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  • Writer's pictureMegan Dunn


Some people notice different smells and tastes in their stored milk. It can remind them of soap, metal, fish, or something gone bad. Sometimes the milk even seems sour or spoiled. This makes them worry if it's still good for the baby.

Also, how the baby reacts varies. Sometimes they're totally fine with drinking milk that's been stored. Other times, they absolutely don't want it. And if the milk has actually gone bad, it's definitely not good for the baby.

So, it's like playing detective with the milk – you gotta trust your nose and maybe have a backup plan if things don't smell right. Making sure the baby gets good milk is super important. WHAT'S HAPPENING AND WHAT TO DO?

So, there could be a few reasons behind changes in the smell or taste of breast milk. What you can do to fix it depends on what's causing the problem. The first thing to do is figure out if the way you're pumping, handling, or storing the milk is the issue, or if it's because of something like strong lipase activity or chemical changes. That's the first step to finding the right solution. So, let's talk about how you're using the pump and taking care of things. It's really important to make sure you're being super careful with hygiene. After each time you pump, you need to clean all the parts really well. This makes sure there's no yucky bacteria hanging around to get into the milk. The Centers for Disease Control and Prevention (CDC) have step-by-step cleaning guides in both English and Spanish When it comes to storing breast milk, you should totally use containers made for this purpose. Lawrence and Lawrence suggest going for hard plastic containers made from something called polypropylene. This helps keep more of the good stuff in the milk, like nutrients and immune stuff, compared to other types of containers (check out page 719 for details).

If you can't find those polypropylene containers, Pyrex glass containers are the next good option. They're not perfect, but they're better than some other choices. Now, plastic bags for breast milk are also a thing, but they're not the best choice. They can mess with the milk's goodness and even break sometimes, so they're kind of a backup option. Okay, here's another thing: some folks have this thing called "excess lipase activity." It used to be thought that they have too much lipase in their milk, but new research says that's not quite true (Lawrence and Lawrence, page 137). So, your milk is actually just fine!

Lipase is this enzyme found in all kinds of milk. There are two types: lipoprotein lipase and bile salt-dependent lipase (you can dig deeper in Lawrence and Lawrence, 2016, pages 136-137). These enzymes are helpful for digestion and giving the baby some immune benefits. So, it's not a bad thing at all! Lipase activity:

  • supports an infant’s ability to digest fats (lipids; Lönnerdal and Atkinson, 1995, p. 361) by ensuring that the fat molecules remain well-mixed into the milk in a small, easily digestible form (Lawrence and Lawrence, pp. 136-137)

  • breaks down triglycerides to release fat-soluble nutrients (Lawrence and Lawrence, p. 137)

  • releases free fatty acids that provide immunological effects (antibacterial, antiviral, and antiprotozoan; Lawrence and Lawrence, p. 136)

  • protects against infection by intestinal parasites such as Giardia and Cryptosporidium (Lawrence & Lawrence, p. 463)

Now, let's dive into something called "high lipase activity." Basically, when there's too much of this lipase doing its job on the fats in your pumped milk, it can make the milk smell like soap or fish, and sometimes even taste that way. But guess what? Your baby might not be a fan of that.

Here's the thing: how fast this happens depends on your milk. Some folks notice the change in less than 12 hours, while others might not see much difference for a few days.

Even if your milk smells a bit like soap, don't worry, it's still totally safe and good for your baby. But if your baby's giving you the "nope" face, you can try mixing the soapy milk half and half with freshly pumped milk. That usually does the trick and the baby is cool with it, especially if the soap smell isn't too strong.

To avoid this happening in the future, just freeze your milk right away. That can help keep the soap smell at bay. Chemical Oxidation

Alright, let's talk about something trickier: "chemical oxidation." This is the third possible hiccup, and once it's happened, there's no going back. If your milk smells sour or rancid, it's likely because of chemical oxidation, not too much lipase action (you can explore more in Mohrbacher, page 461).

This one's a bit complex. There could be a bunch of things causing it. Like, if your diet has a lot of fats that aren't so great or even water with stuff like copper or iron, that can be part of the problem. The thing is, once this chemical change has happened, the milk can't be fixed.

And unlike the other issues, there's not much you can do once it's gone this route. But hey, now you know what to look out for and how to keep things on track in the future. Possible solutions include:

  • avoiding fish-oil or flax-seed supplements, anchovies, old vegetable oils (a smell or taste test can help you determine whether an oil has gone rancid), some nuts (Brazil nuts are especially likely to become rancid) or other foods that may contain rancid fats (Mohrbacher, p. 461; Vieira, McClements, and Decker, 2015, p. 313S – 315S).

  • drinking bottled water or water from a different source than usual to reduce the potential for iron or copper ions being a cause (Mohrbacher, p. 461).

  • increasing one’s antioxidant intake may help prevent this problem, so you might try including beta carotene and vitamin E in your diet (Mohrbacher, p. 461) to see whether it helps.

When chemical oxidation occurs, the milk is spoiled and must be discarded, and scalding the milk will only make the problem worse. However, the above dietary changes should help you resolve the problem completely without the extra work of heat-treating your milk! HOW CAN YOU FIND OUT WHICH PROBLEM—HIGH LIPASE ACTIVITY OR CHEMICAL OXIDATION—YOU HAVE?

So, let's do a little detective work to see what's up with your milk. Once you've confirmed that your pumping gear is squeaky clean, pump or express some milk and give it a good sniff and taste. Is it all sour-smelling or does it seem alright with a touch of sweetness? If it's got that sour vibe, it's a sign of rancid fats and chemical changes. To tackle this, think about changing up your diet to sort things out.

If it's smelling good and has a little sweet taste, pop it into the fridge. Over the next few hours, give it another smell and taste test. (By the way, you can safely keep milk in the fridge at around 39ºF/4ºC for up to 8 days, though sooner is better – more details in Mohrbacher, page 461). You can also freeze a small batch and then after about a week, check the smell and taste again. Most folks find that their milk stays just fine.

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