Matt Allan recently asked about background information on propolis as a
food preservative.  One of the best sources I know of for background
information on propolis and other bee products is Krell, R. 1996.
Value-added products from beekeeping. Food and Agriculture Organisation
of the UN, Agriculture Services Bulletin 124.  The book is available
from the International Bee Research Association (see their web page).
 
According to Krell, the antioxidant, antimicrobial and antifungal
activities of propolis offer scope for applications in food technology.
In Japan, a patent has been registered for use of propolis as a
preservative in packing material.  In that country frozen fish are also
preserved with propolis.  The preservation is said to extend the storage
life of fish by 2-3 times.
 
Krell also gives a reference for use of propolis for post-harvest
treatment and conservation of fruits.  The reference is Sangalli, A.
1990. La propoli. L'Ape Nostra Amica 12(4): 16-25.  Maybe an Italian
subscriber to BEE-L will be able to provide further information about
that reference.