I bought pints of various berries, divided each batch into two samples, and heated one by immersing and swishing its plastic basket in a pot of hot water. I emptied the heated sample onto towels to cool down and dry. Then I repacked it, and encouraged both baskets to spoil by wrapping them airtight and letting them sweat on the kitchen counter. After 24 hours I counted the moldy berries in each basket.

The strawberries fared best when I heated them at 125 degrees for 30 seconds. In two samples from different sources, this treatment gave a total of 1 moldy berry out of 30, where the untreated baskets had 14. I also treated some bruised berries, including one with a moldy tip. After 24 hours none were moldy. The tip mold not only hadn’t spread, it had disappeared.

I tried the same treatment, 125 degrees for 30 seconds, on raspberries and blackberries, and got the same good results. There were many fewer moldy berries in the heated samples.

For thicker-skinned blueberries, a Canadian study recommended a 140-degree treatment for 30 seconds. I tested it twice, with samples of around 150 berries each time. That heat took the bloom off. It melted the natural wax that gives the berries their whitish cast, and left them midnight blue. It also cut the number of moldy berries from around 20 per sample to 2.

Research has also shown that exposure to hot air slows fruit spoilage. But hot air can take several hours, and I found it harder than hot water to apply precisely in the kitchen. I did spread some raspberries out on a sheet pan lined with towels, and put them in a 150-degree non-convection oven for 20 minutes. The berry bottoms got hotter than the tops, which were cooled by evaporation. Still, only 1 out of 48 heated berries became moldy, compared with 7 out of 52 in the unheated basket.

Why is it that delicate berries can survive heat high enough to kill mold and injure fingers? Probably because they have to do so in the field. One study of tomatoes found that intense sunlight raised their interiors to 122 degrees. Such heat hurts the quality of growing fruits, but I couldn’t taste much of an effect on briefly heated ripe fruits.

So if you find yourself plagued by quickly spoiling fruits, start giving them a brief hot bath before you spread them out or chill them. Thermotherapy can be healthy for all concerned.