Researchers still are working out the details of how heat-shocking works, but it appears to change the food in several ways at once. Many of the fruits and vegetables you bring home from the store are still alive and respiring; the quick heat treatment tends to slow the rate at which they respire and produce ethylene, a gas that plays a crucial role in the ripening of many kinds of produce. In leafy greens, the shock of the hot water also seems to turn down production of enzymes that cause browning around wounded leaves, and to turn up the production of heat-shock proteins, which can have preservative effects.
For the home cook, the inner workings don't really matter. The bottom line is that soaking your produce in hot water for a few minutes after you unpack it makes it cheaper and more nutritious because more fruits and veggies will end up in your family rather than in the trash.
The optimal time and temperature for heat-shocking fruits and vegetables varies in response to many factors — in particular, whether they were already treated before purchase. Use these as general guidelines.
— Asparagus: 2 to 3 minutes at 131 F (55 C)
— Broccoli: 7 to 8 minutes at 117 F (47 C)
— Cantaloupe (whole): 60 minutes at 122 F (50 C)
— Celery: 90 seconds at 122 F (50 C)
— Grapes: 8 minutes at 113 F (45 C)
— Kiwi fruit: 15 to 20 minutes at 104 F (40 C)
— Lettuce: 1 to 2 minutes at 122 F (50 C)
— Oranges (whole): 40 to 45 minutes at 113 F (45 C)
— Peaches (whole): 40 minutes at 104 F (40 C)
EDITOR'S NOTE: W. Wayt Gibbs is editor-in-chief of The Cooking Lab, the culinary research team led by Nathan Myhrvold that produced the cookbooks "Modernist Cuisine: The Art and Science of Cooking" and "Modernist Cuisine at Home."