The critical zone in freezing is between −1 and −5 °C: most of the product’s water turns to ice in this range. If this zone is passed slowly, a few large crystals form and tear the cells; if it is passed quickly, many small crystals form and the texture is preserved. Blast freezing ensures that the product core passes through this zone in at most a few hours; it is standard for meat, fish, bakery products, fruit and ready meals.
Methods: blast freezer room and tunnel (air), IQF fluidised bed (small pieces), plate freezer (fish blocks, contact), cryogenic (liquid nitrogen, very fast, expensive). In air blast freezing, what determines the time is the thickness and packaging of the product: a 5 cm thick piece of meat freezes in 4–6 hours, a 20 cm carcass piece in 24 hours; cardboard packaging extends the time.
The capacity calculation is based on the daily quantity to be frozen, entry and exit temperature and the target time; the product heat load is the sum of sensible + latent + frozen sensible heat. Bringing 500 kg of chicken from +4 to −18 °C in 8 hours takes about 17 kW of capacity. The cooling load calculator does this calculation; the food storage conditions guide gives the storage temperatures and times of frozen products.