The soft, unprotected structure of mulberries would break apart in boiling water, so you can’t blanch them as you would firmer fruits like cranberries and blueberries. Instead, use ascorbic acid as it is an antioxidant that protects your hydrated mulberries from growing mould and bacteria after drying, and still preserves their wonderful colour. In a deep pan or a wide bowl filled with 1 quart cold water, mix in 3 tablespoons of ascorbic acid and soak your mulberries for 10 minutes. Strain them gently on a paper towel and wait for them to completely dry.
Before putting them in the oven, remember to stick to one layer of mulberries per casserole dish and use coated, non-stick pans. Trying to save time by filling a 3-inch deep casserole dish with mulberries will prevent even heat distribution and cause the berries to stick together. Baking requires 24-36 hours in an oven of 140 degrees Fahrenheit, and you should turn and shuffle the mulberries around the pan with a spatula approximately every hour to ensure each mulberry dries evenly. Shake the pan side to side gently to test their dryness, as they should make a rattling sound against the pan when done.
However, no matter how carefully you arrange and bake your berries, every batch will contain some uneven moisture, which needs to be redistributed before storing to reduce the risk of mould and spoiling. Fill a glass jar half-full with berries, and for 10 days store them at room temperature, away from sunlight or moisture and with the lid closed. This will cause the drier mulberries to absorb excess moisture from the less dry mulberries, and you should shake each jar for 10 seconds once a day to rotate the berries and prevent them from sticking to each other.