Let’s first address the elephant in the room, a.k.a. the title. It is a common “habit” in industry, literature, and everyday communication to misuse the word “pasteurization” to describe any food processing that involves heating and thermal processing. As described in the following paragraphs, that is technically wrong. There are four different thermal processing and heating types: sterilization, commercial sterilization, pasteurization, and blanching. These heat treatment processes aim to produce a safe final food product, a condition achieved by applying heat and rendering the finished food product free of viable microorganisms of public health significance or causing economically significant spoilage of the food in storage and distribution.
On the other hand, these thermal processing types are differentiated based on two criteria: i) the target microorganisms to be controlled and ii) the storage and distribution conditions of the final food product. As mentioned above, it is essential to recognize that there are various degrees of preservation by heating and those commercial heat-preserved foods are not truly sterile. The following paragraphs provide the definitions of the different methods of heating preservation.
Sterilization
Sterilization refers to the complete destruction of All microorganisms. Because of the resistance of certain bacterial spores to heat, this frequently requires treatment of at least 121.1 °C (250 °F) of wet heat for 15 min or its equivalent. For food sterilization, every food particle must receive this heat treatment (Potter and Hotchkiss, 1995). Suppose a can of food is to be sterilized. In that case, immersing it into a 121.1 °C pressure cooker or retort for 15 min will not be sufficient because of the relatively slow heat transfer rate through the container and within the food; the size of the can used also plays a critical role in the time needed to achieve sterility. Depending on the size of the can, the adequate time to attain actual sterility may be several hours. During this time, there can be many changes in the food, reducing its quality. Fortunately, most foods need not be completely sterile to be safe and keep quality (Potter and Hotchkiss, 1995).
Commercially Sterile
The term commercially sterile or "sterile" (in quotes), sometimes seen in the literature, means the degree of sterilization at which all pathogenic and toxin-forming organisms have been destroyed (David et al., 2023). Commercial sterility is defined as the condition achieved by the application of heat or other sterilizing agents which render the finished food product, packaging, and equipment free of viable microorganisms (including spores) of public health significance as well as all other types of organisms which if present could grow in the product and produce spoilage under normal non-refrigerated conditions of storage and distribution. Most commercially sterile food products have 12-18 months of shelf life under normal storage and handling conditions (David et al., 2023).
The state of commercial sterility of the foods can be achieved by using one of the following thermal processes: i) Aseptic processing, ii) Canning, and iii) Hot-fill-hold. The proper selection and design of the processes to achieve the state of commercial sterility for the final food are based on the type of food and its characteristics (pH, water activity (aw), viscosity, etc.) (David et al., 2023).
Pasteurization
Pasteurization involves a milder (lower temperature and shorter time) treatment than the previously described heating methods, which is essential for food safety, preserving product quality, and extending its shelf life. Pasteurization treatment, typically at a temperature below boiling point, is an effective preservation method to destroy all vegetative pathogens, viruses, and bacteria, but not spore-formers (Ramesh, 2007). Pasteurization is a treatment process also used to inactivate enzymes and kill relatively heat-sensitive microorganisms that cause spoilage, with minimal changes in food quality, such as sensory and nutritional properties. Handling and storage of refrigerated products must be under refrigerated conditions (at or below 4 °C or 41 °F). Pasteurized products have a shelf life of between 1 week to several months, a factor that depends on the product type (Ramesh, 2007).
There are several types of pasteurization: i) in-package pasteurization: inside packages, heating to sterility level is not required; ii) pasteurization before packaging, used for heat-sensitive foods such as milk. Furthermore, non-thermal processes, namely High-Pressure processes, can also produce pasteurized (refrigerated) products, destroying vegetative bacteria, yeasts, molds, and viruses at low temperatures (Ramesh, 2007).
Blanching
Blanching is a heat treatment applied via hot water or steam (~212 °F, ~100 °C) primarily used to reduce surface microbial contamination, inactivate enzymes in fruits and vegetables, and soften vegetable tissues (Potter and Hotchkiss, 1995). This is standard practice when such products are further to be processed. Depending on its severity (temperature-time), blanching will also destroy some heat-sensitive vegetative microorganisms. The heating time depends on the type of fruit or vegetable, the method of heating, the size of the fruits or vegetables, or the temperature of the heating medium.
Selecting heat treatments
Thermal processing is the primary method to produce industrially safe, shelf-stable, or refrigerated food products. Thermal processing is an effective preservation technique that consists of heating the product at a high temperature and keeping it at that temperature for a specific amount of time in an accurately designed and well-executed process to achieve the desired finished product (David et al., 2023). The intensity, how high the temperature, and how long the holding time will be, is determined by nature (liquid, multiphase or solid food) and characteristics of the product (pH, aw), the conditions to be distributed and stored (shelf stable or refrigerated) and the final product quality based on the degradation of product’s characteristics during processing, distribution, and storage. Typically, all products of low-acid foods (pH>4.6 and aw>0.85) or high-acid (pH≤4.6 and aw>0.85) that are designed as shelf-stable products undergo a commercial sterile process (FDA, 2023). While heat-sensitive foods, where the main goal is to maintain most of the nutrients and organoleptic characteristics, undergo the milder heat treatment of pasteurization (David et al., 2023).
Furthermore, tools such as thermobacteriology (the kinetics behind microorganism resistance to heat), coupled with advanced heating methods, such as High-Temperature-Short-Time (HTST) and Ultra Heat Treatment (UHT), and new developments in heating systems, such as more efficient heat exchangers designs, microwave, and ohmic heating systems have enabled food processors to effectively produce safe products while maintaining most of food product quality characteristic (color, nutrients, texture, etc.) (Fig. 1) (David et al., 2023). Figure 1 shows the optimal thermal processing conditions (temperature-time) to achieve commercial sterility of the product while maintaining most of the nutrients (vitamin). Finally, it is important to mention that the proper thermal treatment to be applied to a specific product can be conducted after consultation with experts in the field, known as “Process Authorities.” A process authority is an expert in thermobacteriology, heat, and mass transfer, capable of providing the proper thermal process design to ensure final food product safety and quality.
References
David J.R.D., Coronel P.M., Simunovic J., 2023. Handbook of Aseptic Processing and Packaging 3rd Edition, CRC Press, Boca Raton, FL.
FDA, 2023. Acidified & Low-Acid Canned Foods Guidance Documents & Regulatory Information. Accessed on January 15, 2023. https://www.fda.gov/food/guidance-documents-regulatory-information-topic-food-and-dietary-supplements/acidified-low-acid-canned-foods-guidance-documents-regulatory-information
Potter, N.N. and Hotchkiss, J.H. 1995. Food Science, 5th edition, Springer Science+Business Media, Inc., New York, p. 138-139.
Ramesh, M.N. 2007. Pasteurization and Food Preservation, Chapter 23, In: Handbook of Food Preservation, 2nd edition, M. Shafiur Rahman, editor, CRC Press, Boca Raton, FL, p. 571-573.
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