Sterilization kinetics

The inactivation of spores by CO2 has been described by a single first-order rate constant (54,57). In contrast, a two-step process has been reported for the heat inactivation of spores in the absence of CO2 (54,59). The presence of CO2 during heat treatment could increase the rate of spore heat activation, considered to be an initial step in sterilization (59), and/or the rate of spore inactivation to result in first-order sterilization kinetics (54). 

Finally, on theoretical considerations relating to btoburden, most experi-mental work on sterilization kinetics has addressed the inactivation of pure cultures of microorganisms. Most real life" biobuidens are mixed cultures. An important assumption surrounding the application of experimental pure culture situations to practical mixed culture situations is that each component of a mixed culture should behave independently of the others. As far as is known there is no evidence to doubt this assumption. 

Figure 5.79. Typical plot of sterilization kinetics with viable cells, according to Equ. 5.265 with varying temperature. Death rate constants can be estimated from the slope of the lines.

Figure 5.80. Typical plot of sterilization kinetics with bacterial spores compared with vegetative cells. A deviation is known due to the activation of spores by temperature.

Table 4.18 Thermodynamic activities of ABDAC against Pseudomonas aeruginosa calculated from MIC, sterilization kinetics, and CMC data...

Carboxyhc acid ester, carbamate, organophosphate, and urea hydrolysis are important acid/base-catalyzed reactions. Typically, pesticides that are susceptible to chemical hydrolysis are also susceptible to biological hydrolysis the products of chemical vs biological hydrolysis are generally identical (see eqs. 8, 11, 13, and 14). Consequentiy, the two types of reactions can only be distinguished based on sterile controls or kinetic studies. As a general rule, carboxyhc acid esters, carbamates, and organophosphates are more susceptible to alkaline hydrolysis (24), whereas sulfonylureas are more susceptible to acid hydrolysis (25). 

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. 

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 ... 

Death kinetics are obviously important in chemical or thermal sterilization. The spores formed by some bacteria are the hardest to kill. Problem 12.3 gives data for a representative case. 

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 ... 

A continuous fermenter is operated at a series of dilution rates though at constant, sterile, feed concentration, pH, aeration rate and temperature. The following data were obtained when the limiting substrate concentration was 1200 mg/1 and the working volume of the fermenter was 9.8 1. Estimate the kinetic constants Km, //, and kd as used in the modified Monod equation ... 

ChemicaPPhysical. The hydrolysis half-lives of oxamyl in a sterile 1% ethanol/water solution at 25 °C and pH values of 4.5, 6.0, 7.0, and 8.0 were 300, 17, 1.6, and 0.20 wk, respectively (Chapman and Cole, 1982). Under alkaline conditions, oxamyl hydrolyzed following first-order kinetics (Bromilow et al., 1980). 

First, determination of hydrolysis kinetics for each compound in sediment-free distilled, buffered distilled or natural water systems were measured. Using sterile techniques, concentrations of the parent compounds were determined as a... 

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... 

There is another, nonkinetic parameter widely used when evaluating a heat sterilization process. This is the F0 value, defined as the number of minutes required to kill all endospores present in a system held at a temperature of 250°F or 121°C. It should be noted that, in principle, the kinetic approach implies that sterilization cannot be achieved (because there is no zero on a logarithmic plot) so the nonkinetic offers more hope. The F0 can be measured and there is an interconnection between F0 and D if F0 is slightly redefined as the number, n, of D values required to sterilize a system, i.e., F0 = nD. 

Acid phosphomonoesterase (EC 3.1.3.2). Milk contains an acid phosphatase which has a pH optimum at 4.0 and is very heat stable (LTLT pasteurization causes only 10-20% inactivation and 30 min at 88°C is required for full inactivation). Denaturation of acid phosphatase under UHT conditions follows first-order kinetics. When heated in milk at pH 6.7, the enzyme retains significant activity following HTST pasteurization but does not survive in-bottle sterilization or UHT treatment. The enzyme is not activated by Mg2+ (as is alkaline phosphatase), but it is slightly activated by Mn2+ and is very effectively inhibited by fluoride. The level of acid phosphatase activity in milk is only about 2% that of alkaline phosphatase activity reaches a sharp maximum 5-6 days post-partum, then decreases and remains at a low level to the end of lactation. 

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. 

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