Sterilization processes

It is necessary to estabUsh a criterion for microbial death when considering a sterilization process. With respect to the individual cell, the irreversible cessation of all vital functions such as growth, reproduction, and in the case of vimses, inabiUty to attach and infect, is a most suitable criterion. On a practical level, it is necessary to estabUsh test criteria that permit a conclusion without having to observe individual microbial cells. The failure to reproduce in a suitable medium after incubation at optimum conditions for some acceptable time period is traditionally accepted as satisfactory proof of microbial death and, consequentiy, stetihty. The appHcation of such a testing method is, for practical purposes, however, not considered possible. The cultured article caimot be retrieved for subsequent use and the size of many items totally precludes practical culturing techniques. In order to design acceptable test procedures, the kinetics and thermodynamics of the sterilization process must be understood. 

The Arrhenius rate theory, an empirical derivation, holds for the sterilization process ... 

Culture spores Sterilization process Approximate Z9-value... 

An inherent problem is the location of the sensors. It is not possible to locate the sensors inside the packages which are to be sterilized. Electromechanical instmmentation is, therefore, capable of providing information only on the conditions to which the packages are exposed but cannot detect failures as the result of inadequate sterilization conditions inside the packages. Such instmmentation is considered a necessary, and for dry and moist heat sterilization, a sufficient, means of monitoring the sterilization process. 

The most widely used sterilization method ia the food industry is moist heat. The heat is usually suppHed by high pressure steam, but because most foods already contain moisture the role of steam is to heat the food to the required temperature. The cooking and sterilization processes can frequendy be combined into one. The food may be sealed into impervious containers of glass, metal, or plastic film and undergo terminal sterilization, or it may be presterilized in batches or in a continuous operation and then filled into a presterilized container. The latter process is called sterile filling. 

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 purpose of open unidirectional airflow benches is to protect products from particulate contaminants by creating a controlled environment. These benches are used, for example, in electronic, biological, pharmaceutical, and food industries. It should be mentioned that within pharmaceutical production, aseptic sterile processes must be carried out in a Class 100 environment (U.S. Federal Standard 209 E, Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones). To avoid particle contamination in the bench, horizontal or vertical airflow with high-efficiency particulate air (HEPA)-filtered air is used. The air velocity is normally 0.4-0.5 ra s". Some examples of typical arrangements of open unidirectional airflow benches are shown in Fig. 10.51. 

The existence and possible presence of bacterial spores determines the parameters, i.e. time and temperature relationships, of thermal sterilization processes which are used extensively by the food and pharmaceutical industry. These are defined below (see also Chapters 20 and 23). 

All disinfection and sterilization processes for equipment should be validated, for preference using a microbiological challenge with an organism of appropriate resistance to the disinfectant, sterilant or sterilizing conditions. Once the required log reduction of the challenge organism has been achieved, physical and/or chemical parameters can be set which form the critical control points for the process. 

A sterilization process may thus be developed without a full microbiological background to the product, instead being based on the ability to deal with a worst case condition. This is indeed the situation for official sterilization methods which must be capable of general application, and modem pharmacopoeial recommendations are derived firm a careful analysis of experimental data on bacterial spore survival following treatments with heat, ionizing radiation or gas. 

Application of the F- value concept has been largely lestrieted to steam sterilization processes although there is a less frequently employed, but direet parallel in dry heat sterilization (see section 4.3). 

Drying or cooling. Dressings packs and other porons loads may become dampened during the sterilization process and mnst be dried before removal fiom the chamber. This is achieved by steam exhanst and application of a vacnum, often assisted by heat from the steam-filled jacket if fitted. After drying, atmospheric pressure within the chamber is restored by admission of sterile filtered air. 

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. 

Under no circumstances should living cultures of microorganisms, whether they be for vaccine preparation (Chapter 16) or for use in monitoring sterilization processes (Chapter 23), be taken into aseptic areas. As already pointed out, separate premises are needed for the aseptic filling of live or of attenuated vaccines. 

A product to be labelled sterile must be flee of viable microoiganisms. To achieve this, the product, or its ingredients, must undergo a sterilization process of sufficient microbiocidal capacity to ensure a minimum level of sterihty assurance (Chapter 20). It is essential that the required conditions for sterihzation be achieved and maintained through every operation of the sterilizer. 

This chapter will discuss briefly the principles and applications of the various methods of monitoring and validating sterilization processes. 

Monitoring of the sterilization process canbe achieved by the rrse of physical, chemical or biological indicators of sterilizer performance. Such indicators are frequently employed in combination. 

Biological indicators (Bis) for use in thermal, chemical or radiation sterilization processes consist of standardized bacterial spore preparations which are usually in the form either of suspensions in water or culture medium or of spores dried on paper, aluminium or plastic carriers. As with chentical indicators, they are usually placed in dummy packs located at strategic sites in the sterilizer. Alternatively, for gaseous sterihzation these may also be placed within a tubular hehx (Line-Pickerill) device. After the sterilization process, the aqueous suspensions or spores on carriers are aseptically transferred to an appropriate nutrient medium which is then incubated and periodically examined for signs of growth. Spores of Bacillus stearothermophilus in sealed ampoules of cultrrre medium are used for steam sterilization morritoring, and these may be incubated directly at 55°C this eliminates the need for an aseptic transfer. 

Table 23.1 Examples of chemical indicators for monitoring sterilization processes...

Low temperature steam and formaldehyde Reactive chemical Indicator paper impregnated with a formaldehyde-, steam- and temperature-sensitive reactive chemical which changes colour during the sterilization process Gas concentration, temperature, time (selected cycles)... 

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