Freezing.
A steak frozen in a domestic freezer spends hours between −1 and −5 °C, where ice crystals grow large. In a blast freezer it passes in under an hour.
At −18 °C no microbe can grow: most of the water is ice and the rest is a concentrated syrup (X.10). Freezing does not sterilise — most bacteria survive and wake on thawing — but it stops spoilage and growth for months; quality, not safety, limits frozen storage. Enzymes keep working slowly even frozen, which is why vegetables are blanched before freezing to kill them (XVI.2). And air dries and oxidises exposed surfaces into the grey, leathery patches of freezer burn: wrap food tightly.
Speed matters for texture. Water in food freezes over a range, mostly between about −1 and −5 °C, the zone of maximum crystal formation. Passed quickly, it forms many small crystals; passed slowly, fewer large ones that pierce cell walls, so thawed food leaks juice and turns soft. Thin pieces, a cold freezer, a metal tray and space for air to circulate all speed it. Thawing needs the opposite care: on the counter the surface sits in the danger zone for hours while the centre is still frozen. The workshop freezes and thaws a steak.
Below: the centre temperature of a 25 mm steak freezing in a domestic freezer and in a blast freezer, with the zone of maximum crystal formation shaded, and the time to thaw it in the fridge, in cold running water and on the counter (thermal bench).
- Explain what freezing does to microbes and enzymes.
- Explain why fast freezing gives better texture.
- Prevent freezer burn.
- Thaw food safely.
Fast in, slow and cold out
In this model a 25 mm steak in a domestic freezer spends 4 h 49 min with its centre between −1 and −5 °C; in a blast freezer, 43 min. That difference is why commercially frozen fish and vegetables often have better texture than home-frozen ones. At home: freeze in thin, flat portions, spread out on a metal tray until hard, then pack tightly with the air pressed out; turn the freezer to its coldest setting a few hours before freezing a lot at once.
Thawing the same steak takes about 14 h 22 min in the fridge, 54 min sealed in cold running water and 4 h 19 min on the counter. A thin steak's surface stays cold while it thaws, but a 10 cm roast on the counter takes 21 h 22 min, and its surface sits above 5 °C for 6 h 13 min before the centre has thawed — hours in the danger zone (XII.3). So thaw in the fridge (plan a day ahead), in cold water changed every half hour, or in the microwave and cook at once. Many foods — thin fish, prawns, dumplings — can be cooked straight from frozen. Refreezing thawed raw food is safe if it thawed in the fridge, at some cost in texture.
Freeze two portions of the same berries: one spread on a tray in a single layer, one in a heap in a tub. Thaw both and compare how much juice leaks out.
Terms
- Zone of maximum crystal formation
- Roughly −1 to −5 °C, where most of a food's water freezes.
- Freezer burn
- Dry, oxidised patches where air reached frozen food.
- Blanching
- Brief boiling before freezing to kill enzymes.
- Quick freezing
- Freezing fast enough to form small ice crystals.
Sources
- Y. Choi and M. R. Okos, "Effects of temperature and composition on the thermal properties of foods", in Food Engineering and Process Applications, vol. 1, Elsevier, 1986, pp. 93–101.
- USDA Food Safety and Inspection Service, "Safe Minimum Internal Temperature Chart" (foodsafety.gov), 2024.
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.