I wish I was that rigorous. It's a habit I got into from noticing that no matter how long I settle bio inside before filtering, once filtered and stored in 25l containers outside, there is sometimes a little extra glycerol that drops out. And if I have a soapy batch that refuses to settle all the soaps out despite standing a month, the solution to that is to stand it outside in 25l containers, at which point the soap finally drops.
Whether that is UV or a wider thermal cycle, I can't say.
And then there's the containers I've not used for a year that had a small amount of bio at the bottom. Which I initially thought I had left a little methoxide in, the liquid was so clear and thin - different to oxidised, rancid bio that goes thicker (and stinks).
This study on biodiesel discharge into the marine environment showed that UV exposure of FAME broke it down into the fatty acids and methanol, so it could be that is what happened.
https://digital.lib.washington.edu/researchworks/handle/1773/36524Perhaps if there is limited oxygen in a well sealed container the outcome is different?
The study also mentions isomerisation, which other studies have used to improve cold flow properties of FAME (though they use special catalysts and high temp/pressure). I wonder whether the waste oil I collect has partial hydrogenation and lots of viscosity increasing trans-isomers, which perhaps are rotated into cis-isomers by UV? Just wild speculation that, though
I've read that the FAME process opens the double bonds which allow them to reform into cis and trans isomers in equal quantity, but I don't know how true that is either...
It does make me want to experiment with UV lights and see what happens.
I think you observed that bio that's been outside goes lighter in colour, too?
