To freeze grated zucchini for baking, the National Center for Home Food Preservation recommends:
- Choose young tender zucchini.
- Wash and grate.
- Steam blanch (directions below) in small quantities 1 to 2 minutes until translucent.
- Pack in measured amounts into containers, leaving 1/2-inch headspace.
- Cool by placing the containers in cold water.
- Seal and freeze. If watery when thawed, discard the liquid before using the zucchini.
To steam blanch: Use a pot with a tight lid and a basket that holds the food at least three inches above the bottom of the pot. Put an inch or two of water in the pot and bring the water to a boil. Put the vegetables in the basket in a single layer so that steam reaches all parts quickly. Cover the pot and keep heat high. Start counting steaming time as soon as the lid is on.
Freezing Summer Squash, National Center for Home Food Preservation