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Why does my breast milk smell sour?

As nursing moms we are dealing with breast milk every day! Have you ever thought, why does my breast milk smell sour? Have you left out some pumped milk and smelled it after a few hours and thought my breast milk smells bad or your breast milk may have a funny smell to it? Well, I have the answer for you on why you may have sour smelling breast milk.

Do you know what sour milk smells like? Yup, it’s pretty gross. It’s an awful smell. I first discovered my breast milk smelt sour about a few months into nursing. I had pumped a bottle and let it sit out for maybe 4 hours – much longer than I had done before. Breast milk is good to sit at room temperature for 6 hours so I didn’t think much of it. That’s when I noticed it had a funny smell when I went to get it. It smelt BAD. It smelt SOUR. I quickly did a recalculation on how long it had been out and it hadn’t exceeded the time limit. So what was up? Why does my breast milk smell sour?

Why does my breast milk smell sour? 

That’s when I did some research and googling and found that the likely cause was excess lipase in my milk. After that day though, I didn’t really think much of it and forgot about the issue. In fact, I completely forgot and passed it off as maybe a one-off fluke. I also never let my milk sit out that long so it never really came up.

Until recently. Until I started to thaw my first bag of milk from my freezer stash. I had previously been pumping two times a day in order to have milk to give in a sippy cup during the day. Our baby only would drink about a pump and half worth so there was always a bit left over. This is what I would store and freeze. I was saving it up to use in my last few weeks before returning to work so I could wean off of breastfeeding.

Here’s what my stash looked like – that’s over 100 ounces of breast milk stored!

Breast Milk Smells Sour

The first bag I opened I didn’t think anything of it. I defrosted it and put it in a sippy cup. Well at dinner our little guy almost gagged trying to drink it. I thought immediately what could be wrong? I took a smell and it was SOUR! I then thought, maybe it was because it was the oldest bag I had stored which was 2 months old. I tried the newest bag, not as bad but still a tinge. Then another, and another. All sour! So what was up?

So why does my breast milk smell sour? 

Well, turns out I do have excess lipase in my milk. Lipase is an enzyme in human milk that breaks down the milk fats so baby can easily digest it. Mothers have found that an excess of this enzyme can cause the milk to smell or taste sour or soapy, even though all storage guidelines have been followed. The milk is not harmful and some babies do not mind the taste while others reject it. I had no issues with storing my milk in the fridge and my baby would drink it, but once frozen it turned much more sour.

So what can you do if you have excess lipase? 

If your breast milks smells sour, fortunately you can salvage your milk and still use it if you plan on freezing it. First you must scald the milk. Heat the milk to about 180 F or until you see bubbles around the edge of the pan (Kelly Mom). Be sure not to let it go to a full boil. Then quickly cool the milk and store as desired.

So what will I do with the over 100 ounces of breast milk I have stored? I have contacted a local milk bank to see if they will take it. Unfortunately my local bank declined. However, some milk banks will accept this milk as it is often used for premature babies that do not reject it for the taste.

For more resources on why does your breast milk smell sour, visit Kelly Mom.

Looking to Increase Your Breast Milk Supply? 

Try our Oatmeal Raisin Lactation Cookies Recipe – it’s the BEST lactation cookie recipe you’ll ever try!

And to get into a routine for your baby, see our 4 month breastfeeding schedule and 6 month old feeding schedule.

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5 comments on “Why does my breast milk smell sour?”

  1. Thank you!! I ran into the same problem!! I thawed out some milk to mix with veggies for my 6 mo old, tasted a little and almost gagged. I then thawed bag after bag and was totally blown away and confused by how they all seemed to have gone sour! This post has been super enlightening.

  2. Oh my god… I am so happy you wrote this article… I had the same problem, glad to know I’m not alone. I too was very sad to see all that hard work go to waste. Thanks for the reassurance!

    1. Thanks for sharing Trista! I had never heard of this before and had not had any other moms tell me about it. Glad to know we are not alone!

  3. I read your article and the whole while I was thinking “yep this is/was me!”. I also followed guidelines when pumping/storing and had the exact same situation occur. I pumped twice a day (3/4oz a side each time) for over a month until I realized that every single one was sour upon thawing. My daughter would start to fuss and refuse the bottle only when I would use milk that had been frozen. I researched why this would occur and found out that it seems to happen with a lot of women, however not many in my local area seemed to have heard of it. I was so upset and did not want to throw any of my precious milk out, unfortunately the local milk bank would not accept it either so I ended up doing the unthinkable- threw it all out ๐Ÿ™ My LO is almost 9 months and I am still breastfeeding and pumping, but have to use all my pumped milk within a day and a half (stored in the fridge only) otherwise it spoils. It makes going out for a few hours without her difficult, but I chose to breastfeed and wouldn’t have it any other way ๐Ÿ™‚

    1. Melissa – thanks for sharing your story! It’s nice to know we are not alone in this and how sad that we both had to throw out all of our stored milk! I never had an issue with the pumped milk in the fridge either so I didn’t think much of my frozen stash until I went to go use it. I’m proud of you for sticking through it though and breastfeeding for over 9 months, that’s amazing! Thanks again for stopping by. ๐Ÿ™‚

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