8 min readPathway sheetOften a 10-mark answer; the principle of freeze drying is a 2-mark question

Freeze drying (lyophilisation)

Freeze drying freezes food, then removes the ice by sublimation under vacuum. It gives the best-quality dried food, but is slow and costly.

PrincipleSublimation: ice turns to vapour without melting, below the triple point

Triple point of water0.01 °C and about 611 Pa (about 4.6 mm Hg)

Final moistureVery low: a few per cent

ProductsInstant coffee, soups, fruit pieces, prawns, army and space rations

Principle

Ice, liquid water and vapour can exist together only at water's triple point: 0.01 °C and about 611 Pa (about 4.6 mm Hg, roughly 0.006 of normal air pressure). Below this pressure, ice does not melt when warmed; it turns straight into vapour. This is sublimation. In freeze drying the food is first frozen, then placed under high vacuum and gently warmed so the ice sublimes. Because the food stays frozen and rigid, it keeps its shape, colour, flavour and nutrients, and the escaping ice leaves a porous, sponge-like structure that rehydrates in minutes.

Steps in freeze drying

1
Prepare the food: wash, cut, blanch if needed
liquids such as coffee extract may be concentrated first
2
Freeze solid
quick freezing gives small crystals and fine pores
3★
Primary drying: apply high vacuum and gentle heat
ice sublimes; most of the water leaves here
4
Vapour freezes onto a very cold condenser
keeps the vacuum working
5★
Secondary drying (desorption): raise the temperature a little
removes bound water to a few per cent moisture
6
Release the vacuum with dry nitrogen and pack at once
moisture-proof, oxygen-free packs; the product absorbs water readily

★ one-way, controls the pace

Draw it: a freeze dryer

Heat (from shelves or radiant plates) supplies the energy for sublimation; the condenser and pump keep the pressure below the triple point.

Advantages and disadvantages

AdvantagesBest quality of all drying methods: shape, colour, aroma and heat-sensitive vitamins kept; little shrinkage; porous, so it rehydrates quickly; very low moisture makes it light and shelf-stable at room temperature when well packed.

DisadvantagesVery costly equipment and energy (freezing, vacuum and heat); slow, many hours per batch; the product is fragile and absorbs moisture easily; its large porous surface oxidises easily, so it needs nitrogen-flushed, moisture-proof packing.

Uses of freeze drying

Food group	Examples
Beverages	Premium instant coffee, tea
Fruits and vegetables	Strawberry, mango and banana pieces for breakfast cereals; mushrooms; peas
Meat and seafood	Prawns and chicken pieces for soups and ready meals
Ready meals and rations	Soups, curries and rice for soldiers, mountaineers and astronauts (DFRL, Mysuru has developed freeze-dried fruit-juice powders for defence use)
Biological products	Probiotic and starter cultures, enzymes, vaccines

In food and nutrition

Freeze-dried fruits and vegetables keep most of their vitamin C, carotene and flavour, so freeze-dried amla or mango powder adds real vitamins. The high cost limits its use to premium products, rations for soldiers and astronauts, and probiotic cultures.

Exam angle

2 marks: “What is the principle of freeze drying?” Sublimation: frozen food under high vacuum, below the triple point of water (0.01 °C, about 611 Pa), loses its ice directly as vapour without melting.

5 marks: “Give the advantages and disadvantages of freeze drying.” Advantages: best quality, nutrients kept, porous, quick rehydration, light, shelf-stable. Disadvantages: costly, slow, fragile, absorbs moisture, needs special packing; examples.

10 marks: “Explain the process of freeze drying with a neat diagram.” Principle (sublimation, triple point); steps (freeze, primary drying, condenser, secondary drying, packing); diagram; uses; advantages and disadvantages.

Remember it

Freeze drying = Freeze, Vacuum, Sublime, Desorb, Pack: ice goes straight to vapour.