So my girlfriend recently brought me some Trappistes Rochefort 10 and 8 from home, which I'm very excited about, but I noticed today some build up on the inside of the necks of two of the 4 bottles. It almost looks like mildew or something, which is what I initially thought it was when I saw the first bottle, but the second one has the same thing and I find it very hard to believe that two out of 4 bottles were improperly sealed, but I don't really know much about beer packaging or anything like that. If anyone is familiar with this type of thing then I would like some insight, I would rather now be drinking a tainted beer and regretting it all weekend.
Good news is nothing in that bottle will harm you...might taste nasty but it wont make you sick. Now the caps could be poorly sealed and kept in a humid area and inviting some fungal growth. You won't know unless you pry it open(if it yields without normal effort you know the culprit). That brewery produces almost 500,000 gallons of beer a year or roughly 61 million bottles of beer you might just got unlucky.
It's likely yeast. Real beer contains yeast. Refrigerate it for one week and drink it. Smell it first. If it smells foul, drink it anyway.
Sounds like they were stored at ambient temp and some of the yeast from its bottle conditioning decided to hang out up top. I see it pretty regularly with unfiltered homebrew that is stored at ambient temp. What Brew-Betty said. Stash em in the fridge for a week so the yeast fall asleep and drop out, and give this beer some love. Careful, it might be very highly carbonated though.
Since his GF brought the beer from home which suggests somewhere not close to where the OP lives, I'm going to bet his GF did not store the bottles upright which caused the yeast to settle and stick to the top of the bottle. Be careful though, this beer was fermented with fungus.
