Freezing systems can be classified according to the manner in which they extract heat from the product:
4. cryogenic freezing: the food is maintained below – 60°C by spraying liquid nitrogen (-196 °C) or liquid carbon dioxide (-79 °C) in the cooling room. NB: the drawback is a large management costs: 0.3 to 1.5 kg of liquid nitrogen are required for 1 kg of frozen food;
5. cryomechanical freezing: in this case the food it is exposed to a cryogenic treatment first (fast formation of small crystals) and then cooled with a mechanical process;
The last two methods (cryogenic and cryomechanical) can be considered mixed convection-conduction processes.
In general, the use of air as a secondary fluid in the freezing of foodallows to use rather simple industrial plant design, providing high flexibility of use, allowing in general to treat a wide variety of shapes and sizes, accepting bulk products. It is probably the most suitable for individual quick freezing (IQF).
By contrast, among the associated disadvantages, we can find:
Regarding the last point, the loss of water can degrade the quality of the food, and is the main cause of the accumulation of ice on the coil, making it more costly to manage.
Economic damage that occurs due to the loss of weight of the commodity is not to be underestimated, since it is in relation to both the loss of quality and the decrease of the marketable mass: we must consider that the annual production of a freezing plant may reach several thousand tonnes and, considering for example the price of 1€ per kg, a weight loss of 1% will cost 1000€ for every 100 tonnes of product.
The rate of dehydration depends on the physical characteristics of the product like porosity and diffusion coefficients, as well as on the heat transfer parameters.
About this point, it is clear that the greater the resistance to the diffusion of water vapor from the outer layers of the foodstuff, the less is the loss of water.
Food protected by a leathery integument like peas and beans, lose water in smaller quantities then carrots in small cubes, whose internal tissues are in direct contact with the air.
The heat exchange conditions also influence the phenomenon.
Generally speaking, it is more advantageous to lower the temperature of the refrigerant, since the driving force behind the exchange of sensible heat between the coolant and the commodity is the temperature gradient;
In percentage terms it increases more than the difference between the water pressure at the same temperature, which represents the driving force in the process of evaporation and sublimation:
This is related to the flattening at low temperatures of the water saturation curve.
Freezing air plants are designed in multiple configurations, some of which are extremely basic like cold rooms or ventilated tunnel operated in batch or continuous mode.
In-line freezers, i.e. freezers with continuous flow of food, operating according to the Individually Quick Frozen technique can be considered the more efficient and modern configuration.
These facilities are based entirely on the technique of fluidization: a layer of solid food elements of small size is sustained by aerodynamic forces generated by a crossing stream of cold air, from bottom to top, so that the individual elements are suspended on a fluid bed.
At equilibrium, viscous and pressure forces acting on any single piece equal roughly the weight of the product;
As each element is not at rest but dynamically describing random trajectories, with a tendency to follow the direction of gravity, by placing the feed hopper in elevated position respect to the outfall, we obtain a spontaneous progress of the product, as if it were in liquid phase.
21. Liquid Mixing