Does day-old, leftover kale salad have less nutritional value than fresher kale?
Technically, yes. There is a gradual loss in nutrient content in kale or any fresh fruit or vegetable from the moment it is picked. But the decrease is so minor that the overall health benefits of consuming the leftover salad would still be quite high.
Unless you are picking kale straight from your garden or buying it at the farmers’ market, it has most likely spent days in transit and on display. The additional day in your crisper will make virtually no difference in nutrient levels. And kale is quite hardy. While we tend to treat it as a salad green, it is a cruciferous vegetable like brussels sprouts, cauliflower and broccoli. So you may notice it can be stored far longer than lettuce or spinach.
Temperature is the critical factor in the retaining of nutrient value. Whether the leftover kale is raw or cooked, dressed or not, it is important to keep it refrigerated. One study found that after six days of refrigeration, leafy vegetables lost only 10 percent of their vitamin C content, compared with 20 percent in just two days at room temperature.
“I definitely would hope that doesn’t deter someone from eating the kale salad,” said Jackie Armstrong, a nutritionist at Stanford University. “It’s still packed with nutrition.”
It also helps to chop the kale yourself, as close to mealtime as possible.
“As soon as you chop that kale, you’re basically breaking the cell walls, and that begins the oxidation process,” Armstrong said. “Processing the kale” by chopping increases “the rate at which the nutrients degrade.”
Different nutrients degrade at different rates and are sensitive to different factors. Whereas most minerals retain their value at any temperature, vitamins can be more sensitive, especially the B vitamin folate. “In our studies, we found it’s the thing that goes the fastest,” said Dr. Walter Willett, chairman of the nutrition department at the Harvard T.H. Chan School of Public Health.
If, like many people, you don’t enjoy day-old salad, cook with it instead. “Whatever kind of dressing is on there, like Asian ginger soy sesame dressing, take that as a theme and put it in a stir-fry,” said Anne-Marie Gloster, a nutritional sciences lecturer at the University of Washington. “The flavors will blend.” If it’s an Italian dressing, try it in spaghetti sauce or lasagna; for a sweeter dressing, like raspberry poppy seed, toss it in a smoothie.
Cooking kale may even boost its health benefits. Sautéing the vegetable in olive oil, for example, may help your body absorb some of its nutrients more easily. Heat also breaks down the fiber in kale in a way that, when it interacts with bile in the digestive tract, may help to lower levels of bad cholesterol in your blood, Gloster said.
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