Kinetics of Sterilization

Techniques such as sterilization, pasteurization, or disinfection operate on the basis of the effects of heat, radiation, or chemical agents on viability of cells (bacteria, viruses), while filtration and membrane techniques act mainly mechanically. Even though the mechanisms of desactivation are quite different, similar basic concepts can be applied in most of these cases. 

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Jayaram, S., Castle, G.S.P., and Margaritis, A. 1992. Kinetics of sterilization of Lactobacillus brevis cells by the application of high voltage pulses. Biotechnology Bioengineering 40 1412-1420. 

Sterilizers can be operated discontinuously or continuously. For engineering-scale processes, sterilization is usually done with heat (i.e., steam) on economic grounds. Chemical and physical processes also exist (Aiba, Nagai, and Nishizawa, 1976 Richards, 1968). The stirred processes differ from each other in method of heat exchange. They are far surpassed for continuous operations by tube-type sterilizers, a consequence of the formal first-order kinetics of sterilization (see Chap. 5). Continuous steam sterilizers have been used for a long time in food processing technology (e.g., milk production). 

Sterilization of Media First-order kinetics may be assumed for heat destruction of living matter, and this leads to a linear relationship when logarithm of the fraction surviving is plotted against time. However, nonlogarithmic kinetics of death are quite often found for bacterial spores. One model for such behavior assumes inactivation of spores via a sensitive intermediate state by the mechanism ... 

Wang et al. report the death kinetics of Bacillus stearothermophilus spores using wet, thermal sterilization. Twenty minutes at 110°C reduces the viable count by a factor of lO". The activation temperature, E/Rg, is 34,200 K. How long will it take to deactivate by a factor of lO ... 

These considerations have been extensively explored by producers of canned foods and some simplified kinetics have been derived to allow better control of sterilization procedures. For example, the overall death process in a mixed culture can be described by an exponential decay curve. The equation will follow the form... 

In this section, we discuss the kinetics of thermal cell death and sterilization. The rates of thermal death of most microorganisms and spores can be given by Equation 10.1, which is similar in form to the rate equation for the first-order chemical reaction, such as Equation 3.10. 

Among the techniques discussed, moist heat is the most economical and efficient for the general sterilization requirements of fermentation. Therefore, the following four sections describe cell death kinetics and sterilization operations utilizing moist heat. 

One such application could be delivery of Has2-pDNA, a plasmid that codes for hyaluronan synthase 2 [66]. This enzyme facilitates the synthesis of larger HA molecules and can prevent post-surgical peritoneal adhesions. In one study, DNA-HA films were prepared using previously-described chemistry however, lyophili-zation was replaced with air-drying under sterile conditions and an isopropanol/ H20 mixture was used instead of DMF/H20. The release kinetics of DNA were similar to that from the HA film described previously, but release did not occur until after 7 days. The reason for this delay was not completely clear the authors suggest that a possible way to overcome the delay is to use a crosslinked DNA-HA film sandwiched between two non-crosslinked DNA-HA films. Non-crosslinked film... 

Design and operation of equipment for sterilizing media are based on the concept of thermal death of microorganisms. Consequently, an understanding of the kinetics of the death of microorganisms is important to the rational design of sterilizers. 

The kinetics of the hydroxide ion-catalyzed epimerization of pilocarpine to isopilocarpine and of its hydrolysis to pilocarpic acid have been studied (56). Both forms of degradation lead to loss of pharmacological activity. The importance of possible inactivation by epimerization during thermal sterilization of ophthalmic preparations of pilocarpine was pointed out. It was also considered that some epimerization would always occur during the extraction of pilocarpine from Jaborandi leaves, and that isopilocarpine might, therefore, be an artifact and not a genuine plant alkaloid (55). 

Soil calculated ty, = 4.9 d from first-order kinetic of degradation under both sterile and nonsterile conditions... 

Figure 1 Kinetics of moisture removal from West 890 gray stoppers by application of vacuum in the autoclave post sterilization. Each point represents a separate experiment and at least 10 stoppers were analyzed for the residual moisture content in each experiment. The -1 time point on the x axis represents the washed stopper prior to autoclaving. The 0 time point on the x axis corresponds to sterilized stoppers that were subjected to no drying at all. The later time points represent stoppers treated for various amounts of vacuum drying post sterilization.

Fig. 11 Kinetics of water transfer from stoppers to 25 mg freeze dried lactose. The stoppers were 13 mm finish West 1816 gray butyl stoppers that were steam sterilized and vacuum dried for 1 hour. Circles = 40° C triangles = 25° C squares = 5°C. (From Ref. l)...

The kinetics of dry-heat treatments is comparable to that of moist heat sterilization. The organisms that are considered to be representatives for dry-heat sterilization processes are spores of Bacillus subtilis var. nigerP ... 

The effects of EtO concentration and process temperature were widely studied. Ernst and Shull initially verified that the lethality kinetics of Bacillus subtilis var. niger spores is of zero order at high levels of EtO. With the EtO concentration reduction, the reaction becomes first order. Its sterilization curves at different temperatures show the required EtO concentration, and the duplication of the reaction rate of every 10°C. 

Even though the definition of sterility is an absolute condition, the effectiveness of the sterilization process can be determined by measuring the reduction of microbial population. Such measurements reveal the kinetics of microbial inactivation, and it is from the exponential nature of inactivation that the concept of sterility assurance level (SAL) is derived. This value... 

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