Storing Human Milk

It is important to properly store your expressed (pumped) milk to maximize its nutritional, immunological and antioxidant qualities. Human milk has anti-bacterial properties that help it to stay fresh and when stored per recommendations. Over time nutrients may break down in expressed milk lowering the quality.

Prior to expressing and storing milk ALL nursing parents should:

• Wash their hands with soap and water before expressing;
• Use containers that have been washed in hot, soapy water, rinsed and air dried

MILK STORAGE GUIDELINES

This information is based on current research and applies to mothers who have healthy, full-term babies. Babies who are preterm/ in NICU/or ill should use more conservative guidelines.

General storage tips:

• All milk should be dated before storing. You can use a “first in, first out” rule and use the oldest milk first.
• Storing milk in 2-4 ounce (60 to 120 ml) amounts may reduce waste.
• Refrigerated milk has less fat loss and more anti-bacterial and protective properties than frozen milk.
• When combining milk expressed from different pumping sessions, ensure fresh milk is chilled in the refrigerator before adding it to previously expressed milk.

Preferably, human milk should be refrigerated or chilled right after it is expressed. Acceptable guidelines for storing human milk are as follows.
Are you wondering why these guidelines differ? Read more at Why Do Milk Storage Guidelines Differ, Nancy Mohrbacher, IBCLC 

Academy of Breastfeeding Medicine Human Milk Storage Guidelines for Full-Term Infants 

Read more about the ABM Milk Storage Guidelines Clinical Protocol #8.

CDC Human Milk Storage Guidelines 

Find more info at the CDC website.

STORAGE CONTAINERS

Bottles

• glass or hard-sided plastic containers with well-fitting tops
• avoid containers made with the chemical bisphenol A (BPA), identified with a number 3 or 7 in the recycling symbol. A safe alternative is polypropylene, which is soft, semi-cloudy, and has the number 5 recycling symbol and/or the letters PP. You can avoid the risks of plastic completely by using glass.
• containers which have been washed in hot, soapy, water, rinsed well, and allowed to air-dry before use or washed and dried in a dishwasher
• containers should not be filled to the top – leave space to allow the milk to expand as it freezes

Bags

• freezer milk bags that are designed for storing human milk
• squeeze out the air at the top before sealing, and allow room for the milk to expand when frozen
• stand/lay the bags in another container at the back of the refrigerator shelf or in the back of freezer where the temperature will remain the most consistently cold

Disposable bottle liners or plastic kitchen/snack bags are not recommended. With these, the risk of contamination is greater. These bags are less durable and tend to leak, and some types of plastic may destroy nutrients in milk.

REHEATING HUMAN MILK

Thawing From Frozen

There are several options when defrosting your milk:

• Thaw in the refrigerator overnight
• Run warm water over the sealed frozen container of milk
• Place frozen container in a container of warm water

Avoid boiling and microwaving as these methods will cause loss of nutritional properties of human milk and could unevenly heat the milk, making it dangerous for infants to drink (ABM, 2017).

Warming Refrigerated Milk

While many infants may be content drinking cold or room temperature milk, some may have a preference for warmer milk. It is recommended to warm milk slowly in lukewarm water to protect fat content and nutrients, for example holding it under a running faucet.

PUMPED MILK ISSUES

MILK ISSUES: SOAPY, METALLIC, SOUR, OR SPOILED MILK?

Being able to pump and store one’s own milk has been an enormous benefit for many who go back to work or school after their babies arrive. However, for a small group of people, there is an extra challenge: changes in the smell and possibly the taste of their milk after it has been stored.

Some people describe a “soapy” smell or taste in their milk after storage; others say it is a “metallic” or “fishy” or “rancid” odor. Some detect a “sour” or “spoiled” odor or taste. Accompanying these changes are concerns that the milk is no longer good for the baby. In addition, while sometimes the baby doesn’t seem to care and drinks a bottle of the expressed milk readily, other times the baby refuses to drink the milk. And sometimes, if it is actually spoiled, it isn’t good for the baby. So what could be happening?

WHAT IS GOING ON, AND WHAT CAN BE DONE?

There are several possibilities for why the smell or flavor has changed. Solutions depend on the root cause. The first step to finding a solution is to determine whether expression, handling and storage are the source of the issues, or whether highly active lipase or chemical oxidation are the cause.

Issues related to pump parts

First, look at pumping or expressing, handling, and storage practices. Pump parts should be cleaned thoroughly between pumping sessions to ensure no bacterial contamination is conveyed to freshly pumped milk. The Centers for Disease Control and Prevention (CDC) provide cleaning instructions in both English and Spanish. 

Use storage containers that are specifically designed for storage of human milk as described in “Storage Containers” above.

Excess Lipase Activity

Second, some people have excess lipase activity. Previously, it was believed that some people have excess lipase in their milk, but recent research has shown that to be a misunderstanding (Lawrence and Lawrence, p. 137). There’s nothing wrong with your milk!

Lipases are enzymes found in all milks. Two kinds are well known: lipoprotein lipase and bile salt-dependent lipase (Lawrence and Lawrence, 2016, pp. 136-137). Each has positive roles to play in digestion and immunological effects for the baby. Lipase activity:

1. 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)
2. breaks down triglycerides to release fat-soluble nutrients (Lawrence and Lawrence, p. 137)
3. releases free fatty acids that provide immunological effects (antibacterial, antiviral, and antiprotozoan; Lawrence and Lawrence, p. 136)
4. protects against infection by intestinal parasites such as Giardia and Cryptosporidium (Lawrence & Lawrence, p. 463)

However, when lipase activity is unusually high in expressed milk, its work in breaking down the fats can result in a soapy or fishy aroma and/or taste that may be rejected by the baby. The rate at which this occurs varies from one person’s milk to another’s; for some, the change in smell and taste occurs in less than 12 hours, while others find little or no change for up to a few days.

Even when a soapy smell is detected, the milk is still safe and nutritious for your baby. However, if your baby won’t take it, you might try mixing it half and half with freshly pumped or expressed milk. Often, babies are satisfied with the mixture if the soapy smell isn’t too strong. Sometimes mixing it won’t work, so let’s talk about how to prevent the problem!

You can prevent the problem from occurring with future batches of pumped or expressed milk—no one wants to lose all the milk they’ve worked so hard to produce! Freezing will not prevent lipase activity from altering the aroma or taste of your milk, but scalding will.

Here are the steps to de-activate lipase activity:

1. Place the milk in a clean pan over low heat on the stove (not a microwave).
2. Heat the milk just to the point that it is bubbling around the edges (simmering) but not boiling, as boiling will reduce or destroy valuable immunologic properties (Lawrence and Lawrence, pp. 162, 721-722).
3. Place the pan in a larger bowl filled with ice water (use plenty of ice) to cool it quickly.
4. Store the milk in the fridge or freezer as needed.

Chemical Oxidation

This third possible problem may be more challenging to resolve, and the milk is not salvageable once it has occurred. If the smell is sour or rancid, milk storage experts Jones and Tully suggest the cause is chemical oxidation rather than excess lipase activity (Mohrbacher, p. 461). Several factors may contribute to this problem, including a diet containing polyunsaturated fats or rancid fats, or drinking water with free copper or iron ions.

Possible solutions include:

1. 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).
2. 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).
3. 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 should not be consumer by your baby. In the case of chemical oxidation, 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?

Assuming that you’ve made sure your pump equipment is clean, after pumping or expressing some milk, smell it and taste it. Does it smell or taste sour, or does it smell fine and taste a bit sweet? If it does smell or taste sour, then it indicates the presence of rancid fats and chemical oxidation. Try changing your diet to eliminate the problem.

If it smells fine and tastes a bit sweet, put it in the refrigerator. Every few hours, do another smell and taste check. You can also freeze a test batch, and conduct a smell and taste check after about a week. Most people find that their milk continues to be fine. However, if you’re one of the few who find that your milk begins to smell or taste soapy, fishy, or metallic after a period of time (which can range from a few hours to 24 hours or even longer periods), then you will need to follow the instructions for scalding your milk to de-activate the lipase and prevent future changes in the milk.

If your milk turns quickly (within a few hours) and you’re pumping at work, you will want to figure out a way to scald it before you put it in the cooler to take home. If it seems to take longer for a soapy smell or taste to develop, then waiting until you get home to scald it may be fine. You might conduct a smell-and-taste test as described above on your first batch to be sure if you haven’t already done so.

COMMON QUESTIONS

Does freezing milk ruin it? 

No, frozen human milk retains many of its unique qualities and remains an ideal source of infant nutrition.
From the Academy of Breastfeeding Medicine: “Human milk is a fresh, living food with many antioxidant, antibacterial, prebiotic, probiotic, and immune-boosting properties in addition to nutrients. Although some of these nutrients and health properties change with storage, there is good evidence that human milk storage can be safe, allowing provision of optimal nutrition to the child when breastfeeding or immediately expressed milk is not available. When direct breastfeeding is not possible, stored human milk maintains unique qualities, such that it continues to be the gold standard for infant feeding.”  Read more about the ABM Milk Storage Guidelines Clinical Protocol #8. 

Why does my milk smell or taste soapy?

Sometimes thawed milk may smell or taste soapy. This is due to an enzyme in milk known as lipase (Newman & Pitman, 2014). The milk is safe and most babies will still drink it. If there is a rancid smell from high lipase when the milk has been chilled or frozen, the milk can be heated to scalding (bubbles around the edges, not boiling) after expression, then quickly cooled and frozen. This deactivates the lipase enzyme. Scalded milk is still a healthier choice than commercial infant formula.

Why is my milk separating?

Human milk naturally separates into a milk layer and a cream top when it is stored. This is normal. It is safe to shake or swirl the milk to combine the cream prior to feeding.

Is it safe to refreeze my milk after thawing?

Previously frozen milk that has been thawed can be kept in the refrigerator for up to 24 hours (Lawrence & Lawrence, 2010). There is currently limited research that supports the safety of refreezing breastmilk as this may introduce further breakdown of nutrients and increases the risk of bacterial growth. At this time, the accepted practice is not to refreeze milk that has completely thawed (no ice crystals present).

Can I reuse previously fed milk if my baby does not finish?

If baby does not finish the bottle during a feed, the recommendation is milk may be reused within 1-2 hours. After this time frame, it should be discarded to avoid transfer of bacteria from baby’s mouth to bottle (ABM, 2017). Many families find that storing milk in smaller quantities can help reduce waste if baby does not finish the bottle.

Is it safe to store my milk in a shared refrigerator?

Expressed milk can be kept in a common refrigerator at the workplace or in a day care center. The US Centers for Disease Control and the US Occupational Safety and Health Administration agree that human milk is not among the body fluids that require special handling or storage in a separate container (CDC, 2015).

I have thrush; is my milk safe?  

If you or your baby has a thrush or yeast/fungus infection, continue to breastfeed during the outbreak and treatment. While being treated, you can continue to express your milk and give it to your baby. Label any milk stored while you or baby is undergoing treatment. Be aware that refrigerating or freezing milk does not kill yeast, so offering that milk after treatment may result in another bout of thrush.

What should I do with my pumped milk in an emergency? 

Read more about emergency milk storage here.

REFERENCES

Academy of Breastfeeding Medicine. (2017) Clinical Protocol Number #8: Human Milk Storage Information for Home Use for Healthy Full Term Infants.  Breastfeeding Medicine, 12(7), 390-395.

Centers for Disease Control and Prevention (CDC). (2015). Breastfeeding: Frequently Asked Questions.

Centers for Disease Control and Prevention (CDC). (2017). Proper Handling and Storage of Human Milk.

Lawrence, R.A. & Lawrence, R.M. (2010). Breastfeeding: A Guide for the Medical Profession (7th ed.). Elsevier Mosby, Philadelphia

Mohrbacher, N. (2010). Breastfeeding Answers Made Simple. Texas: Hale Publishing.

Newman, J. & Pitman, T. (2014). Dr. Jack Newman’s Guide to Breastfeeding, Revised Edition. UK: Pinter & Martin, Ltd.

RESOURCES

How to Keep your Breast Pump Kit Clean, CDC

Pumping Milk, LLL USA

Read more about Working and Breastfeeding, LLL USA

Breastfeeding During A Hurricane, LLL USA

Why Do Milk Storage Guidelines Differ, Nancy Mohrbacher, IBCLC

Cleaning and Sanitizing Pumping Accessories, LLLI

Hand Expressing, LLLI

PERSONAL STORIES

Making It Work: Why I Avoid the Freezer Stash, LLL USA blog

Rachel’s Story: Expressing My Milk, LLL USA blog

IS YOUR CONCERN OR QUESTION NOT COVERED HERE?

Please contact a local LLL Leader with your specific questions.

Medical questions and legal questions should be directed to appropriate health care and legal professionals.

Page updated January 2020

Resource partially adapted from LLLI materials.