I brewed my first kettle soured Berliner Weiss and I noticed a huge layer of foam on top. Is this most likely an actual yeast fermentation (infection) happening or is this what lacto brevis (WLP672) looks like as it ferments?
That's normal. Keep in mind that Lacto from White Labs tends to have Sacc in it, this is a contamination issue that WL has.
Agreed that yeast has gotten in there. I have used lacto a half-dozen times and I've never seen any bubbles from it whatsoever, so I'd say it's certain that you've got yeast in there. Probably not ideal for a kettle sour, as you will end up boiling off a lot of the alcohol. To be honest... if I were in your shoes I would treat this as a no-boil Berliner, maybe adding a little extra yeast and moving it to a closed fermenter. You would be exposing your equipment to lacto bugs, but if you're into sours that is hopefully not too big a problem. (Also, for what it's worth, I would consider lacto to be a relatively mild threat compared to something like brett.) [edited for clarity/emphasis]
Very interesting to see so much apparent yeast activity there. Coincidentally I am planning on souring with WLP672 this Friday... What has your pH done ? Gravity ?
I don't really have good pH testing strips, but if I had to guess I would say it really started tasting sour after 36 hours. I would guess it went from 4.5-5.0 down to 3.3 or so in that time. It started at ~7 brix (in 6 gallons) and dropped down to 5.5 brix post boil (5 gallons). Lost quite a bit of sugar in that short time. I didn't think to test the gravity pre-boil because I didn't think there was any yeast in there until I took my post boil reading.
I had actually already done the 30 minute boil before I posted this (I needed to boil off 1 gallon to fit it in the carboy). I didn't think this was yeast until I noticed my gravity was significantly lower than when I started (1.029 in 6 gallons down to 1.019 in 5 gallons). Do you lose any sugars when souring with lacto?
Yes, Lacto bugs eat sugar, but not very much. Here's the Milk the Funk page on "100% Lactobacillus fermentation," which is not really a thing in terms of beer production. As you can see in the "Lance Shaner's Experiment" section, in one experiment various strains of Lacto bugs reduced gravity from 1.037 to about 1.033 or 1.034. Your much more substantial gravity drop is clearly attributable to the presence of yeast. This is arguably unfortunate for you, since you boiled off a fair amount of your alcohol, but on the bright side, a low-ABV sour beer can be a really nice thing. I would roll with it. A tart, refreshing beer that you can drink a lot of without getting too drunk sounds great to me.
