Heat sterilization sterilizer design

Distilled water is often used in the formulahon of oral and topical pharmaceutical preparations and a low bacterial count is desirable. It is also used after distillation with a specially designed still, often made of glass, for the manufacture of parenteral preparations and a post-distillation heat sterilization stage is commonly included in the process. Water for such preparahons is often stored at 80°C in order to prevent bacterial growth and the production of pyrogenic substances which accompany such growth. 

The F-value concept which was developed for steam sterilization processes has an equivalent in dry heat sterilization although its application has been limited. The Fh designation describes the lethality of a dry heat process in terms of the equivalent number of minutes exposure at 170°C, and in this case a z value of 20°C has been found empirically to be appropriate for calculation purposes this contrast with the value of 10°C which is typically employed to describe moist heat resistance. 

The D value is the determining factor if the death rate is, indeed, exponential, an assumption which is not necessarily always valid. Although only conditions which supply a single D value would be sufficient to completely sterilize a solution containing, say, one organism per unit volume, most heat sterilization processes are designed to administer 12D. This is an example of overkill and becomes more evident the fewer organisms there are in the untreated product in the first place. 

Sterilization processes are discussed in detail in Chapter 20. However, it is axiomatic that whatever method is chosen, the process should not cause damage to the product. By reference mostly to moist heat sterilization processes (the reader should remember that there are parallel approaches to other methods of sterilization) this section illustrates the factors that must be considered in the design of a sterilization process. 

Moist heat sterilization is achieved by exposure to saturated steam under pressure in a suitably designed chamber. In these circumstances there is an exact relationship between the steam temperature and pressure, but the pressure is used solely to obtain the temperature required and otherwise contribute nothing to the sterilization process. The time, temperature and pressure must be used to control and monitor the process. 

Great care is needed in the design of autoclaves and sterilization cycles because of the requirement for the presence of moisture. The autoclave must be loaded to allow complete steam penetration to occur in all parts of the load before timing of the sterilization cycle commences. The time required for complete penetration, the so-called heat-up time, varies with different autoclave constmction and different types of loads and packaging materials. The time may not exceed specific limits in order to guarantee reproducibility and, for porous loads, saturated steam. The volume of each container has a considerable effect on the heatup time whenever fluids are sterilized. Thermocouples led into the chamber through a special connector are often employed to determine heatup times and peak temperatures. The pressure is refleved at the end of each sterilization cycle. Either vented containers must be used or... 

There are four types of food sterilization processes terminal sterilization in prefiUed containers in a batchwise process terminal sterilization in prefiUed containers of appropriate design heated to the required temperatures in a continuous process aseptic filling foUowing batchwise cooking in an appropriate retort and aseptic filling in a continuous cooking system equipped with appropriate valves to aUow the necessary pressures for attainment of the required sterilization temperatures. 

The relationship between temperature and pressure holds true only in the presence of pure steam adulteration with air contributes to a partial pressure but not to the temperature of the steam. Thus, in the presence of air the temperature achieved will reflect the contribution made by the steam and will be lower than that normally attributed to the total pressure recorded. Addition of further steam will raise the temperature but residual air surrounding articles may delay heat penetration or, if a large amount of air is present, it may collect at the bottom of the sterilizer, completely altering the temperature profile of the sterilizer chamber. It is for these reasons that efficient air removal is a major aim in the design and operation of a boiler-fed steam sterilizer. 

Other heating equipment. Autoclave used for sterilization can be used to achieve superheating condition at 120°C. For higher temperature heating, a domestic pressure cooker, or a plastic steamer may be used. Some commercial laboratory pressure cookers have been designed for AR-IHC with controlled temperature. A water bath can be used to achieve lower temperature heating condition. 

Furthermore, the transplants are often packed in aluminum boxes, which are sealed by sterile filters permeable to water vapor. Even if the box can be designed with a negligible resistance to water vapor flow, the heat transfer is substantially reduced. 

---