Medically reviewed by Stephanie Nguyen, WHNP, IBCLC. Last reviewed October 2026.
You thaw a bag of milk you worked hard for, and it smells like soap. Or metal. Or it just smells off, and your baby turns away from the bottle. Take a breath, mama. This is one of the most common questions we hear at Modern Milk, and in most cases your milk is still safe and nutritious.
The short answer
If your milk was pumped and stored safely, a soapy or metallic smell is a change in the fat in your milk, not spoilage. It is commonly called “high lipase” milk. It is safe for your baby to drink. The only real problem is that some babies do not like the taste.
What is lipase?
Lipase is an enzyme found in everyone’s breastmilk. It breaks fat down into smaller pieces so your baby can digest it and absorb fat-soluble vitamins. It is supposed to be there.
When milk sits in the fridge or freezer, lipase keeps working. As it breaks down fat, it releases fatty acids that can smell and taste soapy. Home freezers do not get cold enough to stop this, and freezing and thawing can make fat easier for lipase to reach.
Is “high lipase” really the cause?
Here is what is new. For years moms were told they make “too much” lipase. When researchers compared sour-smelling samples with normal ones, they did not find higher lipase levels. So the taste change is real, but “excess lipase” has not been proven as the reason some milk changes faster than others.
There is also a second cause: oxidation. Fats in milk can react with oxygen, which tends to cause a metallic, fishy or rancid smell rather than a soapy one. Things that may play a part include polyunsaturated fat supplements such as fish oil or flaxseed, and iron or copper in tap water.
In real life the two overlap, and the fixes are similar. A rough guide:
| Smell or taste | More likely | First thing to try |
|---|---|---|
| Soapy | Fat breakdown by lipase | Shorter storage time, mixing with fresh milk |
| Metallic, fishy, rancid | Oxidation | Review supplements and water, limit air in storage bags |
| Truly sour, like spoiled cow’s milk | Possible spoilage | Check storage times and pump hygiene; when in doubt, throw it out |
7 things to try
- Test before you build a stash. Freeze two or three small portions. Thaw one after a few days, one at a week and one at two weeks, then smell and taste. You will learn how long your milk keeps its normal flavor.
- Offer it anyway. Many babies drink it without complaint. If yours does, there is nothing to fix.
- Mix it with fresh milk. Start with half thawed and half fresh. If baby accepts it, slowly increase the thawed share.
- Shorten the timeline. Freeze milk soon after pumping rather than after days in the fridge, use the oldest milk first, and thaw in the refrigerator. Warmth speeds up these changes.
- Change the temperature. Some babies take affected milk cold or slightly chilled, when the flavor is less noticeable.
- Look at oxidation triggers. If the smell is metallic or fishy, ask your provider about pausing fish oil or flaxseed supplements, and try filtered or bottled water for a couple of weeks. Squeeze extra air out of storage bags. Results are mixed, but it is low-risk to try.
- Scald future milk, as a last resort. Heat freshly pumped milk in a clean pan to about 180°F, when small bubbles form around the edge but it is not boiling. Cool it quickly in an ice bath, then store. This stops lipase.
What you should know about scalding
Scalding only works on fresh milk. It will not fix milk that already tastes soapy. Heating also reduces some of the protective, immune-supporting parts of your milk. Scalded milk is still good food for your baby, but we suggest it only if your baby flatly refuses stored milk and the gentler options have not worked. It is best for healthy, full-term babies; if your baby is premature or medically fragile, talk with your care team first.
What about the milk already in my freezer?
Please do not pour it out. Try mixing it with fresh milk, offer it cold, or stir it into solid foods once your baby is eating them. If your baby still will not take it, some milk banks accept this milk. It also works well for milk baths.
Frequently asked questions
Is soapy tasting breastmilk safe? Yes. If it was collected and stored within safe guidelines, it is safe and nutritious.
How do I know if it is high lipase or spoiled milk? Soapy or metallic milk that was stored correctly is a taste change. Milk that smells sharply sour or rancid, or that was left out too long, should be discarded.
Does freezing stop lipase? No. Lipase stays active in a home freezer.
Can I fix milk that already tastes soapy? You cannot reverse it, but mixing with fresh milk often makes it acceptable.
Will this happen with every pumping session? Usually it is consistent for you, but the timing can shift with diet and storage. Retest every few months.
How long can I store breastmilk? Up to 4 hours at room temperature, up to 4 days in the refrigerator, and about 6 months in the freezer (up to 12 is acceptable).
We are here to help
If your baby is refusing bottles and you are heading back to work, you do not have to figure this out alone. Our IBCLCs offer in-person consults in Scottsdale and virtual consults anywhere. Book a lactation consult.
Want the full picture on pumping, storing and feeding? Our online course, Mama’s Milk Mastery, walks you through it step by step. Check out Mama’s Milk Mastery.
Stocking up on storage bags or bottles? Visit our Scottsdale shop or browse online. Shop storage and feeding.
Sources
- CDC: Breast Milk Storage and Preparation
- Breastfeeding Support: My Breast Milk Smells Soapy or Sour (research summary, updated April 2025)
- Academy of Breastfeeding Medicine clinical protocols, Protocol #8: Human Milk Storage
This article is for education and does not replace care from your own provider.
