What is freeze-drying? Are freeze-dried foods safe and healthy?

Freeze-drying food has a long history, possibly more than a thousand years old, as peoples living near mountains would leave produce out in favorable locations, where the combination of cold temperatures and high altitudes (lower pressure) would enable the water in foods to freeze, then sublimate, i.e. the water in the food changes from ice to evaporated gas, without passing through the liquid form. The low-water-content food left behind would be much lighter, still resemble the original food (especially with the original colors and thus nutrients), and last much longer.

Modern freeze-drying starts with the frozen produce (just like in supermarket freezers), then puts it in a large vacuum chamber, where the frozen water inside the food sublimates under controlled conditions. The resulting freeze-dried foods are lightweight, retain the original colors and tastes, don’t spoil for years, and most importantly, retain the macronutrients, micronutrients (vitamins and minerals), and phytonutrients remarkably well. Studies have shown that freeze-dried foods often retain over 90% of the original fresh food’s nutrition, much more than other drying techniques (air-drying, heat-drying) which usually damage micronutrients and phytonutrients — as easily seen by the large changes in the food’s colors.

Like frozen food, freeze-dried foods are first harvested at peak ripeness to have better taste and nutrients. But typical store-bought “fresh” fruits and vegetables are harvested early, artificially ripened, and lose up to half of their nutrient value sitting in the fridge.

Freeze-dried food has long been consumed by people in demanding contexts like mountaineering and space flights, where lightweight and high nutrition food is absolutely essential. But you don’t need to be an astronaut to benefit from its convenience and super nutrition!

A broad review of freeze-dried foods vs other drying methods, in preserving antioxidants and phytonutrients: A Review on the Effect of Drying on Antioxidant Potential of Fruits and Vegetables (2016)

Typical studies on fresh vs frozen nutrition include: https://www.sciencedirect.com/science/article/pii/S0889157517300418 and https://onlinelibrary.wiley.com/doi/full/10.1111/j.1365-2621.1999.tb15943.x