The purpose of thermal technics is using thermal energy is to eliminate or minimize microorganisms and to denature (deform) enzymes and toxins to prevent their activity in foods. Thermal methods are the oldest for food preservation. Thermal methods of food storage include blanching, thermization, pasteurization, sterilization, and hot filling.
Objectives of thermal methods to preserve foods
One of the objectives of thermal methods is to eliminate microorganisms, increase food safety, and produce healthy food for consumers. Another goal is to produce ready-made and semi-prepared foods by cooking.
Thermal methods and technics to preserve foods
Blanching
Blanching is a gentle heating process at a temperature of 70-100 °C for 1-15 minutes to fresh foods such as fruits and vegetables by boiling water or steam and is not a complete preservation process alone. Blanching removes a portion of the water from the tissues, improves color, and reduces germs. An important blanching activity is the inactivation of enzymes that can change the color, texture, taste, and nutritional value of materials during storage. This process can be the pre-process or post-process of the main storage process such as dehydration, freezing, and canning.
Thermizasion
Thermizasion is a gentle food storage method designed to increase the shelf life of raw milk. This process is performed at a temperature of 63-65 °C for 15 seconds. This method is used when it is impossible to use raw milk immediately for pasteurization and sterilization. Therefore, this method is used to reduce the microbial load of raw milk, especially psychrotrophic bacteria in milk. These bacteria produce heat-resistant lipase and protease enzymes. These enzymes are not inactivated in the pasteurization process and cause an unpleasant taste in milk. Condensed milk should be stored at a temperature below 8 °C.
Pasteurization
Pasteurization is a heat process with a temperature below 100 °C (65-100) at different times of 1-30 minutes. It is almost classified as a medium heating process. This method is used to preserve liquid and semi-solid foods. In materials with weak acidity (pH> 4.5) such as milk, pasteurization is used to minimize pathogenic microorganisms and increase the life of the product from a few days to a week. In acidic foods (pH <4.5) such as juices, the shelf life of the product increases to several weeks. This is possible by eliminating the damage caused by microorganisms and inactivating enzymes. Pasteurization is not able to remove thermophilic spore bacteria.
Sterilization
Sterilization means the complete elimination of all microorganisms, including spores. These methods are divided into thermal and non-thermal types. This process is mostly used in canning. This process also kills yeasts, molds, vegetative forms, and bacterial spores.
Sterilization is divided into two categories of “in-container sterilization” and “in-flow sterilization”. In-container sterilization involves the canning process. “In-flow sterilization” refers to the aseptic process described in another article. In this type of sterilization, the food is fluid and uses a very high temperature for a short time to sterilize. In this process, a temperature of 130-150 °C is used for a few seconds and fill the previously sterilized containers with a pump.
Hot filling
Hot filling, also known as hot pack, means filling sterilized acidic food into unsterilized containers. These foods are hot enough to commercially sterilize related containers. In this method, the heat of the sterilized foods, because it enters the container before cooling, sterilizes the container. Storage time and process temperature depend on pH and other characteristics of the food. This process is very effective for packaging acidic foods with pH<4.6, because Clostridium botulinum does not produce toxins in this range. If pH>4.6, it should be pasteurized and stored in the refrigerator, and the hot filling method will not be efficient for this type of product. In commercial production, acidic foods must be heated at a temperature of 77-100 °C for about 30-60 seconds and the hot filling process is conducted.
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