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Old 01-07-2013, 08:59 PM   #1
TTB
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Mr Malty gives numbers based on no rehydration (I assume).

Jamil says half of cells are killed without rehydration.

So if I rehydrate...I should be able to use ~1/2 the amount of dry yeast Mr Malty tells me to use.

Thoughts?

I understand the "its not expensive so just buy another pack" case, but beyond that.

Thanks in advance for your input!

 
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Old 01-07-2013, 09:06 PM   #2
maida7
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I assume mr malty's numbers are based on properly re-hydrating the yeast.

 
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Old 01-07-2013, 09:14 PM   #3
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TTB's assumption is incorrect. Maida's assumption is correct.
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Old 01-07-2013, 09:21 PM   #4
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It definitely assumes perfect rehydration. It first calculates the number of cells needed for the proper pitch rate (which is independent of the type of yeast you're pitching). It then assumes something like 18 billion cells per gram for calculating the number of grams of dry yeast required.

This seems to be a fairly generous assumption about yeast density, as both Fermentis and Danstar only guarantee > 6 billion and > 5 billion cells per gram, respectively (at least for US-05 and Nottingham). I'm fairly sure the data sheets are conservative, though, and the Danstar one gives a pitch rate example that suggests that the actual density is normally between 5 and 10 billion cells per gram.

In any case, in case you don't care about the details, this means that MrMalty is assuming that every cell in the packet is viable when pitched. It's worth noting that its calculations call for somewhat less dry yeast than the manufacturers recommend, at least in the case of Danstar Nottingham.

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Old 01-07-2013, 10:28 PM   #5
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wow, great info thanks guys.

So then I should be pitching more dry yeast than mr malty says if I'm not rehydrating.

When pitching dry yeast are you guys pitching more than MM says? or rehydrating and pitching MM amounts?

 
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Old 01-08-2013, 02:40 AM   #6
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For dry yeast, I usually follow the manufacturer's instructions and ignore MrMalty. I always rehydrate.

 
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Old 01-08-2013, 06:08 PM   #7
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Quote:
Originally Posted by TTB View Post
wow, great info thanks guys.

So then I should be pitching more dry yeast than mr malty says if I'm not rehydrating.

When pitching dry yeast are you guys pitching more than MM says? or rehydrating and pitching MM amounts?
You should be rehydrating- you'll lose about 50% of your cells by not doing it, doubling the cost of yeast to get to the correct pitch rate AND you really don't know how many cells you're getting. Rehydrating is cheapest, best practice and only takes about 5 minutes.
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Old 01-08-2013, 07:51 PM   #8
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Ah, for the record, MrMalty documents that they assume 20 billion cells per gram for dry yeast, which is then scaled by the viability number (90% in my tests).

http://www.mrmalty.com/calc/dry.html

(It also confirms that they assume you're rehydrating.)

 
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