Jane Says: Winter Is Here, and That Means It's Kale Season
“Is it true that kale gets sweeter after a frost, or is that just a marketing gimmick to get me (and my kids) to eat more of it?”
Well, there’s no denying kale is good for you—it’s rich in carotenoids such as beta-carotene, zeaxanthin, and lutein, as well as vitamins K and C and, to a lesser degree, calcium. Like other cruciferous vegetables, it has cancer-inhibiting glucosinolates, the sulfur-containing chemicals that also provide flavor.
But you can check your suspicions at the door: Kale does become sweeter after a frost or two, as do a number of other cool-weather vegetables. Now is a terrific time to work hearty braising greens such as kale and collards—not to mention broccoli, brussels sprouts, cabbage, cauliflower, beets, turnips, rutabagas, and carrots—into your culinary rotation, as they are at their best.
Let me revise that—they are at their best flavor-wise. Their looks, on the other hand, may disappoint, especially when compared with their pristine counterparts grown in the milder climate of, say, California’s Salinas Valley. But as far as braising greens are concerned, I’ll take my local frost-damaged kale and collards, with their brown patches and withered edges, any day—simply trim them well before cooking. If you are growing a late crop of kale, don’t panic if it wilts after a frost; the thick leaves will more than likely bounce back when it warms up the next day. It’s not the presence of frost that determines damage, after all, but the formation of ice crystals in the plant.
When it comes to frost, by the way, there are two kinds—advection and radiation. “Advective frosts occur when a cold front sweeps into an area,” reads Cornell University’s handy cheat sheet. “Winds are typically gusty, clouds may occur and the thickness of the cold air layer may reach more than a mile high. One seldom sees the first frost of the season under these conditions. The first frost is typically a radiation frost. These occur under a clear sky and calm winds.”
“On overcast nights, cloud cover acts like a blanket on the Earth, trapping radiant heat from the ground,” the explainer continues. “Any wind mixes the air thus trapped, creating a uniform temperature. However, clear skies and calm winds allow radiant heat from the Earth to rise to the upper layers of the atmosphere. Lack of wind prevents mixing of the air and an inversion layer develops. An inversion means that atmospheric conditions are inverse or opposite of normal daytime conditions when air temperature decreases with height. In an inversion, cold air collects near the ground while warmer air lies above this trapped cold layer.”
According to the “Principles of Frost Protection,” from UC Davis, more economic losses occur owing to freeze damage in the United States than because of any other weather-related hazard. No wonder that great effort is expended to prevent it. Wind machines (which are different from the wind turbines used to produce energy) are one protective measure used in vineyards and orchards to break up inversions that can cause irreversible damage to tender crops. But when it comes to greens, one farmer's frost damage is another cook's added flavor.
The increased sweetness in some vegetables after a frost is called "cold sweetness," and there is a solid biological reason for the phenomenon. “As plants produce sugars through photosynthesis, most are combined and stored in the plant as starches and other large polymers,” explained University of Wisconsin horticulture professor Irwin Goldman in the Wisconsin State Journal. “But in response to cold temperatures, some plants break down some of their energy stores into ‘free’ sugars, such as glucose and fructose, and stash them in their cells to guard against frost damage. Sugar dissolved in a cell makes it less susceptible to freezing in the same way that salting roads reduces ice. It’s wonderful for us. Not only does it keep the plants from freezing, but they taste sweeter too.”
I wrote about cooking kale in one of my first columns for TakePart, and I usually don’t stray too far from the advice I gave there. That said, one of the ways I’ve been enjoying kale recently comes courtesy of Deborah Madison’s Vegetable Literacy. She cooks it with slivered brussels sprouts and radicchio, “plus the softening element of onions.” Once the leaves are tender and glistening, you can use the mélange in all sorts of ways—on risotto or potato cakes, for instance, or on toasted thick slices of country bread. As a pizza topping, it’s a hit with young and old alike, especially if you can refrain from crowing about how healthy it is.
One last note: Sweetness is a desirable characteristic in the cold-tolerant vegetables mentioned above, but in potatoes, not so much. (I’m talking regular potatoes—Solanum tuberosum—here, not sweet potatoes, Ipomoea batatas, which are unrelated.) Potatoes that have been stored in the refrigerator—instead of the cool, dry, dark place where they belong—exhibit an off-putting sweetness that no amount of salt can rectify, and because their extra sugars will caramelize, they also turn brown when cooked.
According to Goldman, all is not lost. ”This can be avoided by moving cold potatoes to warmer storage areas to recondition them and allow the sugars to convert back to starch before cooking them,” he said in the State Journal piece. Who knew? I sure didn’t, so I reached out to Goldman to find out more. “I believe it takes about two full weeks to recondition the potatoes at warm temps,” he replied in an email, “but perhaps this reaction (which is controlled by the enzyme called invertase) will proceed enough at one week to still be meaningful.” Just one more reason to consider the potato an über-tuber.