A quick Google search suggests that rancidification/peroxidation rates under LN2 storage are considered negligible for food storage purposes: LN2 is being investigated as an alternative to freezing for this very reason. LN2 also appears to slow down or stop rancidification in cryopreserved tissue (paper). Nevertheless, there are concerns about rancidification during the freeze/thaw process. Also, this does not give much info about the outcomes of long-term storage (decades and possibly centuries) as required in cryonics.