Any milk expressed within a 24 hour period can be expressed directly into the same container if the previously expressed milk was stored between 32° - 60° F. Storage recommendations are then based on the time and date of the first milk expressed.
When adding fresh milk to already frozen milk:
- Cool fresh milk in refrigerator for at least one hour
- Add only a smaller amount than what is frozen (to avoid thawing the top layer)
Frozen milk should be thawed under cool running water, gradually increasing temperature of water until thawed and heated to room temperature.
Microwaving milk is not recommended because valuable components of the milk are destroyed at temperatures greater than 130° F. Also, the uneven heating process can cause "hot spots" that may burn baby.
As always, shake milk after any heating to help distribute the heat evenly. Once milk is frozen, it can be kept refrigerated for up to 24 hours. It should not be refrozen. It's not yet known (for sure) if it's safe to give a baby breast milk that was left from a previous feeding, or milk that was previously warmed. However, some sources believe the milk can be used for the next feeding, only.