The four stages — read the section about the four stages.
The triple point of a substance is the pressure and temperature that solid, liquid, and gas can exist at the same time (in equilibrium).
Sublimation is when a substance passes from a solid to a gas. Sublimation can be sped up by cooling a substance and placing it in a vacuum to bring its temperature and pressure below the triple point. When the temperature is increased with the vacuum, the phase shift is from a solid to a gas.
Freeze drying can reduce the moisture composition of food down to between 1-4%.
Changes that happen during freeze drying
Physical changes:
- Freeze speed can affect the cell walls of the material — results in a “mushier” reconstituted product
- Some foods will brown, or otherwise change in appearance — e.g. carrots lose some of their orange color
- The obvious benefit: water content and mass decrease
Chemical changes:
Not much for chemical changes, thus the benefit to freeze drying.