High Lipase Milk: Why Stored Milk Tastes Soapy and What to Do

You’ve built up a freezer stash, you thaw a bag, and it smells… like soap. Or metal. Or something you can’t quite name. Then your baby takes one sip and pushes the bottle away. If that sounds familiar, you’ve probably heard the phrase “high lipase milk.”

The good news: this milk isn’t spoiled, and there’s a lot you can try. Here’s what’s going on and how to work out a plan for your stash.

What “high lipase milk” means

Lipase is an enzyme found naturally in all breast milk. Its job is useful: it breaks fat into smaller pieces so your baby can digest it.

The theory behind “high lipase milk” is that lipase keeps working after you express your milk, even in the fridge or freezer. As it breaks down fat, it releases fatty acids that can smell or taste soapy, metallic or even rancid. Parents who notice this after a short time in the fridge or freezer often say they have “high lipase.” La Leche League GB describes the smell as appearing “after a short time in the fridge or freezer” and attributes it to lipase, and the ABM storage protocol says the odor likely comes from oxidation of fatty acids released by lipase-mediated fat breakdown.

The science is less clear than the label suggests. La Leche League International points to research that questions whether lipase is really the cause:

• In a 2019 study (Pitino et al.), researchers tested frozen milk that mothers’ own babies had refused, from 16 mothers. Lipase activity in the samples was at or below literature values for mature human milk, and free fatty acid levels and bacterial counts were not different from typical milk bank donations. The authors concluded that refusal “may not be entirely due” to lipase.
• La Leche League International also cites researcher Justin Silpe, who tested freshly expressed milk from multiple donors and found no clear pattern between donors’ reports of smell, taste or color changes in storage and the lipase levels measured in their milk. This work is unpublished, and he notes that more research and larger samples are needed.
• Exposure to air during pumping and storage can also oxidize unsaturated fatty acids in milk, which can change its smell.

So “high lipase milk” is best thought of as a handy name for stored milk that changes in smell or taste, rather than a diagnosis.

Is it safe?

La Leche League GB says this milk is completely safe and most babies will drink it. La Leche League International adds that there’s no evidence milk with these changes is unsafe, and that stored milk is generally considered safe if it was stored following recommended guidelines.

Soapy is not the same as spoiled, though. Throw milk out if:

• It looks stringy, foul or purulent (ABM Protocol #8 says to discard milk like this), or smells clearly sour or rotten to you
• It’s been stored longer than recommended (see below)
• Your baby drank from it and the leftovers have sat out past the usual limit

Storage times still apply

The CDC’s guidelines for freshly expressed milk:

Where	How long
Room temperature, 77°F (25°C) or colder	Up to 4 hours
Refrigerator	Up to 4 days
Freezer	Best within about 6 months; up to 12 months is acceptable

The CDC says fresh milk should be used or thrown away after 4 days in the fridge. Other groups differ a little (the NHS allows up to 8 days at 4°C or lower, and ABM Protocol #8 allows 5 to 8 days only under very clean conditions), but if your baby refuses stored milk, staying at the short end of the range is the cautious choice. Our breast milk storage calculator works out a use-by time for your milk, and our guide to how long breast milk lasts in the fridge covers the details.

How to test your milk at home

Before you change your whole routine, find out when (and whether) your milk changes. None of the sources we read give a standard home test, so this is a practical idea rather than a guideline. Keep all samples within the CDC storage times. A simple version:

1. Express a fresh batch and split it into several small labeled containers.
2. Put some in the fridge and some in the freezer.
3. Smell and taste a fridge sample at intervals, for example after 12 hours, 1 day, 2 days and 3 days.
4. Thaw a freezer sample after about 1 week, 2 weeks and a month, and check again.
5. Offer your baby a little from the samples that changed, to see whether they actually mind.

This tells you how long your milk stays “fresh-tasting,” so you can plan around it. Some babies don’t care at all, and the problem turns out to be the bottle, the nipple flow or a new caregiver instead. La Leche League International suggests checking those possibilities too.

What you can do if your baby refuses it

Shorten storage time

La Leche League says the longer milk is frozen, the greater the potential for changes in smell or taste, so keep milk in the freezer for the shortest time possible, and consider how much milk you really need in storage. It also says colder temperatures may slow changes, and that if you won’t use expressed milk within 24 to 48 hours, you can move it to the freezer as soon as possible. Store it in the coldest part of the freezer.

Mix it with fresh milk

La Leche League says some babies readily accept freshly expressed milk but not previously frozen milk, and suggests trying a mix of the two. Start with small amounts of the previously frozen milk, so you don’t have to discard much if your baby is reluctant, and gradually add more.

Thaw and warm gently

La Leche League suggests defrosting milk gradually, in the fridge overnight or by placing the closed bag or bottle in a cup of warm tap water, and warming it slowly in lukewarm water. The CDC says never to thaw or heat breast milk in a microwave. Our guides to thawing frozen breast milk and warming breast milk explain the safe methods.

Untested ideas

La Leche League quotes Dr. Ruth Lawrence that a few mothers have noticed improvement after lowering the pump’s pressure and speed, which is anecdotal and not researched, and says the suggestion to add alcohol-free vanilla extract or other flavors to milk has not been researched either. Treat these as experiments, and check with your baby’s doctor before adding anything to milk.

Scalding: what it is and the trade-offs

Scalding is the traditional fix for high lipase milk. The idea is to heat freshly expressed milk to deactivate lipase before you store it. La Leche League International says scalding is believed to slow lipase’s action. La Leche League GB describes it as heating fresh milk to scalding, meaning bubbling around the edges but not boiling, then cooling it and freezing it quickly.

There are real downsides, and sources disagree on whether it’s a good idea:

• Scalding reduces some of milk’s immune factors. La Leche League GB says it “may be the only option for some families.”
• La Leche League International notes that the Academy of Breastfeeding Medicine and other health organizations recommend against scalding because it may destroy other immunological factors in milk, that there are no clear guidelines on how to do it, and that removing lipase removes an enzyme that helps babies digest fat. It adds that if you have tried it and it works for your family, you may decide it is worthwhile. ABM Protocol #8 says heating milk above 104°F (40°C) to deactivate lipase is not advised because it may destroy many of the immunologically active factors.
• Scalding can’t rescue milk that already smells soapy. It has to be done before storage.
• La Leche League International says research shows lipase is not responsible for the changes in smell, so scalding may not help at all.

Because of this, try the other options first. If your baby refuses all of your stored milk and you’re considering scalding, talk it through with a lactation consultant (IBCLC) so you can weigh it against your situation, such as how much of your baby’s diet is pumped milk.

If you end up with a stash your baby won’t take, you may be able to donate it. The 2019 study above concluded that frozen milk refused by the mother’s own infant appeared suitable for human milk bank donation, and La Leche League notes that some parents donate unwanted milk to a local hospital milk bank. Ask a milk bank whether it can accept your milk, because banks have their own rules.

When to talk to a lactation consultant or doctor

Get help if:

• Your baby refuses most or all of your stored milk and you’re returning to work or relying on pumped milk
• You’re thinking about scalding your milk
• Your baby refuses the bottle even with fresh milk
• Your milk smells or looks unusual even when fresh, or you notice blood or an unexpected color (see our guide to breast milk color)

For everything else about keeping expressed milk safe, visit our breast milk storage hub.