Kale has a substantial mineral content, providing manganese as well as some iron, calcium, and potassium and the antioxidant vitamin E in the leaves as well.

Buying
Kale should be kept in a chilled display case in the supermarket, otherwise it will wilt and become bitter if left in a warm environment. Choose the small leaved specimens for tenderness and mild flavour, especially if the greens are to be eaten raw; coarse, oversized leaves are likely to be tough. Look for a fresh green colour; leaves should not be yellowed or browned. Purchase only moist, crisp, unwilted kale that is unblemished by tiny holes, which frequently indicate insect damage. Kale stems are edible, so ensure that this part of the plant is plump and moist, not withered or collapsed.

Storage
Kale develops a stronger flavour the longer it is stored, so use it within a day or two of purchase. Wrap the unwashed kale in damp paper towels, then place it in a plastic bag and store in the refrigerator.

Preparation
Whether serving it raw or cooked, wash kale before using, as the leaves and stems are likely to have sand or dirt clinging to them. Trim off any roots, then separate the leaves and swish them around in a large basin of cool water. Do not soak. Lift out the leaves, letting the sand and grit settle; repeat if necessary.

If the stems are thin and tender, you can just trim off the tips and cook the stems with the leaves. If they are somewhat thicker, but still tender, cut them off, chop them, and cook a few minutes before adding the leaves. If the stems are really tough, remove them, along with the midribs (the part of the stem that extends into the leaf). You can easily stem kale by folding each leaf in half, vein-side out, and pulling up on the stem as you hold the folded leaf closed. If the stems are very tough, you may need to trim them off with a paring knife.

Whenever possible, use the cooking liquid from kale in a sauce, gravy or add it to a soup; a significant amount of the nutrient content of greens is released as they cook.