# Does dry yeast detoriate once in the fridge?



## trustyrusty (12/8/16)

I had a packet of yeast S 04 that I think had 1 / 15 on it (ie 20 months over) - Difficult to read could be 1/16 but has been in the fridge for a year anyway.
I made a starter to see if OK, it was bubbling like a well carbed freshly opened beer in less than a minute.
Used in a brew and all looks OK to me - yeast seems very happy  - Maybe some brands are less susceptible to age than others?
Re 'use by date' are the assuming you have in the fridge?
cheers


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## MHB (12/8/16)

You got it - use by dates are calculated at 20oC (mostly), and yes yeast even dry do die over time, but like most things the hotter the faster.
A good rule of thumb is that you loose something like 20%/year at 20oC and about 4% at 4oC, not exactly right but close enough to give you a fair idea of what to expect.

The cooler and more stable the temperature for dry yeas the better - I have had packets that were 5-6 years old fire off in no time.
Mark


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## trustyrusty (12/8/16)

Huh ? They recommend putting the fridge - is that because they don't want to put use by 2025


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## MHB (12/8/16)

Food labelling laws are pretty strict (and yeast is regarded as a food stuff) if you have a "recommended" pitching rate, there must be at least that many viable yeast in the package by the use by date and under certain conditions
If you vacuum packed a raw chicken drumstick and stored it in liquid nitrogen it would be edible in a hundred years time - but that isn't a practical use by date as it doesn't reflect how the product is stored and transported.
Mark


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## Lyrebird_Cycles (12/8/16)

MHB said:


> A good rule of thumb is that you loose (sic) something like 20%/year at 20oC and about 4% at 4oC, not exactly right but close enough to give you a fair idea of what to expect.


Deterioration rates are quoted as 50% per year at room temperature*, you can assume first order Arrhenius kinetics below that.

That's probably a pessimistic estimate but it's on the companies' spec sheets: see for instance Danstar Windsor .

* usually 20 or 22 degrees


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## trustyrusty (12/8/16)

How much more profit would any company make if they put 'use by date' in 1 month or 2 months


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## MHB (12/8/16)

That's just silly, imagine the waste, lost stock and unwillingness for retailers to stock the item... sales would plummet.
m


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## trustyrusty (12/8/16)

I am not referring to yeast, (I said 'any company') if a company says use 2 spoons instead of one or 'use by date' could be 6 months but they put 3 and use OHS as excuse. You don't think they would do that?


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## Mardoo (12/8/16)

Dunno. What would your company do?


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## MHB (12/8/16)

You clearly picked your name with your tongue firmly planted in your cheek!
No - there are lots of suppliers (lets stay on yeast), the 20%/year number is from independent testing of freeze dried yeast, either vacuum or Modified Atmosphere Packaged (no O2,CO2 or H2O).
Unless all the makers "consorted" and agreed to tell the same lies we (consumers) would choose the product that offered us the best value.
Worth noting that we are talking about "Best Before" not "Use By", there are fairly important differences between the two - might be useful to look the terms up, and not really an OH&S matter either.
Mark


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