Cold Preservation of Foods – Chilling

Chilling is an important activity in food processing. Foods are chilled to extend shelf life by reducing biochemical reactions and microbial activity. Temperature control is essential in order to prevent spoilage and food safety concerns during storage. Foods are chilled using mechanical and cryogenic refrigeration systems.

Mechanical Refrigeration

The diagram of a simple refrigeration system is shown below. The principle it illustrates is the same for both domestic and commercial refrigeration. The purpose of the refrigeration cycle is to pump a refrigerant (coolant) around the system so that heat can be captured and then removed. It is a closed continuous system consisting of the following parts: receiver, metering device (expansion valve), evaporator, compressor, and condenser. The receiver is where excess refrigerant is stored. It is stored in the liquid state and is at high pressure. The high pressure forces the refrigerant through the expansion valve which is responsible for dropping the pressure to a low pressure. A lower pressure results in much lower temperature than its surroundings, which enables heat to be taken up in the evaporator. This is therefore where the cooling of the refrigerator takes place. Cooling is achieved by the absorption of latent heat – the type of heat that is absorbed when a material undergoes a phase change (in this case, going from a liquid to a gas). As a result of absorbing heat, the refrigerant evaporates to a gas in the evaporator. The gas is pumped into the compressor which compresses, or increase the pressure of the gas so that it can return to its liquid state. In the process, the temperature also increases. The heat is removed in the heat exchange component called the condenser which follows. Hence the refrigerant returns to a low temperature, high pressure liquid and the cycle is ready to start again.

Refrigeration system. Image source: berg-group.com

Refrigerants in mechanical refrigerators should have the following properties:

1. Low boiling point – for easy vaporization and hence removal of latent heat
2. High latent heat of vaporization – this means that  it will be able to absorb a large amount of heat before it changes to a gas
3. A dense vapor – to reduce the amount of energy the compressor will need to liquify it
4. High critical temperature – this is the temperature at or above which the vapor cannot be liquified. If the critical temperature is too low, the compressor will not be able to liquify it
5. Low toxicity
6. Non-flamable
7. Environmentally friendly
8. Easy leak detection
9. Low cost

The current widely used refrigerants include hydrofluorocarbons (HFCs) such as 1,1,1,2-Tetrafluoroethane (R-134a), and mixtures of different HFCs such as R-407C and R-410A. These have replaced chlorofluorocarbons (CFCs) which athough are low-cost, non-toxic and non-flammable are found to be ozone-depleting. Examples of other refrigerants include ammonia, propane and CO₂. Ammonia is a good refrigerant but is toxic, flammable and cause corrosion of copper pipes. CO₂ is non-toxic and non-flammable, but at high levels can cause asphyxia (deprivation and suffocation due to lack of oxygen). Ammonia, propane and CO₂ have zero ozone depleting potential (ODP) and low global warming potential (GWP) compared to HFCs).

Cryogenic Refrigeration

Cryogenic refrigeration is spraying or immersing the food directly with liquid nitrogen or carbon dioxide (liquid or solid). This intimate contact with the food improves efficiency of cooling by reducing cooling time and increasing output. Liquid nitrogen is often used for freezing rather than for chilling due to the very large temperature gradient between itself and the food being cooled. This is possible because of its very low boiling point which is -194.4^oC compared to -78.5^oC for CO₂.