The competition
The Dr. Brown’s Deluxe Bottle Warmer is a steam warmer that has been lauded by other reviewers like those at The Night Light and New York Magazine. Unlike the other steam warmers we considered and tested, which require adding water each time, the Dr. Brown’s has an attached water reservoir that lets you load up enough water for several runs. The warming cycles are controlled by a digital timer (you set the length based on the size and temperature of your bottle) that is easy enough to use. But we found the water reservoir leaked all over the counter if we tipped or moved the warmer even slightly. It also won’t fit very wide bottles like the Comotomo.

Philips Avent claims its Nutrient-Preserving Bottle Warmer is safer for heating breast milk because it gradually warms bottles with flowing warm water and uses temperature sensors to prevent overheating. But the warmer actually uses a stagnant water bath, similar to the Chicco warmers we tested. (Philips Avent explained that its claim that the water “flows” is because it is heated from the bottom, which makes warmer water rise to the top; this is basic physics, and it happens with all water-bath warmers.) The Avent warmer does use cooler water (around 100 °F at the hottest setting) to heat bottles; this is good, in theory, because there’s no chance of overheating the milk. But such a low temperature makes for a very slow warm time; after minutes, a refrigerated 4-ounce bottle had barely reached 80 °F. You have to add about a half cup of water to the basin, which you can theoretically use for multiple bottles, but if you want to run back-to-back cycles you’ll have to replace it with fresh, cool water.
We didn’t test the Philips Avent Fast Bottle Warmer because BabyGearLab found it quickly overheated bottles, and it doesn’t automatically shut off at the end of the cycle.
The Chicco Bottle Warmer is a straightforward water-bath warmer that looks like a tiny Crock-Pot. It has a simple switch—you can choose bottles or baby food (we didn’t test baby food). You dump in a half cup of water, put in the bottle, and turn the machine on. The warmer heats the water to about 180 °F, which is 40 degrees hotter than the Kozii, but it uses a thermostat to gauge when the bottle is hot enough, which takes out some guesswork. It took about minutes to heat a 4-ounce Avent bottle and minutes to heat an 8-ounce Avent bottle to 90 °F before going to “keep warm” mode. But it also significantly underheated several bottles, switching to “keep warm” when the silicone Comotomo bottle was at only 60 °F. You have to wait about 1minutes before running another heat cycle, which makes it hard to warm multiple bottles. This warmer wasn’t any faster than the Kozii. It was one of the simplest products we tried and it’s unfortunate the heating performance was such a weakness compared with our pick’s.
The Chicco NaturalFit Digital Bottle & Baby Food Warmer offers more settings than the basic Chicco warmer (you select both the bottle size and temperature), and you have to add water (or tablespoons) for each use. It rapidly boils the water, heating the bottle mostly with steam. It worked consistently, heating 4-ounce bottles in about minutes, and 8-ounce bottles in minutes. But it doesn’t offer any advantage over the First Years warmer, which is a bit faster and much cheaper.

We didn’t test the Boon Orb bottle warmer, because BabyGearLab found it quickly overheated bottles, and you can’t use it with glass bottles. We didn’t test the Cuisinart Baby Bottle Warmer/Night Light either, because it performed poorly in BabyGearLab ’s tests and is more expensive than the First Years warmer. . We also didn’t test the Munchkin Timer Saver or Munchkin Speed or Nuby 2-in-warmers because they are more expensive than the First Years but don’t offer any extra functionality.
Safely heating breast milk
Exactly what temperature is too high? Because breast milk is so complex, this is hard to answer. A study (PDF) from the 1980s suggested certain bioactive properties begin to change at temperatures as low as 10°F to 12°F, with more damage occurring as the temperature increases past 140 °F. Though it’s important to protect breast milk from heat damage, Dr. Thomas cautioned that even overheated breast milk maintains a great deal of its nutritional and anti-infective benefits and is superior to formula. When donor milk is processed for use in hospitals, for example, it is pasteurized at 144.°F.
Dr. Thomas also warned you shouldn’t vigorously shake bottles of expressed breast milk, because rough motion can also damage components of the milk. Gently swirl it instead.
Safely heating formula
Formula doesn’t have the same heat sensitivities as breast milk, but The Infant Nutrition Council of America told us that warming for all types of formula and breast milk should take less than 1minutes—that’s a measure only of the time that it’s heating—to reduce the opportunity for possible bacteria to grow. The standard guidance from INCA and the AAP is to consume the formula within hour of preparation, or refrigerate it immediately and serve it within 2hours.
The WHO (PDF) and CDC advise that powdered formula be mixed with water that is no cooler than 15°F to kill bacteria that could be present in the powder. That means you’ll need to initially prepare the formula much hotter than you could safely serve a baby, and wait for it to cool down (either in the refrigerator, in cool water, or in an ice bath). The Infant Nutrition Council of America says you can prepare a larger quantity of powdered formula in advance, cover it, and store it in the refrigerator below 40 °F for up to 2hours. You can then prepare and rewarm bottles as needed.

Liquid-concentrate formula (which you mix with water) and ready-to-feed formula (which comes in preprepared bottles) do not need to be prepared with hot water or specially heated. We plan to explore the pros and cons of all types of formula in a future guide.
Whether you’re mixing with powdered or liquid-concentrate formula, the water you use needs to be safe from contaminants. Speak with your baby’s pediatrician and local water department to find out whether your tap water needs to be boiled or purified before mixing.

The SAFEHeat technology reduces the risk of hot spots in the milk or in loss of nutrients. Since its operating temperature is not so high, water does not evaporate as quickly in the Kiinde Kozii.
However, this does mean that it takes a slightly longer time than most other bottle warmers to warm up the milk.

This bottle warmer is simple to use, safe and efficient. It has an opening that is wide enough to easily to accommodate even the extra-wide basedTommee Tippee or Comotomo bottles. Although it does not beep when the bottle is heated, it does somewhat alert you with a clicking sound when it is done..