Lately I've been doing some very large boils -- 7 gallons in a 7.5 gallon kettle. To prevent boilovers, I've been using Fermcap-s. It's great, and I've had no boilover problems, but I have no idea how Fermcap works or what's in it. What is this stuff? Other than preventing boilover and excessive kreusen, does it have any effect on my beer?
Fermcap-S is a silicone (Dimethylpolysiloxane) based emulsion that works by breaking surface tension. It has no effect on the finished product if used in the boil at designated dosage. If used in the fermenter, I have read that it will increase residual bitterness by ~10%. This is likely due to the compound binding to the yeast cells before the dissolved bittering compounds do, thereby leaving more bitterness in the beer. I have not verified this myself.
Stick to recommended dosage, or you will exceed FDA guidelines for silicone content in your beer.
This thread at BeerAdvocate covers the topic: Fermcap-S (login required)
The FDA has recently decided that Fermcap S should be filtered before you drink your beer. Fortunately, there's Fermcap AT which is fine to use without filtering.
Here's some info from Birko, which makes a product very much like Fermcap..
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