Modeling Yeast Die Off Rates

Vikeman, ecological population models frequently use exponential models (exponential decay, in this case) for this purpose: N(t)=N(0)e^(kt)

In this case, the constant k in the exponent is negative. I suspect I am not telling you anything you don't know, but this sounds like the model the Wyeast rep was probably alluding to? Other considerations are positive and negative density dependence, and this could be what Chris White was referring to? Negative density dependence implies a negative feedback between population size and growth rate; positive density dependence implies a positive feed back between population size and growth rate. Or maybe White just felt like the data Mr. Malty used were flawed and biased the model towards conservative?

Jack, in your conversation with Chris White, did he suggest how conservative the model was? (e.g.,hypothetically, you will probably over pitch by 10% if the yeast are young; you might over pitch by 30% when using the model to pitch yeast that are close to the expiration date). FWIW, I have emailed White Labs in the past with geeky yeast questions and have gotten nice responses. Perhaps someone there could shed more light onto the issue?[/quote]
Peter, in my conversation with Dr. Chris White he solely mentioned that the die off rate in the Mr. Mallty yeast calculator was conservative. He made no mention of die off rate equations. The only other information he put forward was that the yeast die off rate was yeast strain specific.

Cheers!

Jack

 

Without sounding like a conspiracy theorist I wonder if the die off rate is actually much less than people think. In my twisted mind I see how this would force LHBS to unload very good yeast because it's a few months old and replace it with brand new stock.
There used to be a homebrew store in Lubbock TX that would give 6 month old White Labs yeast away, free, on the premise it was out of date and therefore no good. I used this free yeast for several years and never had an issue with it. I always made a starter, but I do that with brand new yeast and honestly I can't remember any difference. So I would love to see an independent, non-yeast lab report on the viability of commercially available yeasts.

 

Perhaps the Mr Malty calculator is based on a non-representative subset of strains, and Chris White has reason to believe that it could be conservative for others? The details of this stuff tickle the science geek in me, but in my brewing, I have been content with the Mr Malty calculator. A calculator that is conservative, making it more likely to over-pitch, probably serves homebrewers better than a calculator that works the other way. IMO.

 

I once brewed an Oktoberfest with a free Wyeast smack pack that was over 11 months old (I made a yeast starter). That Oktoberfest beer turned out excellent.

Cheers!

 

working on it