Tuesday, December 25, 2012

Refrigeration Effects on Yeast Viability



Some subjects have a plethora of information available allowing the home brewer to evaluate different pieces of data and come to their own conclusion.  However, other subjects have one piece information that misinformation is inferred from.  The effects of refrigeration on viability is one of the latter.   Mr. Malty's slurry viability slider is often misrepresented in this manner.  For a new slurry this slider shows the viability as 94% and for every day it drops 1.6%, and bottoms out at 10% after 53 days.  While Jamil likely had good intentions when designing this into the calculator, it is commonly misused.  Perhaps it was an attempt to show that a slurry cannot be kept in the refrigerator indefinitely.  There are a number of other consideration when deciding to repitch a slurry such as contamination and overall health of the yeast. 

The fact is that it is documented in very reliable brewing literature that yeast stored in a broth can be kept for six months, and yeast stored on an agar slant can be kept for over a year.   (1)(2)

There are much more important considerations when it comes to the viability of a starter than the amount of time it has been refrigerated.  Fruit(3) has a drastic impact on viability as does alcohol.(4)

The linear decay has actually been propagated over to Yeast Calc, another trusted source.  Rather than simply propagate information, I did the tests for myself.  Over the course of a month data was collected on seven different slurries from two different strains used to make a variety of beers.

 
Yeast Strain
Yeast Layer
Viability loss per day
WLP566
Top
-0.07%
WLP004
Top
+0.006%
WLP004
Middle
-0.18%
WLP004
Bottom
-0.77%
WLP566
Top
-0.46%
WLP566
Top
-0.006%
WLP566
Bottom
+0.09%
Average

-0.20%
Conclusion:
While the initial viability can vary greatly, the viability over time dose not change a measurable amount over the course of one month.

(1) The Practical Brewer,Yeast Strains and handling techniques, Sources and Maintenance of Pure Yeast Cultures. p276
(2) Kirsop (1991)
The italic text above is a paraphrase of the texts referenced as (1) and (2)

(3) http://woodlandbrew.blogspot.com/2012/12/how-fruit-effects-yeast-viability.html
(4) http://woodlandbrew.blogspot.com/2013/01/abv-effects-on-yeast.html




13 comments:

  1. Very interesting stuff. How could the yeast viability go up over time though?

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    1. Good question. The table shows the calculated slope based on a linear best fit. Just by the nature of a cell count there is some error. Although some numbers are positive and others are negative they are all roughly zero.

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  2. Hi there,
    cool blog of yours. I would like to ask you some questions. You mention source (1) and (2) telling that yeast can be stored for several months up to a year. I would agree here. However, is there any connection mentioned in those sources to viability? If not, I don't see evidence to conclude that the viability stays the same over the period of time.

    I kind of struggle to believe that the viability stays the same for months. Your WLP004's viability is in general pretty low. And I would not expect any decrease of viability at such low levels anymore. This yeast is in pretty bad conditions already. It nearly can't get any worse. Therefore the viability stays the same... My question here is, how did the viability got to 20% in the first place?

    You mention in your yeast storage post that "Viability does not drop much over time". If I look at your data from WLP566 you have a daily drop of viability of roughly 0.5%. Sure the viability does not drop much after one day. However, after a months the viability dropped by 15% already. And that's for sure a significant drop!
    Cheers, Sam

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    Replies
    1. Thanks!

      I'll add the quoted text to the post. They do not mention viability but rather the usefulness of the yeast after storage.

      The viability is what I measured with a cell count over that time. I would appreciate seeing other peoples data they have collected to correlate to my measurements.

      A wide sample of viabilities was chosen to cover a broad spectrum of situations. Yes, there was a 0.5% drop per day in one of the worst case sample, although as you can see, this is not representative of all of the data.

      Viability isn't everything when it comes to pitching. Vitality could be even more important.

      My point is that the drop in viability over time is marginal. It is much smaller than the popular calculators would indicate.

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    2. Thank you for your reply. Please forgive me to ask further questions. Your conclusions are just very contradicting to my results. I will publish my data in the future on my blog.

      Sure viability is not everything. Testing vitality however is not that easy and sophisticated equipment is necessary. I don't see any reason why homebrewers or even brewers should invest in equipment to measure vitality.

      The yeast calculator from MrMalty predicts a daily drop of roughly 0.7%. Your measured drop of 0.5% is not that far away.

      Cheers, Sam

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    3. Just a couple points of clarification. The drop I see is 0.2% per day on average. Mr. Malty, on the slurry calculator, shows 1.58% per day.

      I agree vitality testing can be much more complicated.

      In part I agree with you about microscopes. It is probably not the best investment for every home brewer. If a brewer is not interested understanding microbiology, it's only going to be a very expensive hunk of metal that's only good for stubbing your toe on. However, In my opinion, it is a wonderful tool for a homebrewer who is interested in understanding yeast better and is scientifically oriented. It has really opened my eyes to a whole new understanding of brewing.

      I'm looking forward to seeing you data.

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    4. Steven, I have to apologize. I did not notice before that MrMalty uses different viability drop rates. The 0.7% refer to liquid yeast. Sorry for the mix up. Well, this puts everything in another perspective. There seem to be a bigger difference between your results and the calculator indeed. I will have a further look into yeast viability again.

      Thank you for your clarifications.
      Cheers, Sam

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  3. I am always searching online for articles that can help me. There is obviously a lot to know about this. I think you made some good points in Features also. Keep working, great job! Whirlpool

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  4. Digging this one up.
    This is something I've wondered about as I look at my mason jars of WLP565 and 515 sitting in my brew fridge. I;d have to check my notes of when those were used, but it has to be easily over a year and possibly two. I'm of the opinion (completely unscientific) that if I were to make up a starter and use whats in those jars, they would grow and I could build up a pitchable amount in short order. I've done it several times with yeast that was considered "very far out of date" and had success. The big question is the condition of those yeast, or if they create anything undesirable? The beer I've made with "old" yeast was excellent, attenuation was excellent, and in my eyes the starters were creating new healthy yeast. Can vitality or mutations create a full starter of unhealthy yeast and if so, what would be the detrimental effects that a home brewer would observe?

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    Replies
    1. Yes, the newt grown yeast will be healthy if given the appropriate nutrient. The health of the crop is independent of the viability and vitality of the parents. The mutation rate of yeast is very low and shouldn't be a concern with yeast stored for years in the refrigerator.

      A bigger concern would be contamination. Depending on your sanitization some bacteria may be able to grow in the cold environment and over years grow large enough to impact the flavor of the beer.

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  5. Would a simple smell test reveal unwanted invaders? Then there would be smell and taste test of the new starter as well. In the oldest samples in my fridge, there are certainly darker stratifications than in fresher samples. There is also still a layer of lighter creamy looking yeast.

    My LHBS said White Labs actually takes back yeast after a certain amount of time and credits them. I'm a frequent flier, so she gave me a vial of way out of date WLP510. It started up perfectly and smells awesome.

    [I have no idea why Google is calling me SANDWICH]

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    1. A smell and taste test would reveal some bacterial contamination, but not all. It would increase your chance of success.

      Your WLP510 starter sounds like it is doing well. You can pitch with confidence.

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