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I always put my dry yeast directly into the wort, even though the packaging says to rehydrate it first. I've never had (noticed?) any problems with fermentation.

What could this habit cause to go wrong with the yeast, that rehydrating would prevent?

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5 Answers

One other vital point not mentioned is rehydration temperature. While each strain and manufacturer has different guidelines, they are all in a range of 80-105F, 25-40C, with most being at the top end of that range - much hotter than typical wort temperatures.

Rehydrating at room temperature can cause a loss of 60% viability compared to rehydrating at warmer temperatures. Why rehydrate?, An interview with Dr Clayton Cone, discusses how the temperature of the water affects viability and explains why rehydrating in wort (at any temperature) causes cells to die.

For examples of rehydration procedures, see

While you can get away without rehydrating yeast, you'll be vastly underpitching. You'll still get beer at the end, but it won't be as good.

As to why the beer won't be as good - Wyeast says this on underpitching:

Effect of Pitch Rate on Beer Flavor

Pitch rates, in addition to strain, temperature, and gravity, make a dramatic difference in the final flavor and aroma profile of any beer.
The pitch rate will have a direct effect on the amount of cell growth during a fermentation. Cell growth decreases as pitch rates increase. Ester production is directly related to yeast growth as are most other flavor and aroma compounds.

A low pitch rate can lead to:

  • Excess levels of diacetyl

  • Increase in higher/fusel alcohol formation

  • Increase in ester formation

  • Increase in volatile sulfur compounds

  • High terminal gravities

  • Stuck fermentations

  • Increased risk of infection

In a nutshell, underpitching stresses the yeast, since they spend a good deal longer propagating, in high gear, rather than fermenting. The increased propagation levels causes the off flavors mentioned above, and as there is initially less yeast, there is more chance of other microbial contaminants getting a foothold. High propagation levels leave lots of scar tissue from budding on the older generations, which reduces the rate of transfer in and out of the cell, making it harder for the cell to consume nutrients and expel waste products, reducing the effectiveness of the cell, contributing to high finishing gravities and stuck fermentations.

In the fermemntor, you want your yeast to spend their energy making alcohol, not babies!

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+1, interesting links. However, I'm still not sure why underpitching is bad. How can it make the beer worse? –  Matt Fenwick Sep 17 '12 at 13:04
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I'm glad you mentioned that, I was going to also include links about the problems associated with underpitching. I'll edit my answer. –  mdma Sep 17 '12 at 13:09
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You should rehydrate before pitching, especially in high gravity worts. See here. Pitching the yeast directly results in fewer viable yeast cells.

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Typically allowing the yeast to rehydrate in wort creates stress on the yeast and roughly half of what you pitch doesn't survive the process. Reason being is that normally the yeast cell regulates what crosses the cell membrane. While rehydrating the cells do not have that control and they end up with too much sugar and nutrients passing into the cells while they rehydrate.

Not to say that you can't sprinkle two packets into your wort to compensate for the viability issues. I've done it this way several times when I am being really lazy. But to be honest I've always found that the fermentation profile is better when I've pitched liquid yeast from a starter or from slurry. I like to use dry yeast to make a simple house beer or session beer. Then use the slurry from that to make really great beer.

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The theory is that the more 'roughly' you treat the yeast, the fewer viable cells will make it through to ferment your wort/must. I almost always rehydrate and I follow the rule of never stirring/disturbing the yeast during this process. I sprinkle it on the top of the starter and let it sit for a few hours for it to naturally hydrate and disperse.

As long as you are pitching adequate yeast, however, it shouldn't make too much difference whether you rehydrate or not.

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You should be rehydrating in water, not a starter. If in fact by starter you mean a small batch of wort. –  brewchez Sep 16 '12 at 18:57
    
shrug I usually proof my yeast when I rehydrate, either with DME or sucrose. I'm not sure it makes much of a difference, honestly. –  bk0 Sep 16 '12 at 22:32
    
A little will be fine, since the gravity will be low - 1.010 or probably lower. But a regular strength starter 1.030 or above will kill off a large portion of the cells. See the first link in my answer for an explanation. If you want to proof, it's best to first rehydrate in water then add fermentables once the yeast have rehydrated. Also, no need to wait hours - the yeast rehydrate in 15 minutes max. –  mdma Sep 17 '12 at 13:19
    
And yet it seems to work wonderfully for wine must, which has gravities in the 1.1+ range. If the theory is correct that a large portion of cells die, then it is not due solely to osmotic pressure. –  bk0 Sep 18 '12 at 11:48
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I'm sure temperature and acidity play a role too. After all, after inoculation the yeast drop the acidity of the wort by nearly 1 pH down to 4pH, while wine is already at that level. You'll have to define "working wonderfully" - most people that direct pitch don't realize there's a problem until someone tells them. –  mdma Sep 18 '12 at 14:42
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Brew Your Own did a study on this. Done side by side, most tasters couldn't tell the difference.

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This would be interesting to learn more about. Do you have a link or issue number? –  Henry Jackson Sep 16 '12 at 21:19
    
I'm sorry, I don't think this answers the question. I wasn't asking about taste, but about what's going on with the yeast. –  Matt Fenwick Sep 17 '12 at 12:56
    
Yes, it doesn't answer the question –  MStodd Sep 18 '12 at 22:48
    
One possible consequence is that you won't be able to tell if you rehydrated or not, hehe. –  Dale Sep 19 '12 at 21:48
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