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All About Breast Milk


Mother's milk is precious to both you and your baby. It represents commitment on your part and ideal nourishment for your baby. Handle your milk with care. The same immune properties in your milk that protect your baby also help protect the milk from bacteria growth while it sits on the refrigerator shelf.

What kind of container should I use to store my milk?

The options include hard or soft containers, with several choices under each category. Each has advantages and drawbacks.

  • Soft containers are plastic bags.
  • Hard containers are made of plastic or glass.

    What kind to use comes down to two issues:

  • protecting the milk and all its valuable components
  • your convenience and that of the baby's caregivers.

Unfortunately, there is not a lot of research about how storage containers affect human milk. One study showed that the leukocytes in milk (the live cells that transfer immunity from you to your baby) sticks to the side of glass containers, but subsequent research showed greater numbers of leukocytes in glass containers than in plastic, as the cells were released from the sides of the containers over time. Research has also shown a loss of antibodies and fat in milk that is stored in plastic bags, but this information applies only to disposable plastic nurser bags, the thin ones you can buy at most stores to use with baby bottles. If you do choose to store your milk in these, use two bags to protect against breakage and "freezer burn." Use twist ties to close the bags.

Plastic bags specially designed for freezing expressed human milk are available from many companies that specialize in products for breastfeeding mothers and babies. These bags are sturdier than those used in baby bottles and have self-closures that are easier to seal and label. They do a better job of protecting milk components than nurser bags. Some types can be attached directly to your pump.

The information currently available suggests that glass or hard-sided plastic containers (the kind of plastic that is clear, not cloudy) provide the best protection for nutrients and immunities. Hard containers should have secure, one-piece tops. If your baby is getting a lot of his nourishment directly at the breast, you don't need to be as concerned about nutrient loss through freezing and contact with storage containers as you do if your baby is getting only expressed milk and not nursing directly at the breast.

Convenience is another issue, and opinions will vary. Plastic bags take up less room in the freezer and are one-use items, so there's no dishwashing involved. However, filling them and pouring milk out of them can be awkward.

How should I wash containers that will hold milk? Do I need to sterilize them?

When you are pumping milk for a full-term, healthy baby, you do not need to worry about sterilizing storage containers or pump parts. Wash your storage containers in hot soapy water, and wash your hands thoroughly with soap and water before you pump. Check the manufacturer's instructions for information on washing parts of the pump. Storage containers and parts of some pumps can be washed in a dishwasher.

Mothers who are pumping milk for a sick or hospitalized baby will need to be more careful about milk handling and sterilization procedures.

Freezer or refrigerator? How quickly does human milk spoil?

Freezing destroys some of the immune properties in human milk, so it's best if your baby is given fresh milk--milk that has been expressed and then refrigerated. The chart in Storage Times for Human Milk tells you how long you can keep expressed milk at room temperature, in the refrigerator, or in the freezer. (Print this file and keep it on your refrigerator door!)

Label each container with the date, so that you can use the oldest milk first and avoid needless waste.

How much should I store in each container?

Store your milk in small amounts, about two ounces in each container, at least at first. (If you're pumping milk for a premature baby, you may want to store it in even smaller amounts.) Breastfed babies take smaller amounts of milk at each feeding than do formula-fed infants, and smaller amounts are also quicker to thaw. Milk left in a bottle after a feeding can be saved until the next feeding, but after that it should be discarded, and you don't want to waste expressed milk. Eventually, you may decide to put more milk in each bottle, based on your caregiver's report on how much your baby takes at each feeding.

You can add more milk to already-frozen milk, but cool the added milk in the refrigerator first. There should be less added milk than already-frozen milk.

Always leave about an inch of space at the top of the container to allow for expansion. Just like water for ice cubes, human milk expands when you freeze it. Hard containers will pop open as the milk expands. Bags will break. Squeeze out the air at the top of the bag and fasten it an inch above the milk.

How do I safely store my milk?

  • Bags containing human milk should be placed inside another container in the refrigerator or freezer. This makes for better protection and easier handling.
  • If you lay the bags down in a container in the freezer, you'll get flatter packages that will thaw more quickly. (But be sure they're sealed well, or you'll have a leaky mess.)
  • You might want to keep all your hard containers of milk together in the freezer or refrigerator inside a larger plastic box.

How long can I keep the stored milk?

Amazingly, research has found that human milk stored in the refrigerator for eight days actually has lower bacterial levels than freshly expressed milk. For more details on recommended storage times, see Storage Times for Human Milk.

Since human milk can be kept in the refrigerator for up to eight days, it may be possible to provide your baby with fresh, not frozen, milk most of the time. This insures that your baby gets the maximum amount of nutrients and immunities. Instruct your caregiver to use the oldest milk first and keep the supply rotating.

Previously frozen milk can be kept in the refrigerator for 24 hours after thawing. This means that you or the baby's caregiver can thaw milk for all of your baby's feedings at one time, or you can thaw the milk in the refrigerator overnight. This can make it faster to prepare a bottle when your baby is hungry. Milk that has thawed should not be refrozen.

How do I safely transport my milk?

Research shows that bacteria do not grow readily in human milk, and that it can be kept safely at room temperature for 4 to 6 hours or more (see Storage times for human milk). So you don't need to worry if you can't rush your milk to the refrigerator right after you express. But it still makes sense to refrigerate the milk as soon as possible and to keep it cool when taking it home or to the sitter's. If you have refrigerator space available at work, you can store your milk there until the workday is over, or use an insulated container with reusable carry-ice to keep it cool.

Heat can destroy human milk's enzymes, immune properties, and other valuable components, so the milk requires gentle care before it is served to baby. Follow these guidelines:
  • Defrost milk by holding it under warm running water.  
  • Or, place the container of milk in a bowl of warm water on the kitchen counter. As the water cools, replace it with more warm water until the milk is thawed and warmed to body temperature.  
  • Do not heat expressed human milk on top of the stove. It's too easy to overheat it this way. Do not boil!  
  • Do not heat expressed human milk in a microwave oven. Even if the overall temperature of the milk stays below body temperature, there may be "hot spots" where the milk is overheated and some of its beneficial properties are destroyed. The uneven heating can also be dangerous when the bottle is given to baby.  
  • Human milk, like any milk that is not processed or homogenized, tends to separate when stored. The cream rises to the top. Swirl the bottle gently to mix the layers.  
  • Human milk has a thin, bluish look to it, quite different from either homogenized cow's milk or the grayish color of infant formula. Your baby's caregiver may need reassurance that this is normal.

These guidelines are for mothers who are expressing milk for a full-term healthy baby. Use clean containers, and wash your hands with soap and water before expressing. or pumping. When providing milk for a baby who is seriously ill and/or hospitalized, check with healthcare providers for instructions.

Where stored Storage temperature
(degrees Fahrenheit)
Storage temperature
(degrees Centigrade)
How long
At room temperature 60 degrees F 15 degrees C 24 hours
At room temperature 66-72 degrees F 19-22 degrees C 10 hours
At room temperature 79 degrees F 25 degrees C 4-6 hours
In a refrigerator 32-39 degrees F 0-4 degrees C 8 days
In a freezer compartment inside a refrigerator 2 weeks
In a self-contained freezer unit of a refrigerator 3-4 months
In a separate deep freeze with a constant temperature 0 degrees F -19 degrees C 6 months or longer

Type of Milk Save or Dump? Why
Milk remaining in the bottle that has been offered to baby Use for next feeding, otherwise discard. Bacteria from the baby's mouth may have entered the milk during the feeding. This may lead to bacterial contamination if it sets too long (though as yet there is no research available).
Milk that has been thawed Save in the refrigerator for 24 hours after thawing, then discard. Do not refreeze. Milk that has been frozen has lost some of the immune properties that inhibit bacterial growth in fresh refrigerated milk.
Milk that has been kept in the refrigerator for eight days Transfer to storage in the freezer, or discard. Bacterial growth is not a problem, but milk sometimes picks up odors or flavors from the refrigerator or the container.

How to Ship Human Breast Milk

Please adhere to the following suggestions when shipping human breast milk

1.) Call FedEx at 800-463-3339 to request a package pick-up and ask for their arrival time.  If you can request a late pick-up, the milk will stay frozen longer.  Pack your milk as close to pick-up time as possible.  Pack only milk that is frozen solid.  If it is convenient for you to take the cooler to a FedEx drop-off site, you will help save money.

2.) If you have a Styrofoam cooler in a cardboard box, you should leave the cooler in that box when you ship your milk.

3.) Put some crumpled newspaper in the bottom of the cooler and on all sides so the milk containers do not touch the sides of the cooler.  Pack all of the milk close together in the center of the cooler.  Put some more crumbled newspaper on top to fill all the space in the cooler.  The newspaper will insulate your milk.  The more milk you ship at one time, the longer it will stay frozen.

4.) Do not put any ice in the cooler. Ice is warmer than frozen milk, and it will cause the milk to start thawing. You may consider dry ice if concerned about the time the milk will be in route between your freezer and the recipients home.

5.) It is suggested that you use a Priority Overnight stamp.  Make sure your name appears on the label.  Put the FedEx label on the lid of the cooler.  Tape a red "Frozen Milk" label next to it.  Use three to four pieces of packing tape to tape the cooler shut.

6.) Call the your recipients and inform them of the FedEx tracking number on the top of the slip.  This will tell them when to expect the arrival of your milk and enable the recipients to locate it if it doesn't arrive.

7.) It is suggested that you only ship milk Monday through Thursday to ensure that someone will be in the area to receive the package when it arrives.

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