During the peak blackberry season in summer, you can either pick a large quantity of fresh, ripe, plump blackberries, or buy them from a nearby produce store. Just make sure you buy the freshest berries available, without any blemishes.
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Once you bring them home, you will need to wash them thoroughly, to get rid of any dirt or debris. Pour them all out into a colander, and rinse them by holding them under cold running water. Work the water through the pile of blackberries with your hands, to ensure that they are all washed.
Next, put the colander aside for around 10 minutes, to allow the excess water to run off and drain out of the blackberries. To help this process along, you can shake the colander a few times, to get rid of the water.
The next step is to freeze the blackberries. Spread them out onto the cookie sheet (make sure this is level) in a single layer, and slide this into the freezer for around 6 hours, to allow them to freeze thoroughly. You do not need to cover the tray.
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After the berries are completely hard and frozen, take the tray out, pick the berries off the tray, and put them all in a plastic zipper freezer bag. Now, the crucial trick is to remove as much air as possible from the bag, as any air left inside will cause freezer burn. In addition to squeezing the air out of the bag with your hands, you can also stick a straw into the bag, zip it close as much as you can, suck excess air out of the straw with your mouth, then quickly slide the straw out and zip the bag close as fast as you can.