Microbial populations, inactivation

High hydrostatic pressure (HHP) processes have been used mainly for sauces or seafood and proven effective at reducing microbial populations without adverse effects on product quality (Considine et al., 2008 Brinez et al., 2006). HHP treatment causes bacterial inactivation by damaging the cell membrane, which affects membrane permeability and intracellular enzyme inactivation and possibly ruptures the plant cell wall (Kniel et al.,... 

This exponential order of inactivation of microbial populations (Fig. 2, the exponential survival curve) has subsequently been demonstrated to be a general characteristic of microorganisms in all processes of sterilization. The logarithmic axis of the exponential survival curve has no zero point. Thus, there can be... 

Exponential inactivation is the basis of the concept of sterility assurance. If the behavior of microbial populations in response to a particular sterilizing procedure is regular and exponential over the region of the survival curve within which their response can be monitored, then the treatments required to achieve SALs of 10 can be extrapolated. 

Fig. 2 Deviations from first-order kinetics of the inactivation of microbial populations by treatment with chemical agents due to (A) an initial lag in the rate of killing or (B) a decrease in the rate of killing with time of exposure.

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... 

Since the inactivation of bacteria follows an exponential decay process with a limiting value tending towards zero, the absolute sterility can never be obtained. The D,g value is the absorbed dose required to reduce a microbial population to 10% of its initial value, so that in industrial applications, a SAL value of 10" is reached after 3 x D value. For drugs, a dose of 25 kGy (or kJ kg ) is generally higher than 6 x D,g value and achieves the minimum SAL required of 10 . Many reviews demonstrated that for drugs with low bioburdens (initial contamination by microorganisms), sterilization was achieved with doses even lower than 15 kGy. 

Buzrul et al. (2008) described high-pressure (400-600 MPa, 22°C) inactivation kinetics of E. colt and L. innocua using the Weibull model. A parameter Zp was defined as an increase in pressure resulfing in 1-log reduction in microbial population. 

Sterility Assurance Levels The concept of the sterility assurance level (SAL) not only considers the kinetics of inactivation of microbial populations but also addresses the numbers of contaminants on product items prior to sieril-... 

SALs for terminally sterilized products are substantiated (validated) by extrapolation of measurable responses of microbial populations at sub-process treatment levels to process treatments that ought to be providing the specified nonmeasureabie SALs. Extrapolation can only be justified when a response takes a regular form and can be supported by theory. This is clearly the case for the kinetics of inactivation of microbial populations. 

The terminology used in association with dry heat inactivation of microbial populations exactly parallels that of sterilization by saturated steam (see Chapter 4). The Df-value is the time required to reduce a population to 10% of its initial numbers when held at a constant temperature T. The f>-value of spores of B. subtilis var niger at 170 C (/ i q) has been variously reported from around 8 s to... 

The kinetics of inactivation of microbial populations exposed to ethylene oxide are exponential [2] when the logarithm of the number of survivors is plotted against time with all other factors (e.g.. gas concentration, humidity, temperature) held constant. Shouldered curves have been occasionally noted instances of tailed" inactivation kinetics have been ascribed to clumping or environmental protection. Good experimental data are not easily obtained. Experimental design should concentrate on rapid attainment of the gas concentration intended. Inactivation from residual sierilant may also lead to misleading results. 

Therefore, the researcher can conclude, at the 95% confidence level, that the initial microbial population j3o) is between 5.94 and 6.32 logs, and the rate of inactivation (Pi) is between 0.046 and 0.036 logio per second of exposure. 

Exposure of E. coli to microwave treatments results in a reduction of the microbial population in apple juice. Canumir et al. (2002) determined the effect of pasteurization at different power levels (270-900 W) on the microbial quality of apple juice, using a domestic 2450 MHz microwave. The data obtained were compared with conventional pasteurization (83 °C for 30 s). Apple juice pasteurization at 720-900 W for 60-90 s resulted in a 2- to 4-log population reduction. Using a linear model, the D-values ranged from 0.42 0.03 minutes at 900 W to 3.88 0.26 minutes at 270 W. The value for z was 652.5 2.16 W (58.5 0.4°C). These observations indicate that inactivation of E. coli is due to heat. 

An alfemative way to model microbial inactivation by means of HP nof using the first order kinetics model is the model of Fermi. This model demonsfrates fhat population decreases because of exposure fo lefhal agents such as heat, electric pulses, and radiation or ozone doses. The equation of Fermi relates the fraction of survivors S(P), fhe pressure applied P (MPa), the critical pressure Pc at which the survivors fraction is 0.5, and the constant k that indicates the rate at which inactivation is occurring (Palou et al., 1998). 

Adaptability of Shewanella oneidensis MRl and Escherichia coli in these experiments indicates that microorganisms can continue to metabolize substrate at pressures far beyond those previously reported [34, 35,41], Although an evolutionary component to the adaptation of microbial communities to temperature and salinity is well known [71], whether there might be any evolutionary component for pressure adaptation is still in question. Shewanella MRl belongs to a genus that contains a number of piezophiles however, E. coli clearly does not. Despite this, there is evidence that exposure of E. coli to pressures up to 800 MPa selects a population of cells less sensitive to pressure inactivation [71]. Furthermore, it is well known that the increase in pressure tolerance is also associated with heat tolerance [71]. 

Microbial inactivation has been well researched. This is particularly true for populations of bacteria. Although each type of nuoioorganiw responds differently to the various available sterilization processes, the form of inactivation... 

The death of a single microbial cell is a biochemical process (or series of processes) the entrapment of individual microbial cells in or on filters is due to physical forces. These effects on individual cells are peculiar to individual sterilization processes. On the other hand, the effects of inactivating processes and filtering processes on populations of microbial cells are sufficiently similar to be described by one general form—exponential death. Exponential kinetics are typical of first-order chemical reactions. For inactivation this can be attributed to cell death arising from some reaction that causes irreparable damage to a molecule or molecules essential for continuing viability. 

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