I've just gone to rack bottle a witbier that has been in primary for about 5 weeks and noticed this white gunk floating on top of the beer.
It doesn't look too healthy, what is it? Is it safe? I'm hoping it's just some undisolved dry wheat extract or dry krausen.
EDIT: After bottling / kegging the brew tasted smelt fine - great in fact.
I believe everything written in response to the query here is correct. It's a form of yeast called Kham yeast. As long as there is no mold it is safe. Try to avoid bottling it with your brew of course. If mold is present, dispatch of the whole brew and sanitize all equipment. Make sure that your fermenter is air tight and that your airlock is clean and filled for your next batch.
Could be a film yeast or some other infection but you'd do best to RDWHAHB if the beer tastes OK.
It won't hurt you if there's any amount of alcohol in there. Smell it and taste it after primary. If both pass inspection, carry on. I forget the organism's name, but this happens in wine too sometimes. It's not ideal, but it (most likely) won't cause you to unexpectedly soil yourself a few hours after drinking it... so it's no big deal in terms of the big picture if this is just homebrew and not for commercial sale.
Next time, leave less headspace in your secondary fermentor and resist the temptation to ever open it up. You want to keep oxygen off the surface, leave it protected by the CO2 blanket that the fermentation emits. O2 will foster the growth of scum like that and will degrade your hooch. CO2 will restrict the growth of oddities.
It looks like normal Wit yeast byproducts to me. Wit yeast is a weird one anyway, imho. Does it smell like vinegar at all? A Wit should taste a little tangy, but you just need to verify that it's not infected.
I'd recommend bottling now, but start checking the bottles for over-carbonation starting in about a week, and if you start getting gushers, then you might need to chuck the whole batch.
Its impossible to tell what it is per se from a photo unless someone has had the same thing happen to them, even then... But I had something similar to this and I think it was lipids (fats) on the surface. The only place that would have come from in any great quantity would be the yeast.
Five weeks in primary may be the culprit if it wasn't under great temp control and you also didn't have supremely healthy yeast to start.
The lipid thing is just a guess though, based on some practical experience.
I'd say its safe. No reason to think its not safe. It might taste fine it might taste funny. There's only one way to find out.
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