Sterilizing value, calculation

When the filtrate is sterile, the number 1 is substituted for the average total recovery and the value is expressed as greater than the calculated value. 

The resistance of an organism to a sterilizing agent can be described by means of the D-value. For heat and radiation treatments, respectively, this is defined as the time taken at a fixed temperature or the radiation dose required to achieve a 90% reduction in viable cells (i.e. a 1 log cycle reduction in survivors Fig. 20.2k). The calculation of the D-value assumes a linear type A survivor curve (Fig. 20.1), and must be corrected to allow for any deviation from linearity with type B or C curves. Some typical D-values for resistant bacterial spores are given in Table 23.2 (Chapter 23). 

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 calculated hydrolysis half-life at 25 °C and pH 7 is 262 d (Ellington et al., 1988). The hydrolysis half-lives of methomyl in a sterile 1% ethanoEwater solution at 25 °C and pH values of 4.5, 6.0, 7.0, and 8.0 were 56, 54, 38, and 20 wk, respectively (Chapman and Cole, 1982). In both soils and water, chemical- and biological-mediated reactions transformed methomyl into two compounds — a nitrile and a mercaptan (Alexander, 1981). 

Several features of the PCBB experiments are different than those for chlorpyrifos. The hydrolysis reaction proceeds via a different mechanism. The rate enhancements observed for chlorpyrifos in natural waters and the aqueous phases of the sediment/water systems (as compared to sterile distilled water) are not observed for PCBB. The values of kj and k calculated for PCBB are slower than those for chlorpyrifos anS similar in magnitude to the hydrolysis rates. 

Cell yield was calculated from values of wet weights of harvested cells after growth in the presence or absence of inhibitors. Each inhibitor was added as a filter-sterilized solution after 4 hr of cell growth. Yield of control cells was 3 g/liter. Each culture was inoculated into fresh normal growth medium after inoculation with inhibitor, to see if growth recovery occurred. From Payne et al. (1990a). 

Calculate the steady-state substrate and biomass concentrations in a continuous fermenter which has an operating volume of 25 1 when the sterile feed stream contains limiting substrate at 2000 mg/1 and enters the vessel at 8 1/h. The values of Ks and ftm are 10.5 mg/l and 0.45 h " respectively, and the yield coefficient may be taken to be 0.48. 

Regardless of the type of lethality induced by a sterilization process—whether it be heat, chemical, or radiation—micro-organisms, upon exposure to adequate levels of such treatments, will die according to a logarithmic relationship between the concentration or population of living cells and the time exposure or radiation dose to the treatment. This relationship between the microbial population and time may be linear or nonlinear, as seen in Figure 1. The D value, or the time or dose required for a one-log reduction in the microbial population, may be calculated from these plots. 

Knowledge of the D value at different temperatures in heat sterilization is necessary for the calculation of the Z value. (See p. 87.)... 

Mathematical derivation of the Z value equation permits the calculation of a single quantitative expression for effective time exposure at the desired temperature for sterilization. The F value measures equivalent time, not clock time, that a monitored article is exposed to the desired temperature (e.g., 121°C). F values are calculated from the following equation ... 

Accuracy of thermocouples should be 0.5°C. Temperature accuracy is especially important in steam sterilization validation because an error of just 0.1 °C in temperature measured by a faulty thermocouple will produce a 2.3% error in the calculated F0 value. Thermocouple accuracy is determined using National Bureau of Standards (NBS) traceable constant temperature calibration instruments such as those shown in Figure 6. Thermocouples should be calibrated before and after a validation experiment at two temperatures 0°C and 125°C. The newer temperature-recording devices are capable of automatically correcting temperature or slight errors in the thermocouple calibration. Any thermocouple that senses a temperature of more than 0.5°C away from the calibration temperature bath should be discarded. Stricter limits (i.e., <0.5°C) may be imposed according to the user s experience and expectations. Temperature recorders should be capable of printing temperature data in 0.1 °C increments. 

If one knows the D value, the BI concentration or population A and the desired probability level of nonsterility B, the minimum F0 value that must be achieved by the sterilization cycle for the particular load can be calculated. For example, if A = 106 and II = 1 (f6 and laboratory studies determine the D value for B. stearothermophilus in the product to be sterilized to be 0.4 min (F0 = 0.4(12) = 4.8 min), a minimum F0 value of 4.8 min should be achieved at the worst case location during heat-penetration studies. The USP requires a steam sterilization process to deliver a lethality input of 12D for a typical overkill approach. 

For dry-heat temperatures other than 170°C, Fh values are used. The Fh concept is comparable to the Fq concept for moist heat sterilization and references lethality to equivalent times at 170°C. Fh values are shown in units of minutes or seconds, and the calculations of Fh use the same equations as the calculations of Fq. The only difference is that a z value of 20° C is substituted for 10°C.t ... 

Another possibility, proposed by Rodriguez et considers the concept of accumulated lethality (Fo), used for thermal sterilization, but applied to optimization of EtO sterilization technology. A mathematical model of the inactivation of biological indicators spores by EtO was developed, along with two formulas a response equation for calculating the number of survivors of a sterilization cycle, and a formula for determining the accumulated lethality of exposure to EtO. Experiments verified that the equations are applicable to processes with relative humidity values between 15% and 90%, enabling users to compare the lethality of dissimilar EtO cycles. 

It is evident that small temperature variations cause dramatic variations in the rate of the sterilization reaction. It is easy to calculate that a variation by only 1°C in the vicinity of 121 °C causes a variation of approximately 24% in the value of D, i.e., of the sterilization rate. 

The Idnetic rate constants for CO2 hydration determined in the laboratory in sterile seawater (Table 4.6) are known sufficiently well that this value should create little uncertainty in the above calculation. However, in natural waters the reaction rates may be enzymatically catalyzed. Carbon dioxide hydration catalysis by carbonic anhydrase (CA) is the most powerful enzyme reaction known (see the discussion in Section 9.3). The catal5dic turnover number (the number of moles of substrate reacted, divided by the number of moles of enz5mie present) is 8 x 10 min for CA (Table 9.7), and marine diatoms are loiown to produce carbonic anhydrase (Morel et al, 1994). The calculations presented in Fig. 10.14 indicate that increasing the CO2 hydration rate constant by 10-fold should increase the gas exchange rate of CO2 in the ocean by 10%-50%. 

For measurement of intersex intensities, the mean intersex stage of all females is calculated to provide the intersex index (ISI). This index provides the best results for assessing the reproductive capability of a population (Bauer et al., 1995 Bauer et al., 1997). A value of 0 indicates that only normal females occur, and no restrictions of the reproductive capability are to be expected. ISI values above 0 show that intersex-affected females can be found and that reproductive success may be reduced. An ISI above 1 indicates the presence of some sterile females, and values above 2 indicate that most females in a population are sterilized due to intersex development. 

It is instructive to calculate ASO - H20(%o), which is the difference between the 5 0 values of sol and H2O in mine water (Fig. 12.22). In the absence of isotope fractionation and with Fe as the oxidant, a zero difference would be expected. Taylor et al. (1984b) propose that Fe dominates pyrite oxidation in submersed environments. For such conditions ASO - H20(%o) values exceed zero due to isotope fractionation, ranging from about 4 to 6 in sterile systems to about 11 in the presence of T. ferrooxidans. Fractionation is caused by the greater reactivity of light 0 than heavy 0, with thus favored in product sulfate. Bacterial activity (in this case T. ferrooxidans) causes more fractionation than takes place in the abiotic oxidation reaction. ASO - H20(% ) values range up to about 18 when O2 is the chief oxidant in the presence of bacteria. S uch conditions are expected in aerated streams and in shallow unsaturated materials (Taylor et al. 1984a, 1984b). 

Probably the most widely known method of calculating D-values from quantal response experiments is the Stumbo 19 method. Each sample should con.sist of at least ten (and rarely more) replicate items carrying the same number of contaminants Nq), Samples should be exposed to increments of sterilization treatment, and then each replicate should be tested for viable microorganisms separately in suitable recovery conditions. The number of sterile replicate.s q is scored after incubation. The Z>-value may be calculated from... 

A separate )-value is usually calculated for each increment of sub<process sterilization treatment and an overall D-value taken from the arithmetic mean. 

The same type of experimental design, allowing that each increment of sub-process sterilization treatment d is equal, may be used to calculate O-values by the Spearman Karber method [20]. The total duration of sterilization treatment should be chosen to cover the whole of the quantal region from /]. which is defined as the longest lime at which every replicate is still found to be nonsterile, to time tfi, which is the shoitest exposure at which every replicate is found to be sterile as tested. The Speannan Karber equation allows estimation of a factor u as... 

If the proportional band of the controller is 75 and the pressure in the sterilizer suddenly drops by 5 psi, calculate the instantaneous value of the controller output and the new value of the steam flow rate. 

Let us do an actual problem, Example 3.1, the data for which are from an actual D value computation for steam sterilization. Biological indicators (strips of paper containing approximately 1 x 10 bacterial spores per strip) were affixed to stainless steel hip joints. In order to calculate a D value, or the time required to reduce the initial population by 1 logio, the adequacy of the regression model must be evaluated. Because bo and bi are unbiased estimators, even when serial correlation is present, the model y = bo + biX + et may still be useful. However, recall that e, is now composed of e,- i + di, where the 4s are N 0, 1). 

---