When you boil or steam a vegetable briefly, but not until it is fully cooked, that is blanching. It is an essential step before freezing many vegetables including broccoli, leafy greens, string beans, okra and asparagus.
Vegetables that are frozen without having been blanched are safe to eat, but have "off" colors, textures and flavors. If you've ever stuck a bunch of raw spinach into the freezer and taken it out later to find you had a darkened, gooey mess, you know what I am talking about.
What blanching does is stop enzymatic activity that decays vegetables. These enzymes can survive freezing temperatures and continue the decaying process even though the food is frozen. Pre-treating the food in boiling water or steam kills off the enzymes.
How long to blanch different vegetables before freezing depends on the food's density and size. Below is a list of some vegetables that freeze well and their blanching times.
After blanching, there are two more essential steps before you put the food into the freezer. The first is to immediately plunge the blanched veggies into ice water or run cold water over them. You want to cool the food down as quickly as possible so that it doesn't continue cooking from the residual heat.
After the food has been quickly cooled, drain it well. With leafy greens, squeeze out as much liquid as possible. Excess liquid will result in poor quality frozen food.
Although some sources suggest blanching times for sweet peppers, onions, corn and tomatoes, I find that these can be frozen without blanching. I also find that most root vegetables, especially potatoes, do not freeze well even when they are blanched first. Here is a list of vegetables that freeze well but need to be blanched first, and how many minutes they need to be blanched:
Artichoke(Hearts) 6 minutes
Asparagus 2-4 minutes (depending on how thick the stalks are)
Beans-Green or Wax 3 minutes
Broccoli (1-inch pieces) 2 minutes
Brussels Sprouts 3-5 minutes (depending on size)
Cauliflower (1-inch) 3 minutes
Kohlrabi (1-inch cubes) 1 minute
Leafy Greens 1-2 minutes (use the longer time for collards and cabbage)
Okra 2-3 minutes (depending on size)
Peas (Pod) 2-3 minutes (depending on size)
Peas (Shelled) 1.5 minutes
Squash (Chayote) 2 minutes
Squash (Summer) 3 minutes
Remember that even perfectly blanched and frozen vegetables do lose some of their nutritional content over time, so it is important to label your frozen foods with the date they were frozen and make note of how long foods can be frozen.