I am finally quite confident I have identified the source of my problem. The culprit appears to be the malt I bought at a nearby HBS. Given that I crushed and brewed with this grain the day I bought it, it definitely looks like it was bad when I bought it. It was the very fact that it was bad from the day I bought it that made me suspect other things first. That is quite a shame given how much time and beer I’ve wasted.
After I again began to suspect the malt I decided to buy some more fresh, and do a comparison. It wasn’t a perfect apples-to-apples comparison, with the newer malt being Maris Otter and the problem malt US 2-row. But the rest was the exact same: I mashed 60 grams of each in 300mL of the same water for 30 minutes (in the oven), filtered through a fine sieve and then boiled without any hops. I added some yeast slurry I had in the fridge, about 75mL in each flask. I did my best to clean off the residue on the mill, especially given that the only grain I’ve ever used it on had been this questionable malt. I am fairly sure I ended up with some of it in the Maris Otter half, though.
In a comment, Sui Generis mentioned that a time when he brewed with questionable grain it was clear from the beginning that something was wrong. That hadn’t been the case for me before. But this time, both due to having something to compare to and to having no specialty grains or hops to mask the aroma, I noticed the 2-row flask smelled a bit off while boiling, just like what I smelled in the bad beers. This intensified after fermentation began but was obvious even when still.
The Maris Otter half smells very clean in comparison. If I really look for it, I think I detect some of the off smell there, too. But that could be in my head, or it could be due to some residue from the mill. Since the difference is so obvious, I am very confident this malt was the problem. And since my other questionable batches not involving this bag of 2-row did have old, precrushed malt, it’s reasonable to assume the culprit there.
Ultimately, I am glad it was something simple like this. It’s a shame it took several months and lots of frustration to figure it out. I suppose it’s good that I am paying more attention to the water before brewing even though that wasn’t the problem. I do regret having posted that first entry with its bogus deduction. It’s no surprise the problem wasn’t something that exotic. It is surprising how convinced I was this had a plastic character to it, when thinking about it now that’s not at all what it is. Smoky, maybe, but it’s definitely not plastic. I added a note to that post just in case somebody stumbles upon it from a web search or something.
For reference, the bag of malt in question was a 10 lb. bag prepackaged by one of the bigger ingredients distributors (I actually don’t remember which one) that I bought in July. I don’t recall seeing any dates on the package. The HBS has since redone their grain setup completely, so I will probably give them another shot later on.
Hopefully I can now get back to blogging about more interesting things. Thanks to everyone who helped out.