One of the things that’s become readily apparent to me living in New Orleans is just how fast everything can melt. I like to take a sip of water in the night, but I’ve woken up to find my frosty glass lukewarm within a couple of hours. I started putting a glass full of ice cubes next to my bed every night, but it still became room temperature liquid too fast, even with the air conditioning pumping.
Beverages like lemonade or iced tea barely survive the lunch hour; I am always asking for a separate glass of ice to keep my drink cold. I rarely ran into this problem during northern summers, which is how I came to understand that it’s not just the heat—it’s the humidity.
As I’m writing this, there’s a thunderstorm going on outside, my dehumidifier humming away next to me. The humidity regularly ranges about 70% here in Louisiana, and it is absolutely true that humidity causes things to melt faster, according to physicists. Humid air holds more heat and deposits condensation. When those warm droplets hit the surface of a drink, or even the outside of a food or drink container, that heats up the contents.
Given that there has been unusually hot and humid weather across the country this summer, from Boston to Chicago, Iowa to Louisiana, how much time do we have before our ice melts in our glasses? Can we get our groceries home fast enough? And should we even bother transporting frozen foods?
There’s been lots of research on how long it takes for an ice cube to melt. A one-inch ice cube lasts between 45 to 60 minutes in a 75-degree room, probably a little longer if you keep your AC at a lower temperature. But if you put it in a cup of cold water or something else cold, it only lasts 15 to 20 minutes. Sugar and salt speed up the melting process as well. (That’s why rock salt is scattered on icy sidewalks.) This is all assuming a typical humidity level, which is under 60%. Higher than that, and you can expect the windows of time to shrink.
I had hoped that insulated tumblers, like the ones made by Tervis, might prevent the ice in my drinks from melting quite so fast. My home experiments showed that they didn’t: My glass of ice was quickly reduced to a puddle. The main benefit was that the outside of the glass didn’t sweat the way a single-walled glass might.
New Orleans abounds with artisanal ice cream makers and delicious gelato producers. But I’ve had trouble maintaining its solid texture as I transport and eat it. Ice cream begins melting in 15 to 20 minutes at a temperature of about 72 degrees, because the solid ingredients absorb the heat around the scoop.
Ice cream is in danger of becoming a complete blob as the temperature climbs, and probably won’t survive more than a few minutes in the heat of a parked car. While a pint of melty ice cream can and will re-freeze, its creamy consistency will not remain; instead, it might have more of a thin, icy texture due to broken water molecules. To keep it recognizable as ice cream, either eat it fast or transport it with a cooler to maintain a consistent temperature.
Cow’s milk will stay good for roughly two hours at room temperature of 72 degrees, although not everyone will see a need to throw away milk that’s been left out longer than that by accident, especially if the carton is sealed. Again, if it’s at average humidity levels, and your milk passes the “sniff test,” you might determine that it’s fine.
What about non-dairy milks? While you might think that they have more staying power than cow’s milk, it turns out that they are also susceptible to heat and humidity. One sign is whether the liquid in the container starts “clumping,” which basically means it is thickening up. Oat milk, which is my plant-based favorite, can actually thicken up to resemble drinkable yogurt if it’s exposed to too high a temperature.
If you want to be absolutely safe during the summer and preserve the condition of your groceries, get everything home as soon as possible, and safely stowed. To give your frozen things the best shot at surviving, save the refrigerator bags from food deliveries or meal kits and tuck them into your shopping tote bags. If you drive to the store, you can even make a habit of keeping a small cooler in the trunk.