It's extremely unlikely to be air residue, since what they're finding is grain and yeast protein. That stuff won't be flying around in the air, particularly as the Danes did their brewing indoors.
But, yes, the Danes did brew a lot, into the 20th century, since beer was the daily drink for Danish farmers.
They also brewed a LOT in the 18c there. I think it'd be more plausible that air-residue landed on the paintings while they were being made.
It's also an art authenticity check too. Destructive, sure, but can be done carefully.