Take the time to blanch bok choy before freezing to ensure the best flavor and color later. Blanching before you freeze slows the natural enzyme action in the leaves that would otherwise cause them to become discolored and bitter upon thawing. Frozen bok choy, also called Chinese cabbage or pak choi, has loose heads of large green leaves, and is best suited to cooked dishes, such as a cooked green in Asian-influenced stir-fries and vegetable medleys.
Separate and wash the bok choy leaves in cool water. Trim off any wilted leaves and the tough stem ends of each leaf, then cut or tear the leaves to the desired size. For large heads of bok choy, cut off the white portion near the leaf base, as these don't freeze well.
Bring 2 or 3 inches of water to a full boil over medium-high heat.
Place the bok choy leaves in a steamer basket, and set the basket in the pot so the bok choy is sitting above the water. Place a lid on the pot and steam the bok choy for 3 minutes.
Remove the steamer basket from the pot and run cold water over the leaves in the basket until they are completely cool.
Shake the excess moisture out of the leaves and pat them dry with a paper towel. Pack the leaves into a storage bag, leaving 1 inch of headspace. Press out the air and seal closed. Store the bok choy in a 0-degree Fahrenheit freezer for up to six months.
If you don't have a steamer, boil the bok choy leaves in the water for 1 ½ minutes. Cool the leaves in a bowl of ice water before draining, drying and freezing.