Freezing: principle and methods
Freezing turns most of a food's water into ice at −18 °C or below, stopping microbes. Quick freezing forms small ice crystals and keeps texture better than slow freezing.
How freezing preserves
Most foods begin to freeze a little below 0 °C, at about −1 to −3 °C, because dissolved salts and sugars lower the freezing point. As water turns to ice, the liquid left behind becomes very concentrated, so water activity falls and microbes cannot grow; the cold also slows enzymes. Microbes generally stop growing below about −10 °C; −18 °C is the standard storage temperature because enzyme and chemical changes are also very slow there. Freezing is bacteriostatic: many microbes survive and grow again after thawing. Vegetables are blanched first to stop enzymes; fruits are packed in sugar or syrup with ascorbic acid to prevent browning.
Slow (sharp) freezing vs quick freezing
Industry prefers quick freezing; home freezers freeze slowly.
Methods of commercial freezing
| Method | How it works | Examples |
|---|---|---|
| Still-air (sharp) freezing | Food placed in cold rooms or home freezers with still air | Meat, home freezing |
| Air-blast freezing | Very cold air (about −30 to −40 °C) blown fast over food on trays or trolleys | Most frozen foods and ready meals |
| Fluidised-bed freezing (IQF) | Cold air blown up through a bed of small pieces so each floats and freezes separately (individually quick frozen) | Peas, sweet corn, diced vegetables, prawns |
| Plate (contact) freezing | Packed food pressed between hollow metal plates through which refrigerant flows | Blocks of fish, prawns and meat |
| Immersion freezing | Packed food dipped in cold brine, sugar solution or glycol | Poultry, cans of juice concentrate |
| Cryogenic freezing | Liquid nitrogen (−196 °C) sprayed on or food dipped; or solid or liquid carbon dioxide (−78.5 °C) | Shrimps, strawberries, mushrooms; very fast, tiny crystals |
Freezing vegetables, e.g. green peas
- 1Select tender, fresh produce; shell or cut
- 2Wash and grade by size
- 3Blanch, then cool at onceinactivates enzymes
- 4Drain wellsurface water forms ice lumps
- 5★Quick-freeze, e.g. IQF in a fluidised-bed freezer
- 6Pack in moisture-proof, airtight packsprevents freezer burn
- 7★Store at −18 °C or colder; keep the cold chain unbroken
★ one-way, controls the pace
Quick-frozen green peas, sweet corn and mixed vegetables often hold more vitamin C than 'fresh' produce that has sat in a market for days. Frozen foods help hostel and hospital kitchens plan vegetables out of season, provided the cold chain does not break.
2 marks: “Differentiate slow and quick freezing.” Slow freezing takes 3-72 hours and forms large ice crystals between cells, causing drip; quick freezing passes −1 to −5 °C in 30 minutes or less and forms small crystals inside cells.
5 marks: “What is IQF? Explain with examples.” Individually quick frozen: small pieces frozen separately in a fluidised-bed freezer so they stay free-flowing; peas, corn, prawns; advantages (quality, portion use).
10 marks: “Explain the principle and methods of freezing foods.” Principle (ice formation, low aw, −18 °C); slow vs quick freezing; methods (still air, blast, fluidised bed, plate, immersion, cryogenic); pre-treatments; steps for vegetables.
Freezers: Air, Blast, Fluid bed, Plate, Immersion, Cryogenic: 'A Big Frozen Pea Is Cold.'