white film on top of beer?

So I have made the switch to 15 gal plastic conicals. I have brewed probably 12 batches and probably three of them have a thin layer of white on top that almost looks like soap scum. It has solidish components in it that to not dissipate but stay in the beer. Two of three times I had white mold inside the fermenter after three weeks, with no noticeable flavor contribution or off flavors. So what's going on here?

You are growing something you don't want to be growing. Check sanitizing procedures.

Also worthy of note is that these conicals don't create a perfect seal. They breath around the threads at the top where the lid screws down.

coldrice said:

Also worthy of note is that these conicals don't create a perfect seal. They breath around the threads at the top where the lid screws down.

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Oxygen allows mold to grow, as well as other microbes (like aceterobacter that turns alcohol into vinegar).

Yes, but if he has a clean, healthy ferment going the only thing that should be going through that leak is outbound CO2. Even at completion there should be a blanket of CO2 covering the beer and that should stay in place for quite a while as it will take a long time for air to diffuse through the threads.

ajdelange said:

Yes, but if he has a clean, healthy ferment going the only thing that should be going through that leak is outbound CO2. Even at completion there should be a blanket of CO2 covering the beer and that should stay in place for quite a while as it will take a long time for air to diffuse through the threads.

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Yes, but I'm not sure it's a "long time", or rather, I'm not sure how long a time that would last. I have more experience with wine, and a wide headspace is an issue when fermentation slows down and ends. But how long it could be before I have to worry about this, I can't say. I know it would depend on the amount of cracks/openings in the fermenter, the environment (like temperature), and so on. But I have seen mold take hold in a fermenter with lots of headspace in less than three weeks, and that was without any opening to the outside air.

I wasn't very specific about what a 'long time' is but I'd call 3 weeks stretching it IOW I wouldn't be comfortable depending on the fermentation's CO3 blanket to protect beer that long. I do leave beer in unitanks that long but always under a bit of positive CO2 pressure.

Aren’t those plastic conicals polyethylene, as in oxygen permeable? Even as the oxygen is being floated out of the headspace it would be diffusing directly into solution.

I suppose it’s kinda slow, plenty of people ferment in buckets. I’m wondering if that might be enough oxygen to support whatever pathogen has invaded the OP’s system.