A blast cabinet involves two temperatures: the air it blows and the centre of the product. The air is markedly colder; the literature gives -26 to -40 °C for blast freezer air. The product has to reach -18 °C or colder at its centre, and Codex does not count the process as complete until the thermal centre has stabilised at that value.
Why the air is so cold
The rate at which heat leaves the product depends on the difference between product and air and on air speed. Around 5 m/s has been found economical in batch cabinets, and 10-15 m/s is recommended for continuous lines. Loading matters as much as temperature: without air channels between cartons and pieces, the inside of the load freezes slowly despite fast air and low temperature.
Deciding the end
The decision to stop is made from a probe pushed into the thickest part of the product, not from the cabinet panel.
The probe goes into the centre of the thickest piece; a surface reading comes out lower and makes the job look finished early. The target and endpoint for each product group come from process validation. The batch record carries cabinet air, probe point and result, start and finish time, load quantity and tray spacing. Once freezing ends the product moves to cold storage as quickly as possible, and the transfer time is added to the room record.