This weekend my hubby wanted to give our son a bottle, so I thawed some milk that was pumped mid-February (2 months ago). When I was putting it in the bottle it smelled sour like spit-up. Why is that? He still ate it and it didn't make him sick. Is my milk bad? That was the oldest milk I have so I thought it would be fine still.
There are many reasons why your milk may smell sour.
What are you storing your milk in? Sometimes the breastmilk storage bags can let in smells from the freezer, which may make your milk smell less than desirable. Try storing in air-tight glass containers to see if this helps. Also, store your milk in the back of the freezer, rather than the door, so that the milk doesn't defrost as the freezer door stays open. For terrific information about milk storage guidelines, check out Mother and Child Health's article: Storage and Handling of Breastmilk.
Your milk may have excess lipase in it, which can cause stored breastmilk to smell sour or soapy. Lipase is a beneficial enzyme found in breast milk that helps break down milk fat. If your breastmilk has excess lipase, the longer the milk is stored, the stronger the smell becomes. The milk is totally safe for the baby to consume, but some babies may reject the milk because of its strong scent and change in flavor. To break the cycle of the excess lipase, you can scald your milk after you express it, so that you can store it and use it at a later date. Here are two great resources, one by Christina Williams and one by Simply Rebekah, about scalding breastmilk to get rid of excess lipase.
For a more detailed explanation of excess lipase, check out Kellymom's article: My Expressed Breastmilk Doesn't Smell Fresh.
Natalie, as long as you are following proper breastmilk storage guidelines, sour milk doesn't always equal spoiled milk. Hope these resources help!