Explain the principle and methods of freezing foods.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Open in two lines. Freezing preserves food by turning most of its water into ice and holding it at −18 °C or colder, so microbes cannot grow and enzymes slow down.
2 · Principle.
Foods freeze at about −1 to −3 °C; ice formation lowers water activity.
Microbes stop growing below about −10 °C; −18 °C slows enzymes too.
Bacteriostatic: many microbes survive thawing.
3 · Slow vs quick freezing.
Slow: 3-72 h, large crystals between cells, drip loss.
Quick: through −1 to −5 °C in 30 min or less, small crystals, better quality.
4 · Draw an ice-crystal sketch. [Sketch: cells with large crystals between them (slow) vs small crystals inside them (quick)]
5 · Methods.
Still-air and air-blast freezing.
Fluidised-bed (IQF) for peas and prawns.
Plate (contact) freezing for blocks.
Immersion in cold brine; cryogenic with liquid nitrogen.
6 · Pre-treatment and packing. Blanch vegetables; sugar or syrup with ascorbic acid for fruits; moisture-proof packs; unbroken cold chain.
7 · Nutrition link. Quick-frozen vegetables keep much of their vitamin C and let institutions serve vegetables out of season.
8 · Close in one line. Quick freezing with proper packing is the closest thing to fresh food over months.
Scoring tip: The slow-vs-quick crystal sketch is worth drawing in every freezing answer.
