It’s Getting Hot in Here

When we propagate yeast at Bootleg, we work in sterile conditions, under ideal temperatures and timeframes. The yeast is at its peak viability, and as close to perfect as it can be for brewing beer.

Counting Yeast Cells With a Hemocytometer

Of course, to get it in your hands it has to be shipped. Shipping times, temperature and “puffiness” of homebrew packs are common brewer concerns.

But what happens when our little yeast babies leave the nest?

To answer this question, we set out to determine effects on yeast viability when shipping takes extended amounts of time.  If you remember our previous endeavors into abusing our homebrew packs, you know we don’t hold back. Many thanks goes out to the indomitable Faye Johnson, who executed this experiment. 

THE GOAL: To determine the effects of high temperature on the viability of homebrew packs. 

THE PROCESS: Four identical homebrew packs each of OSLO, NEEPAH and Classic American (Chico) were stored at ambient warehouse temperature (~85F/29C) for two weeks to determine how temperature affects the viability of cultures. 

One homebrew pack of each culture was opened and counted using a viability stain on a hemocytometer at four time points: immediately out of the refrigerator shortly after packaging, 48 hours at warehouse temperature (80-90F/26-32C fluctuations), seven days at warehouse temperature, and 14 days at warehouse temperature. 

The fluctuations that seem to indicate an increase in viability for some time periods for BBXNPA and BBUSA1 reflect that a new pack was opened and tested on those days that had not suffered as much loss as the previous pack tested — not that viability increases with time spent at ambient temperature. 


While temperature variability is not ideal for culture health, we found that cell viability was not impacted enough after 14 days to prevent a healthy fermentation with a direct pitch, even in very warm conditions.

If extreme shipping/logistics issues were to occur, the results of this trial give us confidence the viability of the culture in question should be more than 90%. With certain cultures there should be nearly no viability loss. 

Should you still make a starter after receiving your culture? Absolutely! We recommend making a starter every time you brew because it helps verify culture health and gives your homebrew the healthiest fermentation possible.

As could be expected, we also learned not all yeast are created equal. OSLO suffered essentially no loss in viability despite the horrendous conditions, while the Classic American (Chico) ale yeast had the most noticeable drop. Is it possible that yeasts that perform better at warmer ale temperatures (think Kveik, saison and Belgian cultures) do best in these situations? To answer that question more research is definitely required. 

Sam Wineka

Bootleg Biology Lab Manager

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