Like other berries, mulberries don't ripen after they're picked and are highly perishable, but you can enjoy them year-round by freezing them. Unsweetened dry-pack freezing is the simplest method, provided you don't mind the mulberries clumping together. Unsweetened dry freezing is easy too, and works well if you want to keep the berries single for later use in baked goods or as a topping. Sugar packing provides sweetened berries in a syrup that work well for pies, cobblers, sauces and some batters. The syrup provides additional protection against air exposure for slightly longer storage, too.
Unsweetened Dry-Pack Freezing
Put the mulberries into a colander and rinse them under a gentle stream of water, tossing them lightly. Shake out the excess water and place the berries in a single layer on an absorbent towel. Turn them over periodically and allow them to dry completely.
Place the berries gently into a freezer-grade plastic container or bag, leaving at least 1/2 inch of space at the top for expansion. Suck the air out of freezer bags with a straw before sealing them; attempting to press the air out risks smashing the berries. Close the container or bag tightly.
Label the package with the date and place the bag or container into the freezer. For optimal flavor and texture, use mulberries frozen in this way within 6 to 8 months.
Unsweetened Dry Freezing
Wash the mulberries under a light stream of cold water in a colander. Gently turn them during rinsing, then shake off as much water as possible. Transfer the berries onto an absorbent towel without piling them on top of each other. Flip them over every once in a while until they're dry.
Line the mulberries up in a single layer on a rimmed cookie tray. Put them into the freezer uncovered for up to 24 hours, making sure the tray is level, until the berries are frozen.
Put the frozen mulberries into a freezer-grade plastic bag or container. As long as the berries are fully frozen, there should be no additional expansion, so you can fill the package. Use a straw to remove air from bags and seal the package securely.
Write the date on the container or bag so you know when it was put into the freezer. Berries frozen with this technique are best used within 6 to 8 months.
Sweetened Pack Freezing
Rinse the mulberries in a colander under a gentle stream of cold water. Dry them off thoroughly in a single layer on an absorbent towel, turning them over once in a while to speed the process.
Put the mulberries in a bowl and add in 3/4 cup of sugar for each 1 quart of mulberries. Stir until the sugar dissolves. Be slow and gentle if you want to preserve the individual berries or mash them to form a paste.
Transfer the sweetened berries into a freezer-grade plastic container or bag. Leave 1/2 inch or more of space for the berries to expand into. Use a straw to remove air from freezer bags and tightly close the package.
Label the package with the date and put it into the freezer. Mulberries frozen this way are best used within 9 to 12 months.
Fresh mulberries keep for up to a week in the refrigerator.
Don't leave mulberries uncovered in the freezer for more than a day or two, or they develop freezer burn.