Food sterilization is a process used for the destruction of all micro-organisms, including spore-forming bacteria. It also extends the shelf life of food. Finally, sterilization reduces the risk of microbiological contamination.

Zoom on the characteristics and the 4 steps of the food sterilization cycle.

The sterilization process: characteristics and objectives

Nicolas Appert developed the method of sterilization during the 18th century. Subsequently, scientists use steam to sterilize food. This led to the invention of sterilization by the autoclave in the early twentieth century.

Some reminders on the principles of this process of food preservation by heat.

The purpose of food sterilization

The purpose of food sterilization is to increase the shelf life of products. To do this, they are exposed to a temperature higher than 100°C. All types of food products can be sterilized: vegetables, fruits, milk-based recipes, etc. Once sterilized, the preserves can be stored at ambient temperature. Their shelf life can be more than 12 months, depending on the recipe.

Food safety and hygiene

The process of food sterilization allows consumers to be protected from hazards related to food processes. This method guarantees the safety of food. All pathogenic micro-organisms in the food are destroyed by sterilization.

Water spray sterilization vs. steam process

Two different processes can be used for sterilization by the autoclave: the water spray process and the steam process. Here is a comparison of these two techniques.

The process of sterilization by water spraying

The process by water spraying has in general a better heat distribution in the retort. It is used rather for fragile products, in order to avoid the impact of the more “aggressive” heat of steam.

The steam process for sterilization

The steam process is used rather for products to be heated quickly and strongly (example: pet food). It is likely to consume less energy than water spraying, but it all depends on the installation.

Food preparation before sterilization

Before sterilization, the food  must be prepared according to the rules of hygiene and food safety. The products must then be packed in appropriate containers (e.g. cans, Weck jars). They are then hermetically sealed to ensure effective sterilization.

Phase 1: purging and filling process of the food retort

The purging phase allows the air to be removed from the retort (for the steam process). Air being insulating, its presence causes a bad distribution of heat in the retort.

Phase 2: temperature rise of the food retort

This phase allows the temperature of the retort to rise. The heating time depends on the energy input. Each temperature phase is associated with a back pressure. As the products heat up, the pressure inside the packaging increases. To counteract the swelling and avoid deformation of the package, a counter pressure is added.

A degassing phase may be necessary. It allows the vacuuming of glass jars with a rubber (Weck or Le Parfait jars). A first heating phase takes place up to 90°C at 0 bar. Then, these parameters are maintained for 10 minutes. This phase allows the air to escape from jars. A bar of back pressure is then added as quickly as possible to close the jars. They will then be under vacuum.

Phase 3: temperature maintenance stage of the food retort

The phase of maintaining the sterilization temperature corresponds to the cooking phase. It is the most important and critical phase of the heat treatment. The respect of the time/temperature couple is mandatory. The diffusion of heat in the products is done in two ways.

• By conduction: thermal conduction is carried out from the hottest areas to the coldest. The heat transfer is slower and less homogeneous in the product.
• By convection: natural movements in fluid products combined with a forced diffusion by agitation allow a fast and homogeneous heat transfer in the product.

Phase 4: cooling of the food retort

The cooling depends of the cold water flow injected, but also of the products to be sterilized. It allows to decrease the temperature of the retort and the products. The beginning of the cooling phase is critical. Indeed, the thermal shock can deteriorate the packaging or have an influence on the texture of the products.

The advantages of the sterilization technique for the quality of canned goods

The technique of appertization or sterilization has many advantages for the preservation of food. The jars or preserves produced can be stored easily, without spending more energy; no need to put them in the refrigerator or freezer, for example. The glass only needs to be protected from violent shocks to ensure the safety of the food contained. The variety of products that can be subjected to this heat treatment is also among its advantages.

At the end of the process of sterilization by the autoclave food, the sterilized canned food must be stored in appropriate conditions. The objective is to maintain their quality. If the steps of the cycle are well respected and the lid is watertight, you can keep them for several months. If you have any questions about  the sterilization by autoclave process , contact us !