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.
Someone run this by Ramon Alex Hurtado. He might be the best scholar alive on 19th C. painters. He actually knows how to paint, which counts for a lot when art has devolved into so much pomo meaninglessness.
You mean the photographs taken of the jars that are exhibits behind a glass display case in a museum in australia... Funny how light reflects like that.
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.