ONE of summer’s great pleasures is eating berries of all kinds by the basketful. One of summer’s great frustrations is having baskets of berries go moldy overnight, or even by nightfall.
Over the years I’ve come up with various strategies for limiting my losses, but this summer I came across a surprising one, the most effective I’ve ever tried. Thermotherapy, it’s been called. A very hot fruit bath.
Fruits go moldy because mold spores are everywhere, readily germinate on the humid surfaces of actively respiring, moisture-exhaling fruits, and easily penetrate the smallest breach of their thin skins.
The first thing I do with a haul of berries, after eating my fill straight from the basket, is to unpack the rest and spread them out on kitchen or paper towels, so they’re not pressing against one another and trapping moisture.
If I want to keep them overnight or longer, I refrigerate them, because cold temperatures slow fruit metabolism and mold growth. I repack the berries as sparsely as possible, nest each basket in a second empty one to leave an air space at the bottom, and inflate and tie off a plastic produce bag around the baskets, so there’s room for the berries to breathe and the bag itself doesn’t cling to their surfaces.
Even with these precautions I’ve had baskets mold overnight in the refrigerator. So I followed up right away when I saw a reference in an agricultural journal on extending the shelf life of strawberries not with a chemical treatment or gamma irradiation, but with heat.
I gathered a dozen or so reports that hot-water treatments suppress mold growth on berries, grapes and stone fruits. The test temperatures ranged from 113 to 145 degrees, with exposure times of a few minutes at the lower temperatures, and 12 seconds at the highest.
I found it hard to believe that any part of a plant could tolerate 145-degree water. My finger in the same water would get a third-degree burn in less than 5 seconds, and eventually reach medium rare.
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.
