Milk straight from the breast is often described as tasting sweet, like ice cream or bananas. Most breast milk retains a similar smell during storage. But sometimes parents become worried and/or shocked to find the smell has changed during storage. There are two causes for this change: high lipase and chemical oxidation.
Let’s look at the most well know cause first–high lipase. Breast milk contains an enzyme called lipase. It’s in all breast milk. The purpose of lipase is to break down fats to make the milk easier for your baby to digest. This is a good thing! However, sometimes a mother’s milk contains excess lipase which causes the fats to break down faster than normal. This causes the milk to have an unpleasant smell or taste, often described as soapy. It’s still safe to drink, but your baby may reject it.
The other cause of smelly milk is chemical oxidation. Milk that is effected in this way is often described as smelling/tasting metallic, or vomit-y. Chemical oxidation is caused by a reaction between breast milk and the presence of free copper or iron ions in the water source (hence the metallic description). It can also be caused by fats in the mother’s diet including polyunsaturated fats such as fish oil and flaxseed. Rarely, it may be caused my storage or defrosting. Just like milk that is high in lipase, this milk is safe to drink, but your baby may reject it.
If you are planning to store and use stored milk regularly, take time to perform a smell/taste test. Whatever it smells/tastes like freshly expressed is your baseline. Now store some milk in the refrigerator and freezer. Check the smell/taste in a few days. Has it changed? Don’t panic if it has. Go ahead and offer some to your little one. If they will accept, you might be safe to go ahead and store it. But if your baby rejects it, you will want to discover the underlying issue and address it.
Chapter 17 of Balancing Breast and Bottle is devoted to the topic of milk storage, and addresses both high lipase and chemical oxidation–how to detect it, and what to do about it. We hope you find it helpful!