Explain the process of freeze drying with its advantages and disadvantages.

1. 1 · Open in two lines. Freeze drying (lyophilisation) dries frozen food by turning its ice directly into vapour under high vacuum. It gives the best-quality dried foods.
2. 2 · Principle. Below the triple point of water (0.01 °C, about 611 Pa), ice sublimes without melting.
3. 3 · Steps.
   • Prepare and freeze solid.
   • Primary drying: vacuum and gentle heat; ice sublimes; vapour caught on a cold condenser.
   • Secondary drying: slight warming removes bound water.
   • Release the vacuum with nitrogen; pack moisture-proof.
4. 4 · Draw the freeze dryer. [Diagram: vacuum chamber with heated shelves of frozen food → cold condenser → vacuum pump]
5. 5 · Uses. Instant coffee, fruit pieces, soups, prawns, army and space rations, probiotic cultures.
6. 6 · Advantages and disadvantages.
   • Shape, colour, flavour and vitamins kept; porous; quick rehydration; light; shelf-stable.
   • Costly, slow, fragile, absorbs moisture; needs nitrogen packing.
7. 7 · Nutrition link. Freeze-dried fruits keep most of their vitamin C and carotene; used in rations for soldiers and astronauts.
8. 8 · Close in one line. Freeze drying trades high cost for the closest-to-fresh dried product.

Scoring tip: Write 'sublimation' and the triple point in your first lines and draw the chamber-condenser-pump layout.