Storing milk powder in freezer then using it question

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I read where you can keep milk powder in the freezer. Then after reading that I was told that when keeping it in the fridge or freezer that it should be kept in an airtight container to protect it from oxidation and moisture. I have had mine in the freezer for about 4 weeks. Do you think it has gone bad in that time period? How can you tell if it has gone bad? Also, when it has been in the freezer and you want to use it in milk bath, can you use right out of the freezer immediately or should you let it get to room tempature before mixing? Thank you.