Put the bottle you pumped into into the fridge to cool off before adding it to the milk
already there. Adding warm milk
to cold milk
results in warming of the milk
. You would have to use it within 1 hr or 2.
a word of caution- certain bags tend to leak, especially defrosting. or you may just get a bad batch of a normally good brand. a good way to avoid tears over spilled milk is to drop the bag into a small ziplock as you defrost it. any leaks are caught by the extra baggie.