Author: Jake Huolihan

Of the multitude of advice given to the new brewer looking to up their game, the most common suggestions often relate in some way to yeast health, particularly the importance of pitching the proper amount of the freshest yeast possible. In fact, many believe the overall improvement in homebrew over the last couple decades is largely due to the availability of higher quality yeast.

When it comes to yeast freshness, the primary concern has to do with viability, which refers to the total amount of living cells in a given volume of yeast. While it’s impossible to determine an exact rate at which yeast cells die, it’s commonly accepted that a specific volume will lose approximately 20% of its remaining cells on a monthly basis. So, if we assume a fresh pouch of yeast has 200 billion viable cells on the date it is manufactured, one can assume it will be at 160 billion cells a month later, 128 billion cells a month after that, 102 billion cells after 3 months, and so on. It’s for this reason yeast labs include the date it was manufactured on the package as well as recommend using it as soon as possible.

For the most part, I’ve adhered to these recommendations by propagating older pouches in a starter, or occasionally pitching multiple packs of yeast to improve viability. However, based on some surprising results from previous xBmts, I’ve found myself being a bit more lax and have been pitching single pouches of yeast directly into fresh wort, some not quite as fresh as they could be. While my anecdotal experience with these beers has been rather positive, I was curious what, if any, impact this arguably shoddy practice was having on my beer and put it to the test!

| PURPOSE |

To evaluate the differences between beers fermented with a direct pitched pouch of yeast packaged either 1 month or 4 months prior to use.

| METHODS |

For this xBmt, I went with a fairly clean British Bitter fermented Imperial Yeast A09 Pub, a strain known for imparting a certain degree of esters.

Baby Back Bitter

Recipe Details Batch Size Boil Time IBU SRM Est. OG Est. FG ABV 5.5 gal 60 min 53.4 IBUs 9.6 SRM 1.055 1.012 5.7 % Actuals 1.055 1.008 6.2 % Fermentables Name Amount % Special Pale (Cargill) 11.5 lbs 95.34 Crystal, Medium (Simpsons) 9 oz 4.66 Hops Name Amount Time Use Form Alpha % Pekko 15 g 60 min Boil Pellet 15 Pekko 15 g 30 min Boil Pellet 15 Pekko 15 g 5 min Boil Pellet 15 Yeast Name Lab Attenuation Temperature Pub (A09) Imperial Yeast 72% 64°F - 70°F Notes Water Profile: Ca 91 | Mg 0 | Na 8 | SO4 149 | Cl 55 Download Download this recipe's BeerXML file

After collecting my grains the night before brewing, I prepared the water by adjusting it to my desired profile then scheduled my heat stick to turn on a few hours before I planned to start the next day.

The following morning began with the milling of grain.

With the water properly heated, I incorporated the grist then checked to ensure it hit my target mash temperature.

The mash was left to rest for 60 minutes during which I stirred every 15 minutes with my large whisk.

With the mash step complete, I collected the sweet wort in a kettle.

The wort was then boiled for 60 minutes, during which the hops were added at the times stated in the recipe.

Once the boil was complete, I quickly chilled the wort.

A refractometer reading showed the wort had achieved my target OG.

Equal amounts of wort were then transferred to a pair of identical fermentation vessels.

After connecting the filled fermentors to my glycol chiller set to maintain both at 66°F/19°C, each was direct pitched with a single pouch of Imperial Yeast A09 Pub, one packaged a month earlier and the other packaged 4 months prior. Based on commonly accepted assumptions about viability loss over time, this would equate to the fresh and older pouches having approximately 160 billion and 82 billion cells, respectively.

I observed fermentation activity in the beer pitched with fresh yeast just a few hours after pitching, while the beer pitched with older yeast didn’t get going for another 6 hours. After a week of fermentation, I pulled samples and noticed a rather stark difference in appearance.

Hydrometer measurements at this point suggested the beer pitched with fresh yeast was likely done fermenting while the beer pitched with older yeast still had a ways to go.

I left the beers alone for another week before taking a second set of hydrometer measurements indicating both had attenuated to the same FG.

The beers were then racked to separate kegs.

The filled kegs were placed in my keezer where they were burst carbonated overnight before I reduced the gas to serving pressure. Even after 10 days of conditioning, the beer pitched with older yeast appeared noticeably more hazy than the one pitched with fresh yeast.

When it came time to collect data 12 days later, the beers looked much more similar.

| RESULTS |

A total of 20 people of varying levels of experience participated in this xBmt. Each participant was served 1 sample of the beer pitched with fresh yeast and 2 samples of the beer pitched with older yeast in different colored opaque cups then asked to identify the unique sample. While 11 tasters (p<0.05) would have had to accurately identify the unique sample in order to reach statistical significance, only 8 (p=0.34) did, indicating participants in this xBmt were unable to reliably distinguish beers fermented with yeast packaged 1 month prior to use from the same beer fermented with yeast packaged 4 months prior to use.

My Impressions: Over the 5 triangle tests I attempted, being fully aware of the variable and even with the slghit difference in clarity, I was unable to consistently identify the odd-beer-out. To me, the beers smelled and tasted identical, which was a great thing because it was a killer Bitter!

| DISCUSSION |

It goes without saying that freshness of ingredients, whether in food or beer, is universally viewed as a prime contributor to quality, and this couldn’t be more true in the case of yeast. It’s well established that yeast loses viability over time and, if too old, can lead to poor results. However, many modern brewers fret over using yeast that’s anymore than just a couple weeks old for fear of sluggish or even stuck fermentations as well as off-flavor development. The fact tasters in this xBmt were unable to reliably tell apart beers pitched directly with yeast that was either 1 month or 4 months old suggests age, at least within reason, may not guarantee a bad outcome

Despite the apparent similarities between these beers in terms of aroma, flavor, and mouthfeel, there were some easily observable objective differences when it came to fermentation. Not only did the beer pitched with fresher yeast have a shorter lag phase, which ostensibly reduces the risk of other microbes having an impact, but it finished fermenting sooner than the beer pitched with older yeast. Moreover, fermenting with fresh yeast also seemed to expedite the beer clarification process, which may or may not be considered a positive depending of stylistic preferences.

Considering these results along with those from previous xBmts on both pitch rate and yeast freshness, I’m curious at what point age would have a perceptibly detrimental impact. Would 2 year old yeast that’s propagated to the same viability as a fresh pouch produce good results? Either way, I’ll continue to use the freshest yeast possible, as it seems the best way to ensure strong fermentation and quality beer.

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

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