Hi Tim, chemically, the fructosamine protein adduct should be pretty stable. It is held together by a secondary amine bond, which is hard to break at normal pH.

As for the storage at -80oC, are these samples frozen serum or plasma?

You should be OK with serum. If you have plasma, then there could be proteinase activity and/or protein precipitation after thawing and this might affect the assay.

The only specific degradation process on these modified proteins that I can find is intracellular:

http://www.icp.ucl.ac.be/grm/Carbohydrate/fructosamine.htm

I think this publication answers at lest partially to your question.

Best regards