Restoring mechanical instruments is just a hobby so keeping hide
glue good is tough.  I will make a batch on Monday, cover a few
bellows and cover my glue pot.  By Thursday the mold is there.  Being
a Chemist/Pharmacist I found by adding 1/2 a teaspoonful of Urea USP
powder keeps the mold away for about 14 days or more...  It is very
humid in Louisiana.

Good luck...
Chris Kalstone

P.S.  Does long-time storage (refrigeration) of hide glue
destroy some of its bonding properties?