Both kale and spinach are leafy greens that wilt as they cook. This wilting occurs with a variety of cooking methods, from sautéing to braising. Despite the fact that both types of greens are prone to wilt, spinach is among the most delicate of greens while kale is among the most hardy. This relative frailty and sturdiness affects the timing with which they wilt as well as the amount of volume they lose in the process.
Degrees of Wilting
Wilting is a process with degrees and gradations. Spinach will wilt lightly when you simply add it to a pan of cooked meat or vegetables and turn off the heat, or it can wilt a few minutes more until it's tender. Heating it past the point where it's just tender will not make your spinach unpalatable but it will be fully cooked rather than wilted. Wilted kale similarly ranges from just heated through and starting to droop to cooked down to about half its original size.
Wilting is a method of cooking that preserves flavor and vitality. It is used with leafy greens and also some firmer, crisper greens such as cabbage. Wilting spinach allows you to enjoy it just barely cooked and just barely tender. Wilting kale can make the difference between a green that is too tough to eat and one that is easier to chew and digest. The minimal cooking involved in wilting also allows you to enjoy kale's robust texture.
Types of Kale
There are multiple varieties of spinach available throughout the growing season, but they all wilt almost immediately. In contrast, different varieties of kale wilt quite differently. Curly kale can take a while to wilt, but red Russian kale is more tender and only takes a little longer than spinach. Younger, more tender leaves of any type of kale will wilt more quickly than tougher, mature leaves.
Bright Green Color
Both kale and spinach intensify in color as they wilt, but then they lose color as they overcook. Because kale is hardier than spinach and takes longer to wilt and cook down, it also takes longer to overcook and lose color: spinach starts fading after just two or three minutes, while kale starts losing its visual vitality after five to 10 minutes, depending on the degree of heat you use.