Here is everything you need to know about freezing and defrosting breast milk.
The word natural is often used to describe breastfeeding, and in many ways, it certainly is. Some women quickly discover, however, that just because something is natural does not mean that it is easy.

Breastfeeding can be one of the most challenging aspects of becoming a new mother.

Moreover, just when you thought you'd got the breastfeeding thing down, pumping breast milk is even more complicated. Any mom who pumps her breast milk will tell you that it sometimes feels anything but natural. Instead of producing oxytocin as you gaze down at your adorable baby, you have a top-down view of your nipples being sucked into plastic breast shields. Do I have the right size flange? How did I get milk into the tubing? Why am I only getting an ounce every time I pump?

The questions don't end when the pumping is over. In fact, that is where the majority of the issues arise. The rules around breast milk storage are complex. From how long can breast milk last in the fridge to what are the rules around freezing breast milk, there could be an entire college course covering the ins and outs of breast milk storage.

As more and more women return to work after having babies, securing an ample stash of milk in the freezer becomes a must. In addition to learning how to best freeze, store, and thaw frozen breast milk, many women also wonder about the impact that freezing milk has on the nutritional value of their precious liquid gold.

Does Frozen Breast Milk Retain Its Nutritional Value?

As with all food in life, the closer that it is consumed to its original source, the more of a nutritional punch that it packs. This is undoubtedly the case for breast milk. Breast milk is a complete and ideal food for infants, containing all of the fat, carbohydrates, protein, and nutrients needed at any given time. While frozen breast milk remains a healthier choice than formula for your infant, there are some downsides to freezing milk that cannot be ignored.

Breast milk is extremely dynamic. Not only does the composition of breast milk change during a breastfeed, but it also changes throughout the course of a day. A 2009 study found that components of breast milk change every 24 hours to meet the various needs of a baby.

"Milk is so incredibly dynamic," says Dr. Katie Hinde, "There are hormones in breast milk, and they reflect the hormones in the mother's circulation. The ones that help facilitate sleep or waking up are present in your milk. And day milk is going to have a completely different hormonal milieu than night milk."

A 2011 Israeli study confirmed these findings, illustrating that melatonin, a natural hormone that regulates the sleep-wake cycle, follows the circadian rhythm and is released in higher quantities in breast milk at night. Further, breast milk has been shown to have a higher fat content in the morning, and higher protein content in the evenings. There is, for reasons unknown, more lactose in breast milk in the late afternoon.

Therefore, it's safe to say that the milk produced by a mother is exactly what the infant needs at that given day and time. While most women date their breast milk upon freezing, very few consider recording the time that it was pumped. Some experts recommend feeding an infant milk that was pumped around the same time of day that it was expressed, thus ensuring that the milk received mimics the milk produced at that time of day.

Not only does breast milk change over the course the course of the day, but the composition changes over time as well. As an infant grows, fat and calorie content of the milk changes to meet the needs of the baby. The milk that you pumped when your baby was four months old, might not be exactly what your baby sees when served at eight months old.

Related: Milk Composition Changes with Mother's Circadian Rhythm

Further, when an infant or mother becomes sick, the composition of a mother's breast milk changes to adapt to needs of the infant. Researchers believe that when an infant is exposed to illness, a cue is sent via the infant saliva through the breast, signaling the mother to produce more illness-specific antibodies.

Research has shown that specific immune components, enzymes, and proteins are decreased when breast milk is frozen. A 2012 study found that human milk, when frozen and then thawed, saw up to a 9% reduction in fat content. Further, the vitamin content of breast milk decreases when breast milk is frozen, with Vitamin C deteriorating the most. Surprisingly, there is has been only one study examining the quality of human milk after six months of freezing.

Given the current research, it's safe to say feeding a baby straight from the breast provides the most significant advantage. While it's perfectly safe, and even very nutritious, to give an infant previously frozen milk, you lose the benefits of the many dynamic ways that breast milk adjusts to the needs of each infant over time. Still, when compared to formula, which does not adapt whatsoever to the changing needs of an infant, any form of breast milk is an excellent choice.

What Is the Best Way to Freeze Breast Milk?

Whether you are hoping to build a milk supply to return to work or simply to have an extra stash on hand in case of an emergency, freezing breast milk is the only way to preserve your milk for the long term.

Ideally, pumped breast milk should make it to your freezer within 24 hours of expression, although refrigerated breast milk can be frozen within four days. While some women choose to use glass or plastic containers to freeze their milk, most decide to utilize freezer bags made specifically for breast milk.

If lying the milk storage bags flat, they can easily be stacked in the freezer, saving valuable room for the much-needed ice cream. Some women choose to freeze their breast milk in ice cube trays, which allows for easy measurement. Either way, be sure to freeze in small batches of no more than four ounces. It's much less painstaking to thaw out more milk than to throw away unused portions.

Be certain to date and time all of your breast milk before storing it, leaving ample room at the top for expansion of the milk as it freezes. While there are various milk organizing systems, find one that's right for you. Some mothers store several individual bags of expressed milk into one larger Ziploc bag, recording the dates on the front of the pack.

Some women wonder if they can add newly expressed milk to frozen milk. Milk should only be combined if it was pumped in the same day. Further, the freshly expressed milk needs to be completely cooled in the refrigerator before being added to the frozen milk. Adding warm milk to the frozen milk will risk thawing the previously frozen milk.

According to the CDC, breast milk can be stored safely in the freezer for up to one year, although it's recommended that it be consumed within six months of storage. The CDC recommends avoiding using the door of the freezer for storage, as that area is subject to frequent temperature changes from opening and closing the door.

Here is everything you need to know about freezing and defrosting breast milk.
What Is the Best Way to Thaw Breast Milk?

When removing breast milk from the freezer, always thaw the oldest milk first. The CDC recommends remembering "First in, first out" when it comes to thawing breast milk.

Anytime you freeze or heat a product, there is a natural loss of nutrients. Further, slower thawing decreases the amount of fat lost from the milk. Therefore, it's best to thaw breast milk out slowly in the refrigerator. However, this process can take up to 12 hours, so planning ahead is required. Some women remove the breast milk for the next day before going to bed at night.

For quicker thawing, Kellymom recommends holding the container under cool running water and gradually increasing the temperature. Never utilize the microwave or a stove to heat breast milk. Not only does this decrease some of the healthy nutrients, but it can also create dangerous hot patches that can burn the mouth of the baby.

Can I Re-Freeze Previously Frozen Breast Milk?

The opinions surrounding the re-freezing of breast milk are varied. The CDC recommends that thawed breast milk should be consumed within 24 hours of the time that it is no longer frozen. Under no circumstances, says the CDC, should one refreeze breast milk once it has been thawed.

According to the Academy of Breastfeeding Medicine's 2017 protocol, "there is little information on refreezing thawed human milk. Bacterial growth and loss of antibacterial activity in thawed milk will vary depending on the technique of milk thawing, duration of the thaw, and the number of bacteria in the milk at the time of expression. At this time no recommendations can be made on the refreezing of thawed human milk."

It's abundantly clear that more research is needed surrounding the topic of refreezing breast milk. It's safe to say that if there is any doubt, consulting with your baby's pediatrician is the best approach.

Why Does My Thawed Breast Milk Look or Smell Funny?

Whenever milk is cooled, the heavier cream naturally rises to the top of the milk. This completely normal phenomenon sometimes surprises nursing moms. To remix the milk, just swirl it round and round, creating a tornado of milk. Most experts advise that mothers do not shake the milk, as vigorous shaking is thought to denature the proteins. However, there has been no scientific evidence to support this.

Kellymom addresses the issue of shaking milk on her blog. She writes, "To play it safe, use the smallest amount of force needed to mix the layers, keeping in mind that the layers will mix better as the milk warms. If you do shake the milk, it might not be a problem at all-and even if it turns out that shaking makes a difference it will still be the best nutrition for your child."

Some moms report that their breast milk changes in color once frozen and thawed. This is also completely normal. Often expressed milk will look white, but once frozen appears yellow. While there has yet to be a scientific explanation for this, the milk remains healthy for babies.

Related: Mother Breaks World Record by Donating Over 53,500 Ounces of Breast Milk

It's not uncommon for some mothers to report that their thawed breast milk smells odd, such as metallic or soapy. The distinct smell may be a result of higher productions of lipase, a highly-beneficial enzyme normally present in breast milk. Once expressed, lipase breaks down the fat in breast milk, sometimes impacted both the taste and smell of the milk.

Milk that smells soapy is perfectly safe to drink, although some babies might not like the new flavor. If your baby is refusing the previously frozen milk, try shortening the storage time to see if this makes a difference. You could also try mixing the frozen milk with fresh milk and serving it to your baby.

Here is everything you need to know about freezing and defrosting breast milk.
How Can I Tell If My Frozen Breast Milk Has Gone Bad?

If you have followed all of the proper storage guidelines, it's unlikely that your breast milk will become rotten if used within a year. Some breastfeeding experts, such as Leigh Anne O'Connor, IBCLC, recommend that mothers become familiar with the smell and taste of their milk. "I encourage my families to taste breast milk and get a sense for what your personal milk tastes like," she told Romper, "If the milk smells fine and tastes fine, it is likely fine."

Breast milk that has gone bad will have a distinct sour smell, such as cow's milk that has spoiled. If in doubt, it's best not to use the breast milk.

Pumping, Freezing, and Thawing - Is It Worth It?

There are undoubtedly benefits to breastfeeding that pumping can't replicate. From a decreased incidence of ear infections to the production of more milk, putting the baby to breast is always best. However, when this is not possible, pumping breast milk is the next best thing.

Pumping breast milk is certainly a labor of love, but well worth the long-term benefits provided to your baby. While the steep learning curve can feel overwhelming, most mothers feel satisfied with their choice.