Hot hide glue is a protein glue so it is very good food for both mold
and bacteria.  Perhaps the best way to deal with hide glue is to make
use of a small jar such as the Kraft Marshmallow Cream jar with the
wide base.  The glass marshmallow jar glue is mixed in smaller batches
than you would normally leave in your glue pot.  Your glue pot is then
used simply as a water bath to heat the glue in the jar and when the
hide glue is not in the water bath you may keep it tightly covered with
the original lid for storage in the refrigerator. You may store it in
the refrigerator for months but take care that it not freeze.

For a quick warm up you may use a microwave oven set on low while
periodically taking the glue out to stir it in successive heating
cycles until it is up to temperature for placement in the glue pot
water bath.  This has the added advantage that you will have much less
cleaning to do on the metal glue pot kettle.  For very small jobs I
keep medical syringes filled with hide glue in the refrigerator and
just microwave them as I need them.  This has the advantage of keeping
the air away from the glue and slowing any microbial infestation to a
negligible level.

I would not advise adding any ingredient to the glue to prevent the
mold or bacteria from growing.  Liquid hide glues made by manufacturers
have these additives and the strength is never as good as freshly made
hide glue from dry crystals.  Either keep the glue pot hot or refrigerate
the glue when not being used.  Storage temperature is your best defense.

Good luck,

Al Sefl - Who loves the smell of hot glue in the morning...

PS: The hot water in the glue pot water bath is also good for thinning
the glue without causing a temperature drop in the glue.