Yeast - viability

[CodeDaemon]

Hi,

I'm trying to understand how the different calculators out there calculate the yeast viability, and what among these is the proper way to do it. I have found that there are a lot of differences here, but no literature to point me in the direction of what is the correct approach.

Almost all the calculators operate with a ~5% loss of cells per week, but the big difference is if the 5% should be of the start amount of yeast cells, or the remaninig amount of yeast cells.

BeerSmith, yeastcalculator.com: These use 5% of the remaining number of cells so the viability drop gets lower and lower every week, and will never hit 0%. After around 5 months they end up around 25% viability.

Mr.Malty, Brewers Friend: These use 5% of the original amount, so every week the same number of yeast cells die. That means that after around 20 weeks all yeast cells will be dead. One difference between these two is that Mr.Malyt keeps the viability at 10% even if the calculation ends up lower, and Brewer's Friend lets it go down to 0%

Do anyone ahve any documentation on what is the correct approach here, as the difference gets higher and higher as the weeks go between these methods. I know that a lot of people use Mr. Malty's calculator, but it seems like a simplification at best to use the original amount and not take into consideration how many cells you actually have left.

Any help are appreciated here!

 

[Pratzie]

http://tobrewabeer.com/index.php/2015/06/24/yeast-viability-over-time/


Someone shared this with me when I started looking into yeast starters awhile back.

Specifically, what I'd think you should take away from it is that the decrease is non-linear so to answer part of your question, the 5% isn't static. Meaning 5% after of the first week, the next 5% is only of the remaining 95%, which means it decreases. Less product to work with. Hence the non-linear.

There's a lot of literature quoted here to read up on but even if it doesn't help answer the questions you're asking, it paints a pretty good picture about the shelf life of liquid yeasts and why starters are good when you are working with them.

When he mentions that Wyeast recommends only 2 week storage time, you have to question their reasoning for putting that on their product. Obviously they benefit from you buying more yeast from them but it really isn't necessary to throw a 3 week old pack, especially if you are going to use a starter. I'm not trying to knock Wyeast but its worth mentioning their viability is longer then advertised for anyone on a budget or who can't spend a lot of time on brewing.

 

[CodeDaemon]

Thanks for your quick reply! I guess I have some reading to do then

 

[dmtaylor]

MrMalty is wrong. Experience shows you can revive a pack of liquid yeast that is about 8-9 months old without trouble. MrMalty would declare it 100% dead. Not correct. I wouldn't go longer than 9 months though, as I've experienced problems when I've tried.

 

[rhys333]

Check out the simple yeast collection sticky in the fermentation/yeast forum. Further, you can estimate about 1-1.5B cells/ml and 15% loss per month. I never bother with the calculators anymore as this simple rule of thumb works really well

 

[dmtaylor]

Or, switch to dry yeast and never worry again. Dry yeast lasts for YEARS instead of months. Plus, if fresh, one 11g pack is good to ferment 10-11 gallons, or 5 gallons if really old.

 

[ericbw]

While I believe that yeast dies off if it is not fed, I also think yeast viability is grossly underestimated. You can brew a beer with a tiny amount of yeast. Wyeast and Whitelabs aren't even trying to get you to buy more yeast. They say they sell a pitch-able quantity of yeast. They also say to use starters.

I have a jar of bottle harvested yeast that I revive once a year. Sometimes I brew a real beer, sometimes I just want to get it going again.

I just used a pack of White Labs that expired in February. It fired right up in the starter and fermented strong.

 

[KeyWestBrewing]

Or, switch to dry yeast and never worry again. Dry yeast lasts for YEARS instead of months. Plus, if fresh, one 11g pack is good to ferment 10-11 gallons, or 5 gallons if really old.

Don't make that assumption! I screwed up 250L worth of beer for the first time in 3 years because of the yeast being expired. Didn't notice the brick was about a month expired until after I had pitched. What was supposed to be a cream ale came out tasting like a Hefeweizen with off flavors. Not a lesson you want to have to learn the hard way trust me!!

 

[dmtaylor]

Don't make that assumption! I screwed up 250L worth of beer for the first time in 3 years because of the yeast being expired. Didn't notice the brick was about a month expired until after I had pitched. What was supposed to be a cream ale came out tasting like a Hefeweizen with off flavors. Not a lesson you want to have to learn the hard way trust me!!

Refrigeration helps. Unfortunately we also don't always know how the yeast was handled prior to us buying it.

 

[KeyWestBrewing]

Refrigeration helps. Unfortunately we also don't always know how the yeast was handled prior to us buying it.

The yeast I pitched was kept in a walk in the entire time and next day mailed from the supplier when originally purchased. Just saying, if your pitching expired yeast at least be aware your rolling the dice.

 

[CodeDaemon]

Check out the simple yeast collection sticky in the fermentation/yeast forum. Further, you can estimate about 1-1.5B cells/ml and 15% loss per month. I never bother with the calculators anymore as this simple rule of thumb works really well

But that is really what I'm asking here: 15% of what? As I said the main differences is if that 15% is calculated from the initial amount or the remaining amount. For example in 4 months, do you calculate with a viability of 40%, or 52%?

 

[CodeDaemon]

MrMalty is wrong. Experience shows you can revive a pack of liquid yeast that is about 8-9 months old without trouble. MrMalty would declare it 100% dead. Not correct. I wouldn't go longer than 9 months though, as I've experienced problems when I've tried.

Do you have anything to back this up? Any research to back up this claim other than personal experience?

 

[rhys333]

But that is really what I'm asking here: 15% of what? As I said the main differences is if that 15% is calculated from the initial amount or the remaining amount. For example in 4 months, do you calculate with a viability of 40%, or 52%?

Your loss rates are calculated from your initial amount. E.g.: 200 ml slurry @ 1.5B cells/ml = 300B cells initially. At 4 months: 300B x 0.4 = 120B (40% viability). From there you have a figure to use as a starting point for building up to say 200B needed for a 1.050 5 gal batch.

Is it precise? No. Its a reliable way to consistantly estimate a reasonable amount of yeast needed though. I use it in conjunction with a how healthy the stored yeast appears. If I pull it out of the fridge and it shows activity via bubbles in a few minutes, then its a good sign the yeast is still quite viable. Instead of 40% I'll estimate 60ish. If its a little sluggish and darker coloured, then I'll stick closer to 40. Using this method I see activity in 8 -24 hours after pitching (possibly after building a starter, depending), which is right where I want it.

 

[hottpeper13]

Do you have anything to back this up? Any research to back up this claim other than personal experience?

Experience is all that matters here because we are all splitting hairs. This is my experience, I over make starters and then save ~250 ml for making another starter. Depending on the yeast i've been able to revive 1 yr old samples but i'm doing it as if taking from a plate or slant and doing the 15 ml- 150 ml- 1500 ml steps. All of the yeast i've used works this way and some or most of the Belgian ones can just go to the full 1500 ml but I like to step those using half the wort per step. I have a 3 mo expired The Yeast Bay Saison Blend II and plan on using it for a holiday beer to serve alongside my Barrel aged RIS. No worries!

 

[dmtaylor]

Do you have anything to back this up? Any research to back up this claim other than personal experience?

Call it anecdotal if you prefer, I don't care. To me and my experience, it is my truth. Accept my 3 cents and 17 years experience for whatever they are worth to you.

 

[ScrewyBrewer]

If you just figure 10% cell loss each month on average you'll be fine, when the yeast is refridgerated.

100 billion when yeast is fresh
90 after first month
81 billion after second month
etc..

 

[CodeDaemon]

Okay guys, thanks for all your insights here! I really appreciate it

 

[Moebbels]

MrMalty is wrong. Experience shows you can revive a pack of liquid yeast that is about 8-9 months old without trouble. MrMalty would declare it 100% dead. Not correct. I wouldn't go longer than 9 months though, as I've experienced problems when I've tried.

The oldest Wyeast lager yeast I have used successfully was 3.5 years old when I put it to use
Right now i have 1214 on the stirplate that was made in april 2015, I believe.