored opaque cups then asked to identify the unique sample. While 13 tasters (p<0.05) would have had to identify the unique sample in order to reach statistical significance, 16 (p=0.002) made the accurate selection, indicating participants in this xBmt could reliably distinguish a pale lager fermented with a full yeast cake from one fermented with a yeast starter.

The 16 participants who made the accurate selection on the triangle test were instructed to complete a brief preference survey comparing only the beers that were different. A total of 5 tasters reported preferring the yeast cake beer, 7 liked the yeast starter beer more, 1 person had no preference despite noticing a difference, and 3 reported perceiving no difference.

My Impressions: During early samplings, I wasn’t sure I was going to be able to tell these beers apart, but out of the 5 triangle tests I attempted, I chose the odd-beer-out every single time. To me, the yeast cake beer was dominated by a green apple character reminiscent of acetaldehyde, pushing any malt and hop flavor to the background. I perceived the yeast starter beer as having a bready malt flavor with low spicy and herbal hop character, and an appropriate touch of sulfur in the nose.

| DISCUSSION |

Pitching an adequate amount of yeast is thought by many to be one of the most important components to producing good beer, hence the popular use of yeast starters. While underpitching yeast is ardently believed to be the cause of many faults, the potential perils of pitching too much yeast is a bit understated, with some claiming it’s nearly impossible to achieve on the homebrew scale. Arguably, reusing the entire yeast cake from a previous batch of beer is one way to do it.

Practically speaking, racking wort onto a yeast cake has numerous benefits– the yeast has high viability, the brewer isn’t spending money on a new pack of yeast, and less cleaning since the (hopefully uncontaminated) fermentation vessel gets reused. The numerous positive reports from brewers using this approach clearly suggests it’s a valid option, even if tasters in this xBmt could tell apart a beer made in such a manner from one pitched with a standard yeast starter.

Considering these results, I can think of a couple possible contributors to the difference noticed by tasters, the first being overpitching. Some tasters noted a distinct tart apple-like character in the finish of the yeast cake beer, which matches my experience, leaving me wondering if perceptible levels of acetaldehyde were present. Meanwhile, this character was absent in yeast starter beer, which I experienced as cleaner overall. Based on the results of the latest kettle trub xBmt, it also seems plausible all of the residual trub in the fermentor was the culprit. The fact the yeast cake beer ended up noticeably more clear provides some support for this hypothesis.

Adding this to my past experiences with reusing yeast cakes, I’m not convinced it’s the method for me. That said, I am curious to see how things would play out with different yeast strains or higher OG beer styles. Until then, I’ll be sticking to more normal pitch rates.

If you have any thoughts about this xBmt, please do not hesitate to share in the comments section below!

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