How to properly store your breast milk • Combining breast milk: According to the CDC's and AAP's most recent statements, breast milk should not be combined at different temperatures. However, you can combine breast milk from the same day. Ex: You have breast milk in the fridge that was pumped in the morning, you just pumped that afternoon and want to store the milk. Before combining it to the container in the fridge from that morning, wait until the freshly expressed milk has cooled down. You can place the freshly expressed milk in a separate container in the fridge then combine them once they reach the same temperature.put at room temperature HOWEVER, they seem fo be going back and forth on this. If you want my two cents, as long as baby is full term and healthy you can combine them