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If you are just storing the milk for a short period of time before your baby gulps it down, no need to freeze it. Store it in extra glass or stainless steel bottles until your baby drinks it, which also helps to cut down on dishes. Even bottles that say BPA free can have hormone-altering chemicals that can get into your breastmilk, so check out our roundup of glass and stainless steel bottles. And since substances that are fatty (hello milk!) or acidic can leach more of these plastic additive chemicals into the food/liquid, it's a good idea to avoid storing your milk in plastic bottles. Additionally, heat and UV damages plastic (like when you sterilize or wash the bottle) which makes it even more likely that the bottle is leaching chemicals into the milk.
If you are building up a stash for when you head back to work or when you'll be away from your little one, then you'll need to freeze your milk.
One of the easiest ways to freeze breast milk is in a plastic breast milk storage bag. A lot of moms freeze these bags laying flat, which allows them to be compactly stored. These storage bags are thicker than normal food storage bags and usually have really good seals on them to prevent spillage. They are generally made of food grade LDPE (low density polyethylene), which falls under the #4 recyclable category. LDPE plastics are also commonly found in bags for dry cleaning, frozen foods, bread, and the linings of hot beverage cups. The good news is that LDPE plastic does not contain additives like BPA or similar chemicals and is generally considered a safer material (1). Make sure to buy high quality breast milk storage bags, because cheap versions may be mixed with other types of plastics. The bad news? There's not really any right now, but some people are still concerned because it's plastic.
If you're looking for a plastic free option for freezing and storing breast milk, some moms have had good luck with 4 oz mason jars or other small baby food containers that are made of glass. There are a couple on our recommended baby food and storage roundup. Just make sure to leave enough room at the top of the container for the milk to expand a bit as it freezes.
Another option is to freeze breast milk in silicone freezer trays and then when the milk is frozen solid, to pop the frozen milk out and store them in the freezer in either freezer ziplock bags, silicone bags, or large glass jars. This requires an extra step, but can be a handy way to only defrost the exact amount of breastmilk you want at any given time. Our baby food and storage roundup also includes a silicone freezer tray if you want to go this route.
One of the easiest ways to defrost milk is by putting it in the fridge overnight. This works no matter how you've frozen your milk. If you are looking to more quickly defrost milk, or you are a busy mom and having to remember to do something the night before is a non-starter, then it depends on how the milk is frozen. If you stored it in a breast milk storage bag, we recommend quickly running it under lukewarm water (not hot!), dumping it into a glass or stainless steel container or bottle when it's still in icy chunks, and then defrosting and heating up the milk in a bottle warmer or under hot water. That way you can minimize any risk that heating up the plastic bag will release chemicals. For those using glass containers, we recommend a warm bath to defrost. Just keep in mind that it will take a bit longer for the middle to fully defrost. Lastly, if you've got your milk in frozen cubes, take the cubes out and put them in a glass or stainless steel bottle and then defrost in a warm bath. If the bottle you use to feed your child is plastic (no judgement if you just can't make stainless steel or glass work!), heating up the milk in a different glass or stainless steel container and then pouring it into the plastic bottle is a simple way to limit exposures to chemicals that could potentially leach from the bottle.