Our main goal for testing bottle warmers is to assess safety and to see if each could warm a bottle of milk consistently and efficiently without overheating the contents. We also looked for ease of use and cleaning. We tested each warmer over several months to find the best products.
Health and Safety
We performed two different controlled tests of each warmer using the same protocol; that is, we filled 4 ounces of refrigerated milk in a 4 ounce Lifefactory Glass Bottle with silicone sleeve. Each test started with milk in the bottle between 42 F / 5.6 C to 45 F / 7.2 C. A professional-grade digital thermometer was used in the center of the bottle to record temperatures.
A big issue we noted during testing is the tendency of bottles to continue heating if left in the unit after shut-down. So, we kept the bottle inside the unit and continued measuring temperatures every 30 seconds for a full 12 minutes.
Please note — In our tests, we used a glass bottle with silicone sleeve which takes longer to conduct heat than glass alone or plastic. So, the milk temperature measurements in our tests are likely to be lower than if you use a plastic bottle. However, the times are still important as they indicate which warmers heat faster than others.
Manufacturer Warming Recommendations
Following the manufacturer's recommendations, we tested the bottle contents for each warmer every 30 seconds. We also noted the milk's temperature when the unit turned off.
Maximum Heat Settings
We also tested each warmer by cranking a fully cooled warmer to its maximum settings and let it do its thing with the same set up of 4 ounces of refrigerated milk in a small Lifefactory glass bottle with the included sleeve. We recorded temperatures every 30 seconds for a maximum for 12 minutes.Why did we do this?
We don't condone stepping outside the instructions outlined by the manufacturer, but let's face it…not everyone is going to play by the rules. We suspect some parents will try to find the fastest way to heat the bottle. So, in this event, we wanted to see how high the temperatures ascended with each warmer at full-throttle.
Ease of Use
For ease of use we took into account the ease of set up out of the box, the number of pieces, ease of day-to-day use, and quality of instructions. We carefully read the directions, reviewed the initial assembly, and then took notes on how easy it was to use on a daily basis, over several months considering the warming chart, its location, adding water, and whether or not a caregiver could use the unit without significant coaching.
For warming efficiency, we took into account three factors. In our testing, we used body temperature as a standard to which to warm a bottle of milk. Reason? because that is roughly the temperature of breastmilk coming from the breast and we assume this is the ideal temperature for feeding a baby.
Manufacturer Recommendations: Auto Shut-Off Time & Temperature
Following each manufacturer's warming recommendations, we recorded the time and temperature at auto shut-off.
Consecutive Warming Wait Time
Last, we considered whether or not a warmer could be used in tandem, as in warm one bottle, put another right in to warm. This might be important for multiples or daycare scenarios.
Ease of Cleaning
Each recommends basic wipe-down cleaning at least every other day. And, they all require occasional de-scaling for removal of hard mineral deposits to keep in good working order.