Time Dependence of Effects

The reaction rates for different types of enzymatic reactions have been discussed in section 2.5.1. The inactivation of enzymes and the killing of microorganisms can be depicted as a reaction of 1st order  

The co-called D-value represents the time needed to reduce the initial concentration (activity, germ count) by one power of ten. It refers to a certain temperature which has to be stated in each case. For example Bacillus cereus Dj2i°c = 2.3 s, Clostridium botulinum Dj2i°c = 12.25 s. For a heat treatment process, the D-value allows the easy determination of the holding time required to reduce the germ count to a certain level. If the germ count of B. cereus or Cl. botulinum in a certain food should be reduced by seven powers of ten, the required holding times are 2.3 x 7 = 16.1 s and 12.25 x 7 = 85.8 s. 

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One can differentiate between three types of transformation products of environmental pollutants. First, environmental pollutants can be metabolized during the toxicokinetic phase of uptake/metabolism/distribution/elimination in organisms (Table 1). Here, the observed effect is actually due to the combined effect of different metabolites. Taking these transformation reactions into account will help to understand mechanisms of toxicity, species sensitivity differences, and time dependency of effects. Lee and Landrum [8,9] developed a model to describe the mixture effects of PAH and their metabolites in Hyalella azteca. This combined toxicokinetic/toxicodynamic models convincingly demonstrated the importance of accounting for metabolite formation and how different mixture toxicity concepts can be incorporated into such models. 

Figure 3. Time dependence of effective second harmonic coefficient during poling process for polymers 50P (a), and SOP (b), respectively (T-Tg = 0 ( ), 5 (A), 10 (0), and 15 °C (O)).

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