little advice about breast milk please!!

Francesca
If I want to go on a day out and me and hubby both can feed baby, how do I store the expressed breast milk? I know it can be pumped the night before and left in fridge then put into bags in a cool bag with ice packs, so I just pour the milk into a sterilised bottle then warm it with hot water? Will the sterilised bottle stay sterile in the changing bag?