Potential Nutrients Lost Because of Dehydration
Some vitamins, such as A and C, are decreased if they come into contact with air or heat -- while others, such as certain minerals, the B vitamins and vitamin C -- can leach out into cooking water. Vitamin A is light sensitive, so it might be lost if you dehydrate foods in the sun or if you don't store dehydrated foods in a dark place. For example, green, leafy vegetables that are steam-blanched for 5 minutes and then dehydrated in an oven only retain up to 14 percent of their original vitamin C content, between 22 and 71 percent of their original thiamine content and between 20 and 69 percent of their original total beta-carotene content, according to a study published in the Journal of Food Science and Technology in 2013.
Treatments to Limit Nutrient Losses
Using a sulfite treatment -- such as sodium metabisulfite -- will minimize the loss of vitamins A and C but will increase the loss of thiamine and riboflavin; blanching will minimize the loss of thiamine and vitamins A and C due to dehydration, as blanching inactivates the enzymes that increase their loss. Steam-blanching reduces nutrient losses more than blanching in water. Dipping fruits or vegetables in pineapple, orange or lemon juice before dehydrating can help them maintain higher levels of vitamin C throughout the dehydration process, and also helps prevent them from turning brown -- although this isn't as effective as using the sulfite treatment.
Drying Methods to Limit Nutrient Losses
The freeze-drying process results in fewer nutrient losses than using a dehydrator and also results in a product that can be more successfully rehydrated in the future. For example, a study published in the International Journal of Molecular Sciences in 2011 found no significant change in vitamin C levels and -- for most of the fruits tested -- no change in the beta-carotene content in freeze-dried fruits compared to fresh fruit. There were decreases in some beneficial antioxidants called polyphenols, however. This limited loss of nutrients is most likely due to the lower temperatures used in freeze drying, because these are heat-sensitive nutrients.
Keeping Dehydrated Food Safe to Eat
After you dehydrate foods, you need to condition them before you package them for final storage. This means cooling them, putting them in a nonporous container for 10 to 14 days and then stirring or shaking them at least once a day. If condensation occurs, the food isn't fully dehydrated and you need to return it to the oven or dehydrator for further drying. If the food may have been contaminated, it needs to be pasteurized by putting it in an oven at 160 degrees Fahrenheit for 30 minutes or in a 0-degree F freezer in a plastic bag for at least 48 hours. You should store dehydrated foods in airtight glass or plastic containers in a cool, dry, dark place. Foods stored at high temperatures, such as 80 F, will only last a few months, but those kept at temperatures of under 60 F, can last for at least one year. Discard these foods at the first sign of mold or other contaminants.
- University of Missouri Extension: Quality for Keeps: Drying Foods
- Mother Earth News: Freeze-Dried vs. Dehydrated Food
- Ohio State University Extension: Drying Fruits and Vegetables
- International Journal of Molecular Sciences: Effect of Freeze-Drying on the Antioxidant Compounds and Antioxidant Activity of Selected Tropical Fruits
- Journal of Food Science and Technology: Recent Advances in Drying and Dehydration of Fruits and Vegetables: A Review
- Journal of Food Science and Technology: Retention of Nutrients in Green Leafy Vegetables on Dehydration