Prepare vegetables for preservation immediately after picking to prevent color, flavor, texture, sugar content and nutrient changes. Sort and discard any food with decay, bruises or mold. Thoroughly rinse vegetables with running water, using a produce brush if necessary, then drain the vegetables well. Cut foods into c-inch to ½-inch slices. The higher the water content, the larger the slice size should be. Small slices of high-moisture foods would nearly disappear when all the moisture has evaporated.
To preserve quality and color, blanch prepared vegetables in boiling water or a citric acid solution for the times listed in Table 1. Blanching is a heating process that destroys enzymes, which can cause color and flavor issues. Water blanching achieves a more even heat penetration than steam blanching or microwave blanching. Citric acid acts as an anti-darkening and anti-microbial agent.
If using citric acid (available in the canning section of the supermarket), stir ¼ teaspoon of citric acid into 1 quart of water. Citric acid helps prevent discoloration and acts as an anti-microbial agent.
Fill a large kettle one-half to two-thirds full of water. Bring the water to a rolling boil.
Place the vegetables in a wire basket, colander or mesh bag. You can blanch up to 1 quart of vegetables at a time.
Submerge the vegetables in the boiling water, making sure the water covers the vegetables.
As soon as the water reboils, start timing. Adjust the heat to ensure continuous boiling.
Heat according to the time listed in Table 1.
Submerge the container with the vegetables in cold water for the same amount of time as the blanching time).
Drain the vegetables on paper towels.
Drying is not a precise method of food preservation, and the amount of drying time will vary depending on the equipment, moisture content of the vegetables and the humidity in the air.
Spray a cookie sheet or similar flat tray with vegetable spray, or line the tray with plastic wrap or parchment paper and spray with vegetable spray. Another option is to use the specially designed plastic sheets for electric dehydrators, and follow the manufacturer’s directions.
Oven drying: Test your oven to be sure it can maintain a low enough temperature; otherwise, “case hardening” may occur. This is the formation of a “crust” on the food, which prevents the interior from drying properly.
To test your oven, set it to the lowest setting. Place an oven-safe thermometer on the rack where food will be placed. Leave the oven door open 2 to 6 inches. Place a fan near the open door to circulate air. Check the temperature. If your oven can maintain a low enough temperature (140 to 145 F), it may be used for food dehydration. Racks should be 2 inches apart, with at least 3 inches of clearance from the top or bottom to the rack. See Table 1 for approximate drying times. If you would like any more information about freeze drying process, please do not hesitate to contact the professional Vegetable Drying Machine manufacturer - GuanFeng by check below link: https://www.gf-machine.com/product/drying-equipment/