Describes how the actual heat transfer occurs. This is a major consideration in freezer design.
There are three modes of heat transfer:
- Vapor-to-solid heat transfer - This is cold vapor being passed over a warm solid food product (could be N2, CO2, or mechanical refrigeration). This is sometimes referred to as "vapor-stripping", because BTUs are being "stripped" from the cold vapor.
- Solid-to-solid heat transfer - This is solid dry ice particles (CO2 snow) being directed at the food product (CO2 only).
- Liquid to solid heat transfer - Liquid nitrogen can be sprayed on the product, or the product can be placed in a liquid nitrogen bath (LN2 only).
To get a sense of the impact of heat transfer rates, envision this:
- Burying your hand in a bucket that contained 40°F sand (solid-to-solid).
- Putting your hand in a bucket that contained 40°F air (vapor-to-solid).
- Plunging your hand into a bucket of 40°F water (liquid-to-solid).
Obviously, 40°F water would be very unpleasant immediately, the sand wouldn't be far behind, and after a couple of minutes, the 40°F air would be equally uncomfortable. All three modes transfer heat out of your hand, and the temperature in the bucket is the same, but the heat is transferred at different rates.
Offline Freezing and Inline Freezing
Food freezing broad categories:
Offline freezing refers to physically removing the food product from the production line in order to freeze it in a separate manufacturing step or location (such as a cold storage area or commercial blast freezer).
Inline freezing refers to a process that freezes products as part of the continuous manufacturing assembly line, so that products exit the assembly line frozen.
Inline Freezing Systems
Refrigeration can be mechanically produced (in which case you are buying electrical power and transforming that power into refrigeration). Or refrigeration can be purchased as a pre-cooled substance, such as liquid nitrogen (LN2) or liquid carbon dioxide (LCO2) and maintained in storage tanks. Freezing systems that use either of these refrigerants are generally referred to as being cryogenic* freezing systems.
*The National Institute of Standards and Technology has suggested that the term "cryogenics" be applied to temperatures below -238° F. However, correctly or incorrectly, the term "cryogenic freezing" is widely used in food processing to identify freezing systems using either liquid nitrogen (-320°F) or carbon dioxide (-108°F as a solid).
Several important concepts to get you on the right footing
The Freezing Process
Pure water freezes at 0°C. The water in most foods contain dissolved salts and proteins, this causes the freezing point of water to be lowered. The water freezes out as pure ice, thus the concentration increases in the remaining free water-further depressing the freezing point. At about -5°C, one third of the water could still be unfrozen. Even at –30°C, 10% of the water is unfrozen. Thus, some spoilage mechanisms may proceed, but at very much slower rates.
Latent Heat - Changing From Water To Ice
Between about 0°C and -5°C, the water is freezing. To change from free water to solid ice, a relatively large amount of energy needs to be removed. This energy is known as the latent heat of freezing. Latent heat is the energy required to cause a change of state, from liquid to solid.
Sensible Heat - Changing From Warm Water To Cold Water
Complimentary to latent heat is the concept of sensible heat. Sensible heat is the amount of energy removed to change temperature without a change of state. That is, for example, the amount of heat needed to be removed to lower the temperature of water, but not freeze, sensible heat is removed between 40°C and 0°C.
Heat Removal Below Freezing
Below about -5°C, the energy removed is a combination of latent heat and sensible heat. Latent heat is removed to freeze the remaining free water. Sensible heat is removed to lower the temperature of both the free water remaining unfrozen and the ice.
Heat Removal At Different Temperatures
When proposing a freezer, a freezer manufacturer should always state the amount of heat, in kJ/kg, the freezer will extract as well as the product outlet temperature and the time taken to reach this temperature. Note that to change temperature from 0°C to -10°C (or tem degrees) requires SEVEN TIMES more heat to be removed than to change from +10°C to 0°C (or ten degrees).
What is Temperature?
Temperature is a convenient method of measuring the amount of heat (or energy). The higher the temperature, the higher the energy content.
Care must be taken when measuring temperature. Use a thin, needle-like probe. Ensure it is inserted into the product.
Measuring the air or water surrounding a sample of food product is not measuring the temperature of the food. Measure more than one sample. Measure each in a few places, for example, just below the surface and in the middle of the thickest part. If the measurements are different, then a different technique, measuring the EQUILIBRATED TEMPERATURE is required.
The equilibrated temperature is measured when all parts of the product sample are at the same temperature. This can be achieved by placing the product sample, for example, a fish fillet, in an insulated container (such as an Esky) for a few minutes. After a few minutes, all parts of the fillet will be the same temperature.