Bel, i would freeze my milk and then put it in my baby bag's insulated compartment straight out of the freezer. It takes several hours to thaw in there so i knew i had heaps of time and there would be no chance of it going bad. And then, if i needed to use it before it had completely thawed, i would just run the bottle under water (even cold does the trick). The only problem i found was warming it at the other end, cos you can't heat BM in the microwave

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Good luck.