Cookbook:Kale

Kale is a very bitter open-headed member of the cabbage family. It can be cooked in combination with Collard greens or with other milder ingredients to soften its harsh flavour. It can also be substituted for spinach in most dishes.

Kale must be kept cold to avoid bitterness and wilting. Choose smaller leaves because they will be less strongly flavoured, and use them quickly as the bitterness will intensify rapidly after picking. If the leaves are very small and tender, the stems can also be used, but they become woody and tough as the plant grows, so they may need to be removed before cooking.

Further Reading
 * http://www.wholehealthmd.com/refshelf/foods_view/1,1523,127,00.html