You work hard to get milk expressed and stored for your baby; so hard that the idea of milk spoilage is horrifying. You certainly don’t want to waste anything by discarding milk that might not be bad after all. On the other hand, you absolutely, positively don’t want to give your little one milk that is sour. How can you tell?
Things that make you worry:
There are recommendations for milk storage in many places...and they may vary considerably. The reason there are no hard and fast rules is that there are so many variables that contribute to true milk spoilage: how much bacteria was picked up in the process of expressing the milk from pump parts and your skin; how long before it was refrigerated or frozen; how cold is your refrigerator or freezer; was it stored in the back of the refrigerator/freezer or on the door; how much did the temperature of the refrigerator/freezer vary from opening the door or defrosting cycles, etc.
This is what you need to know to make your own decision:
Smell the milk
Breastmilk which has truly spoiled will smell sour, similar to soured cow’s milk. (Have you ever taken a big bite of cereal and milk only to realize the milk was sour? Yeuuck!)
Keep in mind your breastmilk may not smell the same from time to time. Your milk will reflect the flavors of foods you have eaten.
Sometimes moms say their stored milk (frozen or not) has a soapy smell to it. The hypothesis is their milk has a high level of lipase. Lipase is an enzyme in breastmilk which helps emulsify fats, keep fat globules small for easy digestion and help break down the fats so fat soluble vitamins are made available for the baby. There is nothing harmful about a high level of lipase, but the stronger the soapy taste the more likely it is that the baby will refuse the milk.
This is not common, but if this describes your milk, there is a way to avoid the problem. When the milk is freshly pumped, bring it to a simmer (180°, just when you begin to see tiny bubbles around the edge of the pan) and then cool and store it. This will inactivate the lipase enough to stop the pre-digestion. Once milk has developed this smell, there is no way to salvage or reverse it.
A bottle the baby has fed from
What about that partial bottle left after Dad has fed the baby?
A bottle that is 100% expressed never-frozen breastmilk may be used twice before it must be discarded. So make sure to use it up first for the next feed, don’t mix it with more expressed milk. Thawed breastmilk has to be treated just like formula: if it's been used once, throw it out.
By the way, if your baby is getting some breastmilk and some formula, give the breastmilk first. That way you can be sure the baby gets every drop of the breastmilk. Any formula left over needs to be discarded. If the formula and breastmilk were mixed, you might end up having to dispose of some of your precious milk.
Lawrence, R, Lawrence, R. Breastfeeding: A guide for the Medical Profession, 6th ed. Philadelphia, PA: Mosby, 2005
Duke, C.S. New Beginnings, Vol. 15 No. 4, July-August 1998.