Mixing freshly expressed breast milk with already cooled or frozen milk is not advised because it can rewarm the older stored milk. It is best to cool freshly expressed milk before combining it with older, previously cooled or frozen milk. Babies can safely drink cold breastmilk or formula. For healthy, full-term babies, you don't need to be concerned about giving your baby a bottle straight from the refrigerator or mixing formula with cold water. Checkout Ebaby's Marketplace for breastmilk storage bags.