I have an electric pump that I am keen to get in the habit of using to my baby cannhave a bottle of we're out or my OH has him while I go out. I can't express enough in one go to provide a single full feed so my questions are can you mix different batches of milk? If I express today can I add tomorrow's express to it? How do I go about this? Is it better to put it all in the same bottle for a few days then freeze?