To freeze-dry — properly, lyophilise — is to preserve food by first freezing it solid and then placing it under strong vacuum so that the ice turns directly to vapour without melting, a process called sublimation. With water removed this way, the food’s structure, colour, flavour, and nutrients survive far better than with heat drying, while the near-absence of water halts microbial growth and enzyme activity. The result is a light, porous product that rehydrates quickly: instant coffee, expedition meals, astronaut food, dried fruit for cereals, and starter cultures. Freeze-drying is slow and energy-hungry, making it one of the costliest dehydration methods, reserved for high-value products where quality justifies the price.