Author: Phil Rusher

Yeast health is at the forefront of many a brewer’s mind, as it’s solely the responsibility of this single celled organism to transform sweet wort into beer. Because of this, those brewers seeking the highest quality finished product will go to great lengths to ensure optimal yeast viability. For yeast that’s awaiting use, the common recommendation is to store it as cool as possible to impede metabolic activity, though brewers are cautioned against freezing yeast.

When water freezes, molecular crystalline structures are formed that can puncture delicate walls, which in the case of yeast, can lead to cell death and hence reduced viability. While glycerin added to the yeast slurry prior to freezing will inhibit crystallization, the issue for most brewers is more likely the accidental placement of yeast in an environment that’s cooler than intended, often leading to it being pitched in the trash rather than the beer.

I’ve never frozen liquid yeast, whether intentional or accidental, but I’ve heard of many who have made the mistake of tossing a pouch in their freezer rather than the refrigerator. Given the significant results of a recent xBmt comparing beers made with yeast stored either cool or warm, I began to wonder just what might happen to a beer fermented with yeast that had previously been frozen.

| PURPOSE |

To investigate the differences between beers that were fermented with either a previously frozen pack of yeast or a pack of yeast that was kept in standard cool conditions.

| METHODS |

With the hope accentuating any differences caused by the variable, I went with a simple Blonde Ale for this xBmt.

Say it Ain’t So

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5.5 gal 60 min 35.8 IBUs 4.2 SRM 1.050 1.012 5.1 % Actuals 1.05 1.009 5.4 % Fermentables Name Amount % Mecca Grade Lamonta: Pale American Barley Malt 9.5 lbs 100 Hops Name Amount Time Use Form Alpha % Jarrylo 18 g 60 min Boil Pellet 12.6 Jarrylo 14 g 15 min Boil Pellet 12.6 Yeast Name Lab Attenuation Temperature House (A01) Imperial Yeast 74% 62°F - 70°F Notes Water Profile: Ca 50 | Mg 7 | Na 5 | SO4 75 | Cl 60 Download Download this recipe's BeerXML file

After selecting two pouches of Imperial Yeast A01 House with the same date of manufacture, I placed one in my freezer while the other was left in my refrigerator.

Approximately 39 hours later, I removed the yeast from the freezer and noticed it had definitely crystallized. An infrared thermometer reading showed it was at 2°F/-17°C.

At this point, I placed both pouches in my refrigerator to give the frozen one time to melt then proceeded with milling the grain for a single 10 gallon/38 liter batch.

With the water adequately heated, I stirred in the grains before checking to make sure it hit my target mash temperature.

While waiting on the mash, I prepared the kettle hops.

Following the 60 minute mash rest, I collected the sweet wort then proceeded with a 60 minute boil.

When the boil was complete, I quickly chilled the wort to my desired fermentation temperature of 66°F/19°C then took a refractometer reading confirming it was at my intended OG.

Equal volumes of wort were racked to identical Brew Buckets.

With the filled fermenters placed next to each other in my 59°F/15°C basement, I pitched the pouch of yeast stored in my refrigerator into one while the other got the yeast that had been frozen and thawed. After 24 hours, I noticed the beer pitched with frozen yeast was about 4°F/2°C warmer, a possible indicator of stronger fermentation activity.

The beers were left alone for 10 days before I took hydrometer measurements confirming the same FG had been reached in both.

At this point, I proceeded with transferring the beers to CO2 purged kegs.

The filled kegs were placed in my cool keezer, burst carbonated, and left to condition for a couple weeks before they were ready to serve to tasters.

| RESULTS |

A total of 24 people of varying levels of experience participated in this xBmt. Each participant was served 1 sample of beer made with yeast that was not frozen and 2 samples of beer that was made with yeast that was frozen then thawed in different colored opaque cups then asked to identify the unique sample. While 13 tasters (p<0.05) would have had to accurately identify the unique sample in order to reach statistical significance, only 9 did (p=0.41), indicating participants in this xBmt were unable to reliably distinguish a Blonde Ale made with yeast that was stored cool but never frozen from one made with yeast that was frozen then thawed before being pitched.

My Impressions: Out of the 7 semi-blind triangle tests I attempted, I correctly selected the unique sample only 2 times. Despite knowing one of the beers was made with yeast that had previously been frozen, and completely believing it would have a meaty off-flavor, I could not tell them apart.

| DISCUSSION |

Without question, yeast health is imperative in the production of good beer, and so the steps taken by brewers to ensure optimal viability are certainly justified. It’s similarly well understood that the crystallization that occurs when water is frozen can rupture yeast cells, rendering them unable to do their intended job. Surprisingly, tasters in this xBmt were unable to reliably tell apart a beer fermented with yeast stored in a refrigerator from one fermented with yeast that was previously frozen and thawed prior to being pitched.

On top of the apparent lack of a perceptible difference between these beers, the frozen yeast didn’t seem to show any observable signs of detriment in terms of performance– both yeast pitches exhibited identical lag phases, overall fermentation time, and attenuation. One possible explanation for this is that not every cell membrane was ruptured during the freezing process, leaving enough viable yeast for fermentation to proceed unhindered.

It’s not often I find myself shocked by an xBmt result, but this one really got me. There are many publications and even more anecdotes about how freezing yeast will affect its ability to perform so I was absolutely convinced that the frozen yeast would have some sort of negative impact on the beer, be it from a sensory perspective or objectively measurable one. The fact it didn’t hasn’t inspired me to start freezing my yeast, that’d be stupid, but it has forced me to reconsider the common recommendation to those who mistakenly toss a pouch of yeast in the freezer– “in my experience, it worked out fine.”

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

Support Brülosophy In Style!

All designs are available in various colors and sizes on Amazon!

Follow Brülosophy on:

If you enjoy this stuff and feel compelled to support Brulosophy.com, please check out the Support page for details on how you can very easily do so. Thanks!

Advertisements

Share this: Facebook

Twitter

Pinterest

Tumblr

Email



Like this: Like Loading...