Freeze-drying (lyophilization) transforms snacks into unique, crunchy treats. The process removes nearly all water content, extending shelf life while maintaining flavor and structure. When applied to certain candies, this dehydration produces a dramatic result: the candy puffs up. Understanding this requires examining the scientific steps, the internal makeup of the candy, and the physics of the phase change involved.
The Three Stages of Freeze Drying
Freezing
Lyophilization begins with the Freezing phase, subjecting the candy to extremely low temperatures (often -50°C to -80°C) to turn all water content into solid ice. This step is controlled to form small, uniform ice crystals, preventing structural damage. The freezing step traps water molecules in a solid state, preparing them for removal.
Primary Drying
The process then moves into the Primary Drying phase, where expansion is initiated. The frozen candy is placed inside a vacuum chamber where air pressure is drastically reduced below the triple point of water. Reducing the pressure allows the trapped ice to transition directly into water vapor, bypassing the liquid state entirely (sublimation).
Secondary Drying
The final step is Secondary Drying (desorption), which removes any remaining, tightly bound water molecules. This is achieved by slowly increasing the temperature while maintaining the deep vacuum. This ensures the final product contains less than two percent moisture, making it shelf-stable and giving it its final, brittle texture.
The Internal Structure of Candy
The dramatic puffing effect depends entirely on the candy’s physical and chemical structure. Candies that expand well (like marshmallows, taffy, or chewy sweets) typically have a high sugar concentration and a flexible, elastic structure. This high sugar content forms an amorphous solid, or a glassy matrix.
This sugar matrix is a non-crystalline, rigid structure that becomes pliable under specific conditions. Flexibility is enhanced by ingredients like gelatin or other gelling agents, which provide the necessary structural elasticity to stretch. The candy also contains internal air pockets and moisture crucial for subsequent expansion.
When the candy is frozen, the water inside turns into ice crystals slightly larger than the original water volume. This expansion subtly stresses the surrounding sugar matrix, holding it in a stretched position. The strength and elasticity of this frozen, pliable framework determine its ability to inflate rather than crumble under the vacuum.
Sublimation: The Engine of Expansion
The core reason for expansion lies in the rapid phase transition of water during primary drying. Under high vacuum, the ice trapped within the candy sublimates directly into water vapor. Sublimation is forceful because the volume of water vapor is vastly greater than the volume of the solid ice it replaces.
This sudden conversion generates intense internal pressure within the candy. The rapidly escaping water vapor acts as a propellant, pushing outward against the flexible, frozen sugar matrix. Since the structure is held pliable by the low temperature and vacuum, the pressure forces it to inflate dramatically.
The candy expands outward, creating a highly porous structure. The expansion continues until the ice is fully sublimated and the internal pressure equalizes with the vacuum environment.
At this point, the sugar matrix, now completely dry and filled with tiny voids where the ice used to be, hardens into its new, puffed-up form. This final, light, and airy structure gives the finished product its characteristic crunch and lower density.