Sour butter from freezer?
OK, so I left one of the butters (the Kerrygold) out for a little while longer to see if it would affect the taste. I tried it again the day after taking it out of the freezer and the sour notes disappeared!

So I guess my freezer isn't the culprit after all. Here are the possibilities as I see them:


The first butter (Plugra) was, in fact, a bit rancid. The sourness lingered in my mouth and affected the taste of the second butter (Kerrigold).
The first butter was unsalted and I simply wasn't used to the taste of unsalted European butter. (Unlikely, as I just spent a year in Europe! But I also don't eat unsalted butter very often.)
The sourness of cultured European butters is more pronounced when the butter is colder and/or defrosting. (More with the Kerrigold than the Plugra. The Plugra was already room temperature when I tried it.)
It was all psychological.


UPDATE:

Just tried a fresh, unsalted pack of room-temperature Plugra. Well whaddayaknow, it's sour. So I guess the answer really is:


I don't know what I'm doing.