When I was 13, my sister and I spent the summer on a dairy farm. We worked in the creamery, where (twice a day) we bottled fresh raw milk, as well as cleaned all the equipment. It was really a fun summer overall.
One evening we had a very bad thunderstorm. It knocked out the power and curdled all the milk in the creamery. My uncle was visibly distressed, but my aunt made it memorable by serving us all (by candlelight) fresh milk curds and brown sugar (something I was reluctant to try but ended up really enjoying). She matter-of-factly explained that sometimes "electrical storms" did that, and it was just an accepted risk. We had all the curds and brown sugar or maple syrup we wanted for a few days, and a lot was given away as well.
The next morning we had to call all the customers to tell them there would be no milk that day. It was the only time that happened, though there were other storms.
I'm a wife, and I'm not young, but this is not an old wives' tale. It wasn't a matter of spoiling. Our milk was all fresh. Milk that came in from the barn was filtered, rapidly cooled in a special apparatus to (very cold - I'm sure I once knew the temperature but don't recall any more), bottled immediately while cold (the bottles were room temperature), and refrigerated. After we met the demand for the next day's milk with a couple of dozen to spare, excess milk was fed to the calves. Any bottle of milk older than 24 hours was marked, and if not sold within one more day, was tossed. But that evening, every bottle of milk curdled.
I just accepted all these years that thunderstorms could curdle milk, until I ran across it today, where it was labeled an old wives' tale. The only support I could find (besides old newspaper articles discussing it) was a blurb on the Naked Scientists website stating
Electrostatic fields within a certain range can break up emulsions by polarising droplets and causing them to coalesce head to tail. During the build-up to a lightning discharge, the field strength will presumably pass through this range and may cause exposed milk to separate into its aqueous and fatty components. Milk in a metal container would be shielded from the field and remain emulsified.
Our refrigerators had metal cases.
I like yoghurt and sour cream, I've had milk animals and made yoghurt and cheese, but I can't tolerate even slightly sour milk. None of my milk ever curdled, and we have plenty of storms where I live (we have at least a dozen storm-related power outages a year, some lasting days). What I had back then wasn't spoiled milk, it was separated right in the bottle into fresh curds and whey (like when making cheese).
I've googled this and can't find a decent paper, so I'm posting here. How can thunderstorms curdle milk?
EDIT – Reference for notability: "Thunderstorm Curdled Milk Puzzle Still Baffles Science", The Telegraph (June 2, 1937)