Freeze Drying

Freeze drying (lyophilization) is the drying of material in the frozen state. It is usually carried out under vacuum, at absolute pressures that readily permit ice to sublime (change directly from solid to vapor). Absolute pressures used in food freeze drying range between 50 fim Hg and 1,500 /um Hg.

Moist foods contain water that freezes (ie, forms ice) when suitably cooled, and smaller amounts of bound water that do not, even at — 60°C, but instead form part of a solute-rich matrix that becomes extremely viscous and stiffens as temperature drops. In frozen foods, ice is embedded in that matrix. Except for clear juices and extracts, frozen foods also contain other solid components, such as cell walls, membranes, and subcellular particles. As noted in the article Freeze concentration, food solutes depress the freezing point of water. The initial equilibrium—freezing points of foods usually lie between — 0.5°C and -2°C. As foods freeze, the solutes they contain become more concentrated, and the freezing point depression increases. Therefore, successive portions of a food's water content freeze at lower and lower temperatures. Freezing is complete when the last freezable water changes into ice. In many foods, this effectively occurs between -20°C and — 30°C.

During food freeze drying, ice sublimes and bound water desorbs as vapor. Most bound water remains when the last ice has sublimed; almost all has to be removed to provide stable, freeze-dried food. To do this, the product temperatures are raised, and added vacuum drying time is provided after the last ice sublimes. Ice-containing regions in foods remain fully frozen during freeze drying; temperatures well above freezing exist in parts of regions whose ice content has already sublimed.

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