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I experienced few times a gross layer of foam when fermentation finishes. But the beer was fine, no symptoms of contamination neither off-flavors. I never measure the depth of foam but it can have about 2 to 3 cm (among 1 inch).

What can cause this ?

Is that explanation possible? : One of the explanations I found around the internet is that this foam is composed by dead yeasts, died by "hydration shock" when it is directly spread over the wort instead of pure water. This is strange to me, because although I use dry yeast spread directly over wort (at equivalent temperature), I think it would be necessary lots and lots of dead yeasts which isn't all of them within yeast packet. So, they would need to reproduce and dead yeasts don't reproduce, so the explanation about the "hydration shock" is very strange, which makes me think that if the foam is composed by dead yeasts the yeasts died after reproduction and not when spread over the wort.

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Are you talking about kraeusen? –  mdma Jun 20 at 21:15
    
Sure, it's a krausen. But, it sometimes disappear or dissolve remaining few residues over the wort, and not a gross foam during aging process. –  Luciano Jun 20 at 21:29
    
The dry yeast companies recommend re-hydration prior to pitching. It is very simple (Assuming you use 11g of dry yeast). Boil 200ml of water. Add yeast nutrient to boil if you have any. Pour about 110ml of the boiling water in a flask and cover with foil. Allow to cool to room temp. Pour the yeast onto the water. DO NOT STIR. Leave the mix for 15 minutes. Slowly (As not to spill) rotate the flask so that all the yeast is nice and wet. Leave for an hour. Pitch when you are ready. –  Atron Seige Jun 22 at 6:07
    
And remember there are about 220 BILLION yeast cells in a packet of dry yeast. Even if millions are dead, it is still a small amount. –  Atron Seige Jun 22 at 6:08
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1 Answer 1

up vote 4 down vote accepted

I get the same kraeusen when using liquid yeast - it's not from hydration shock. In fact, the dead yeast that don't survive hydration simply fall to the bottom of the fermentor.

The reason for the kraeusen is the constant churning of the wort during active fermentation, which mixes proteins, tannins and hop-alpha acids to create the structure for the krausen. As the krausen forms, it naturally collects yeast on top also, since the yeast become trapped in it.

The yeast are not necessarily dead, an during active fermentation are usually very much alive and highly viable. Brewers harvest yeast from the kraeusen - so-called top-cropping to obtain some very healthy and clean yeast from the fermenting beer. Once the yeast falls back to the bottom of the fermentor, it's mixed with dead yeast and trub, making it less than ideal.

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