Testing baby bottle warmers: not for the faint of hea(r)t

Milk: it's what's for dinner

A baby bottle warmer and an accompanying baby bottle Credit: / TJ Donegan

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Parents rarely want to depend on luck when it comes to picking the right product that they need for their kids. For products related to the care and feeding of babies, it’s no wonder parents are spending hours doing side-by-side comparisons on everything from baby food makers to sippy cups. In keeping with that theme, we recently tested baby bottle warmers here in our labs.

Baby bottle warmers are just what they sound like – water and/or steam baths that are designed to bring frozen/refrigerated/room temperature breast milk or formula up to warmer temperatures. Heating up breast milk is desirable for a few reasons: firstly, some medical research has shown that body temperature (~98°F) may be the sweet spot for breast milk that helps it to maintain its antiviral properties. Overheating breast milk/formula, on the other hand, may kill off helpful digestive enzymes. (Admittedly, other research shows that cooling/thawing breast milk changes the milk, but not necessarily in a bad way.)

Secondly, it takes the pressure off of mom to have to produce milk on the spot for the baby — freeing her up to do things like go back to work or just get some much-needed personal time out of the house. The milk can be stored and preserved for future use, and then heated up right before the baby gulps it down. Lastly, heating up the milk also makes it more enticing for infants used to getting milk straight from the source.

Because it’s so difficult to get direct feedback about the baby’s satisfaction with the milk temperature (because they’re…babies), we had to gather data the old fashioned way - with science!

The bottle

Credit: / TJ Donegan

We used Dr. Brown's Natural Flow bottles for all of our baby bottle warmer testing.

As most parents know, there are a dizzying number of bottle types, shapes, and sizes. For consistency, we stuck with Dr. Brown’s Natural Flow plastic bottles because the tall, skinny shape of the bottle fit all of the bottle warmers we were testing (unlike some of the shorter, squatter bottle shapes, which would not have fit in some of the narrow warming cavities).

The “milk”


Unfortunately, we were not able to use actual breast milk (if it were something we could just pop down to the corner store and buy, believe me, we would have). As a breast milk analog, we used tap water. Because breast milk is mostly composed of water, calcium, and fat, using water as a milk substitute is not unreasonable.

Additionally, the properties related to a liquid’s ability to transfer heat, like the heat capacity (the heat needed to raise the temperature of a fluid by one degree), thermal conductivity (the rate of heat transfer in the fluid), and the density (the mass distributed over the volume of the fluid), are very similar for breast milk and water. Thanks to these similarities, we’re pretty confident that the results we used from heating up water will reflect those using breast milk in real life.

Set up

Credit: / TJ Donegan

The water used as a substitute for milk is kept at refrigerated temperatures.

Before each test, we filled the bottle with four ounces of tap water that had been cooled to refrigerator temperatures (37-39°F).

Credit: / Julia MacDougall

Temperature data loggers are suspended in the refrigerated "milk".

Suspended in the water were two data loggers, or temperature gauges, which are each about the size of a watch battery. With the two data loggers, we were able to measure the temperature over time of the water in both the top and the bottom of the bottle.

Prior to actually starting a warming cycle, we would put a certain amount of water (as per the instructions) into the bottle warmer. A heating element at the bottom and/or sides of the bottle warmer heats up this water, and that hot water (or steam) comes into contact with the bottle, and in turn, warms up the milk inside the bottle.

In each bottle warmer, the time duration of a bottle warming cycle depends on some or all of the following: the starting temperature of the milk, the bottle shape, the bottle material, and the amount of milk in the bottle. We chose the cycle in each warmer that was most appropriate for four ounces of refrigerated “milk” in a tall, skinny, plastic bottle. Differentiating between the varying bottle materials (typically plastic or glass) is especially important because plastic and glass heat up very differently, and could greatly affect the speed at which the milk is warmed.

Let the warming begin!

Credit: / Julia MacDougall

A bottle inside a baby bottle warmer.

With the water and the data loggers in the bottle, we would put the bottle in the warmer. Once the cycle finished (usually after 5-10 minutes), we removed the bottle from the warmer and, after letting it sit for a minute or so, used the tried-and-true method of checking the milk temperature by letting a few drops land on an upturned wrist.


Credit: / Jackson Ruckar

We review the temperature data reported by the data loggers in the "milk".

After collecting the temperature information from the data loggers, we would plot up the results to see how well each bottle warmer actually heated up the “milk”. One such plot is shown below:

temp data

A plot of milk temperature before, during, and after its time in a bottle warmer.

One data logger was suspended in the upper part of the "milk" in the bottle ("top" line in dark blue), and one was suspended in the lower part of the milk in the bottle ("bottom" line in light blue). Due to heat circulation and contact with the air inside the bottle, the uppermost part of the milk always heated up to a higher temperature than the "milk" in the bottom of the bottle.

Most of the bottle warmers could hit temperatures close to body temperature, while some were much mellower, and could barely achieve room temperature. One or two bottle warmers actually heated up the bottle so much that the bottle tip was very hot to the touch. In general, though, as long as the bottle warmer increased the temperature to 80-98°F, it got a pass in our book.

Do you really need a bottle warmer?

Credit: / TJ Donegan

A bottle being warmed manually with hot tap water.

To try to figure out exactly how useful baby bottle warmers are, we also collected data for a bottle of "milk" that had been warmed in the old-school way, under warm tap water (while swishing the liquid around in the bottle occasionally). Surprisingly, the results are very similar to those of a bottle warmer, both in the amount of warming and the time it takes:

milk plot 2

A plot of milk temperature before, during, and after being warmed gradually with warm water from a sink.

We say that's "surprising" because the common wisdom is that a bottle warmer is much faster than simply swishing a bottle around under a hot tap. The reality is they both take about five minutes on average for a four ounce bottle of refrigerated milk. The tap method wastes a lot more water, but the swishing motion ensures you get nice, evenly warmed milk. Here's the thing, though: the tap method only took five minutes, but our tester reported that it felt like eight or nine minutes—and that's without a hungry baby crying in your ear. Using a warmer may not save you much time, but it takes care of the job so you can take care of your baby.

In the course of gathering temperature data, we had to actually use the bottle warmers as any regular, sleep-deprived parent would. This gave us valuable information about the bottle warmer cycle times, setup, and ease of use. While humans naturally desire everything to be done immediately, having short cycle times on a baby bottle warmer when it’s 3:30 AM and your baby decides it’s dinnertime is especially important. We took all of this and more into account when determining the winners, runners up, and losers in our baby bottle warmer roundup.

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