What are the basic principles of thermal processing?

1 Types of thermal processing

Various types of thermal processing exist, depending on the temperature, time, and equipment used. For instance, pasteurization is a mild heat treatment that reduces the number of pathogens and spoilage organisms in liquid or semi-liquid foods. It ranges from 60°C to 100°C for a few seconds to a few minutes. Canning is a more severe heat treatment that aims to achieve commercial sterility, and involves temperatures from 100°C to 130°C for several minutes to hours. Retorting is a type of canning that uses a pressurized vessel called a retort to heat the food and the container simultaneously. UHT (ultra-high temperature) processing is another type of pasteurization which uses very high temperatures (135°C to 150°C) for a very short time (1 to 15 seconds). This method is mainly used for dairy products and requires aseptic packaging.

2 Heat transfer and lethality

The effectiveness of thermal processing is determined by the rate of heat transfer from the heating medium, such as steam, water, or air, to the food. This rate is influenced by factors like the temperature difference between the two, the surface area exposed to the heating medium, the shape and size of the food, its composition (moisture and fat content), and agitation. The goal of thermal processing is to achieve a certain level of lethality, which is expressed as a decimal reduction time (D-value). This value measures how much heat treatment reduces the number of microorganisms in the food. To calculate total lethality, multiply the D-value by the number of log cycles desired. For example, if a microorganism's D-value is 10 minutes at 121°C and 12 log cycles are desired, total lethality would be 10 x 12 = 120 minutes.

3 Quality and safety aspects

Thermal processing can have both positive and negative effects on the quality and safety of food products. Positively, it can inactivate microorganisms and enzymes that cause spoilage, as well as create desirable flavors, colors, and aromas. Additionally, it can improve texture and consistency. Negatively, it can lead to loss of nutrients, formation of undesirable flavors, colors, and aromas, and deterioration of texture and consistency. To optimize the quality and safety of thermally processed foods, it is important to select the appropriate type, temperature, and time of thermal processing for each food product. Additionally, high-quality ingredients and packaging materials should be used that can withstand high temperatures and pressures. Moreover, cooling and storage conditions should be controlled to prevent thermal shock and recontamination. Finally, the thermal process should be monitored and validated to ensure compliance with regulatory standards and consumer expectations.