I broached the subject of cryonics with a family member today. He offered almost none of the normal objections and I've been happy all day about the way the conversation went. One interesting issue that he raised that I'd like to find an answer for is the question in the title.
Butter goes rancid after a while at room temperature. It also goes rancid in the fridge and can absorb the other flavors if things aren't well contained inside the refrigerator. Butter also goes rancid if left in a normal freezer, which mostly is designed to bring things very close to the melting point of water around 273 kelvin.
This suggests that lipid chemistry responds to temperatures in a different way than intuitions mostly educated by other examples of freezing, which is relevant because the brain is mostly made out of fat, with some complicating proteins mixed in. My guess is that developing a "rancid brain" isn't likely to be a serious issue when you get down to the 77 kelvin of liquid nitrogen, but its still something I'd like to be able to answer directly and honestly, after really thinking about it in terms of "safety engineering".
One way to answer the direct question about butter might be to just perform the basic experiment with some butter samples at different temperatures (room, fridge, freezer, -80C freezer in a bio lab) and figure out how long butter stored each way takes to go rancid and then do some curve fitting, but that seems like it would take months or maybe even years, and butter doesn't even necessarily answer neurological questions directly. Even if I learned about butter chemistry, there could be open questions about brain chemistry. I've tentatively googled around for 30 minutes but organic chemistry isn't a primary area of expertise and I wasn't sure out to dig up the specialist scientific literature that might answer my question.
This community seemed like a good place to get help on the subject!
Here are some specific questions I'd love to know the answers to...
1. What are the precise chemical reactions are that are collectively referred to as rancidity in english, and how to they change at cryogenic temperatures? Does butter stop going rancid in liquid nitrogen?
2. Are these or similar reactions possible in the brain, given all the cell membranes and mylenation and so on that are primarily made out of fat?
3. How much personality/memory/mind relevant information might be lost to rancidity, if it happened? If there are brain or neuronal structures that are more likely to go rancid first, would the chemical changes involved in rancidity be likely to change our estimation of the structures "historical operation" or not?
4. The boiling point of oxygen is about 90 kelvin (13 degrees higher than nitrogen's boiling point). If the liquid around a cryo-patient is not changed over time then we might expect the ratio of liquid oxygen to liquid nitrogen to increase over time. Is the presence of the liquid oxygen relevant to rancidty issues or not?