Posted by Old Ale Jay on 7/2/2016 to Homebrew FAQ
Let's start with how long yeast cells will live. Home brewers will regularly build starters from bottles of beer. Often these bottles are Belgian style beers that have been shipped to the USA. A basic time line would include all of the following. The beers are brewed in Belgium and shipped to the USA. They are then sent from a distributor in the US to a local distributor who then dispatches them to your neighborhood store. Most of this will happen without any refrigeration. After this extended time out of a refrigerator, an adventurous home brewer will take this bottle, decant off the beer, and build a small yeast starter from the dregs of the bottle– Most will find live yeast cells. Moral of the story– Yeast are far more hardy than they are often given credit. The six months that Wyeast and White Labs guarantee their yeast is just the tip of the iceberg when talking about how long yeast will live.
Now the question at hand– How hot is too hot? I have seen reports that state 140*F for several minutes will do the trick and temps as low as 103 for several days. Texas heat for several days may kill your pack of yeast but sitting on a porch for four hours is not likely to do the trick. Viability may be an issue but building a yeast starter should remove any concern. Three years back, I decided to test the extremes. I took ten vials of yeast and put them in my garage. I planned on making one starter a month knowing that they would freeze in the winter and get up to the 100* mark in the summer. Results– Ten months in a row, I had every vial of yeast start in its starter. I was excited to report my findings to Chris White at White Labs. He simply shrugged and said this is exactly what he would have expected;)
What to do when shipping liquid yeast in the summer? 1) Build starters to confirm your yeast is viable. 2) Order liquid yeast on Sundays and Mondays to ensure you receive it during the week. 3) Upgrade shipping if you have concerns that it will sit in a truck over the weekend. 4) Make notes on your order to hold your yeast until Monday if you are ordering it late in the week. 5) Most importantly, “Relax, have a Homebrew”
Full disclosure, I am just a home brewer. I have never been trained in brewing microbiology. The above is what I have witnessed in my 20+ years of brewing experience. At the end of the day, your yeast is far more tolerant than you might expect:)