what does top fermenting yeast really mean ?

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bobtheUKbrewer2

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I use safale 04 and for 2 days there is about an inch of white foam on the brew. There can only be contact at the liquid foam interface, surely ? Then on day 3 no foam left and bubbles rising to the surface from within the brew. I have never tried a bottom fermenting yeast but would expect similar for first 2 days.
 
I'm not sure what the contact area has to do with anything, but the difference between ale (top fermenting) and lager yeast (bottom fermenting) is due to it's appearance during fermentation. Ale yeast is used at warmer temps and will actively rise (and fall) throughout the fermentation process, creating a thick foam. Lager yeast ferments at a cooler temp and thus doesn't produce as thick of a foamy surface, and tends to gather near the bottom due to the "slower" fermentation.
 
The yeast is suspended throughout the solution during fermentation. The foam you see on top is caused by the offgassing of CO2 produced by fermentation. That's why fermenting beer ( top or bottom fermenting) looks 'milky' while beer that is done fermenting drops clear. There is no 'interface.'
 
So top fermenting is not an accurate description of what is happening ?

You're over thinking it. Just forget the terms top-fermenting and bottom-fermenting. They serve no real purpose to even know or care about. Just know you picked your yeast because it will provide the characteristics you want for your beer.
 
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