Alright, HBT'ers, Tell Me What This Is!

[Evan!]

3 weeks ago, I brewed an AG Witbier and PM Hefe side-by-side. Fermentation was relatively long...maybe a week or so. I've left them in the primary since then and plan on bottling this week. But here's the thing: after the Wit was done, the krausen fell as is standard. But over the past week, it's acquired this foam on top. Check it:

Samples taste fine. So, anyone seen this before?

 

[bradsul]

I've had that before too. I took it to be offgassing CO2 since my samples all tasted fine as well. My SG was stable for 4 days so I decided to go ahead and bottle and all was well.