30/07/2026
“My frozen milk smells weird. Is it high lipase?”
Sometimes it is. But many times, it’s actually oxidation, which is a completely different process.
Understanding the difference can save you from unnecessary stress and from scalding milk that doesn’t need to be scalded.
HIGH LIPASE
Lipase is an enzyme that is naturally present in all human milk. Its job is to break down fat into smaller fatty acids, making the fat easier for babies to digest. Lipase is a good thing. It helps babies absorb nutrients and efficiently use the fat in breast milk. Everyone has a unique amount of lipase. Remember, your body is expecting your milk to go directly to your baby’s stomach to be digested, not in a fridge or freezer for any length of time
Some mothers notice that after milk has been refrigerated or frozen, it develops a soapy, metallic, or even slightly rancid smell or taste. This happens because the lipase has continued breaking down milk fat during storage. Despite the smell, the milk is still safe and nutritious. Some babies drink it without any issue. Others refuse it.
What can help?
• Offer freshly pumped milk whenever possible if your baby refuses stored milk. Sometimes moms can find the line for how long they can store it before the taste turns. For some that is hours and others a few days.
• Try mixing stored milk with fresh milk, gradually increasing the proportion of stored milk over time.
• Rotate your freezer stash so milk isn’t stored longer than necessary. ALWAYS CHECK YOUR MILK BEFORE CREATING A HUGE STASH!!!!! I’ve worked with too many devastated moms to not warn y’all.
• If your baby consistently refuses milk because of the taste, you may choose to scald freshly expressed milk before freezing. Heating the milk to about 180°F (82°C) (just until tiny bubbles form around the edge of the pan, not a full rolling boil) deactivates most of the lipase enzyme. Cool it quickly before refrigerating or freezing. This is old school advise that can work for some
Remember: scalding is only necessary if your baby refuses the milk. If your baby happily drinks stored milk, there is no reason to scald it.
OXIDATION
Oxidation is not caused by lipase. Instead, oxidation is a chemical reaction that occurs when components of breast milk interact with oxygen or certain minerals, particularly copper or iron. Water quality, mineral content, some vitamin supplements, and even trace metals in pipes or cookware may contribute.
Oxidized milk often develops a smell that parents describe as:
• Metallic
• Rusty
• Penny-like
• Sour
• Rancid
Unlike high lipase, oxidation is not caused by an enzyme breaking down fat. I’ve seen this in my moms who take a lot of fish oil or flax seed oil supplements, when taking certain herbs or medications.
What can help?
• Chill milk promptly after pumping and freeze milk as soon as practical if you don’t plan to use it within a few days.
• Avoid prolonged exposure to air by storing milk in appropriately sized containers with minimal excess air. Try to get large air bubbles out of milk storage bags. Be aware of open bottles in the fridge that are being exposed to other food flavors inside. Breast milk can absorb the odors of other things in the fridge
• If you’re washing pump parts by hand, rinse thoroughly to remove soap residue.
• If oxidation continues to be a problem, consider whether your water supply has high mineral content. Some families notice improvement after using filtered water to wash pump parts and bottles. Analyze any supplements that could be contributing. I’ve had a number of moms stop taking their fish oil supplements and it fixes the issue
• Avoid storing milk in containers that are damaged or scratched.
Can we tell the difference between lipase and oxidation? Not always. Both can produce unusual odors, and the smell alone isn’t always enough to distinguish one from the other.
Is the milk still safe?
In most cases, yes. If milk has been stored according to safe storage guidelines and simply smells different because of lipase or oxidation, it is generally still safe for a healthy, full-term infant.
The challenge isn’t usually safety, it’s whether your baby accepts the taste.
If the milk smells spoiled because it was improperly stored or has exceeded recommended storage times, it should be discarded. Taste your milk. Would you drink it? Don’t make your baby drink something you wouldn’t touch
Breast milk is remarkably resilient. A change in smell doesn’t erase all of the incredible nutrition, immune protection, and biologically active components you’ve worked so hard to provide. Whenever possible, feed fresh. It’s what your body and your baby biologically are expecting
