Freeze drying is a method of food dehydration that has been primarily used in commercial food manufacturing but is starting to make its way into home kitchens.
Freeze dried watermelon.
Adair Hoover, ©2023 HGIC, Clemson University
Freeze drying works in a three-stage process. The food is frozen and then undergoes 2 stages of drying. This is perfectly explained by Hirneisen and McGeehan in their publication “Let’s Preserve: Freeze Drying”. Their description of the process is as follows. Trays of food are placed in the freeze dryer’s chamber. The machine freezes the food to a temperature between -30 °F and -50 °F. Next, a vacuum pump pulls the air out of the chamber and the trays are slightly heated. As the water in the food heats, it sublimates (ice is directly converted from solid to water vapor) and is removed from the product. This process automatically removes up to 98 percent of the water in the food.
Freeze dryers may be purchased for home use but are relatively expensive, ranging from $2000 – $10,000. They are also more expensive to operate. However, they yield the best quality of the types of dehydration. A research study, C. Ratti 2001, reports that the shrinkage of freeze-dried food is 5% – 15% compared to around 80% for air drying.
The final product is a shelf-stable food. Many types of foods can be freeze dried, including fruits and vegetables, hard candies, full meals, and ice cream.
References:
- C Ratti, Hot air and freeze-drying of high-value foods: a review, Journal of Food Engineering, Volume 49, Issue 4, 2001, Pages 311-319.
- Hirneisen, A., McGeehan, N., Let’s Preserve: Freeze Drying, Penn State Extension, 2021 https://extension.psu.edu/lets-preserve-freeze-drying
