Wet heat sterilization is a widely researched and widely used sterilization method. When the temperature exceeds the optimal physiological activity range of cells, as the temperature increases, cell metabolism slows down, and cell growth and reproduction eventually stop. Each type of cell has an upper limit to temperature tolerance. Once the temperature exceeds this limit, the proteins, enzymes, and nucleic acids that play a vital role will be destroyed, leading to irreversible cell death. In short, from a mechanistic perspective, the death of spores in wet heat sterilization is due to protein denaturation, requiring a lower sterilization temperature. In contrast, dry heat sterilization belongs to oxidation in terms of mechanism, therefore, the required sterilization temperature is much higher than that of wet heat sterilization.
Temperature is a measure of thermal energy. Heat is the result of energy transfer between an object and its surrounding environment due to temperature differences.
For products or items that are suitable for wet heat sterilization, wet heat sterilization is a very economical and effective method. Saturated steam is the most common method of wet heat sterilization. Saturated steam sterilization and superheated water sterilization (spray, direct immersion/superheated water rotary sterilization, etc.) involve various forms of energy transfer, including conduction, convection, and radiation. In wet heat sterilization, energy (thermal energy) is transmitted through the container wall to the sterilized drug solution, or directly through the sterilized medium (steam or superheated water) to the sterilized product, which is a form of heat conduction.
In saturated steam sterilization, energy transfer mainly relies on the phase change of steam. When water changes from liquid to vapor state, it needs to absorb a large amount of heat energy (vaporization heat: 540 calories/gram). During the sterilization process, when steam comes into contact with objects at lower temperatures, it releases 540 calories/gram of latent heat, generating condensed water until the temperature of the object is equal to that of the steam. This efficient heat transfer method is incomparable to any other method. In the pre vacuum sterilization process, whether it is high vacuum or pulsating vacuum sterilization, although air in the chamber and sterilized items (such as filters, clothes, masks, filling machine components, hoses, etc.) is eliminated as much as possible in the initial stage of sterilization. But this does not mean that all obstacles to sterilization have been eliminated. Timely and unobstructed discharge of condensed water is equally important as removing air. If condensed water is not discharged in a timely manner, it will greatly affect the effectiveness of steam sterilization.
During the sterilization process of superheated water, the transfer of energy mainly relies on the mass of the superheated water. The large amount of superheated water carrying heat comes into direct contact with the sterilized product and transfers the heat energy to the contents of the container through the container wall, killing the microorganisms inside. Therefore, in order to achieve good uniformity in sterilization, spray circulation is essential. There is data indicating that loading has a relatively small impact on the sterilization process during superheated water spray sterilization. If it is not necessary to maintain balance in the rotary sterilizer, there is no need to use substitutes to fill the sterilizer and process it according to the full load sterilization program.
To improve the effectiveness of sterilization, special attention must be paid to the installation of sterilization equipment and the design of sterilization programs, which are also the necessary steps for successful validation of wet heat sterilization.