Decimal reduction times

A specialized parameter used by certain disciplines in the characterization of enzyme stability is the decimal reduction time, or D value. The decimal reduction time of a reaction is the time requited for one logio reduction in the concentration, or activity, of the reacting species (i.e., a 90% reduction in the concentration, or activity, of a reactant). Decimal reduction times can be determined from the slope of logio([N,]/[No]) versus time plots (Fig. 12.3). The modified first-order integrated rate equation has the following form  

The decimal reduction time (D) is related to the first-order rate constant (kr) in a straightforward fashion  

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As predicted by the Arrhenius equation (Sec. 4), a plot of microbial death rate versus the reciprocal or the temperature is usually linear with a slope that is a measure of the susceptibility of microorganisms to heat. Correlations other than the Arrhenius equation are used, particularly in the food processing industry. A common temperature relationship of the thermal resistance is decimal reduction time (DRT), defined as the time required to reduce the microbial population by one-tenth. Over short temperature internals (e.g., 5.5°C) DRT is useful, but extrapolation over a wide temperature internal gives serious errors. 

D-value (decimal reduction time, DRT) is the time in minutes required to destroy 90% of a population of cells. The D-value has little relevance to the sterilization of medicines for injection, surgical instruments or dressings, where a process designed to kill all living spores must be developed. The D-value is used extensively in the food industry. 

The use of ionizing irradiation on fruits and vegetables has not been intensively studied, but a few experiments have suggested that irradiation has potential for such an application (Allende et al., 2006 Zhang et al., 2006). Results from Buchanan et al. (1998) indicated that low-dose irradiation could easily eliminate E. coli 0157 H7 from fresh apple juice, but they also showed that acid-adapted strains needed higher doses of irradiation in order to obtain a 5 D inactivation in juice (73-value decimal reduction time, i.e. the amount of time it takes at a certain temperature to kill 90% of the organisms being studied (Wikipedia, 2006)). In experiments where sprouts were irradiated... 

Decimal reduction times varied from 6 days to 3 weeks, with the decline being fastest at 37 °C compared to the colder temperatures. The authors suggested that this data should enable predictions of storage conditions that would lead to predetermined levels of reduction of the two pathogens. 

A detailed discussion of these kinetics is out of place in this present text but the above equation does allow some simplified considerations to be made. For example, a useful parameter easily derived is the decimal reduction time, D, for a mixed or single culture is the time it takes for a set of conditions such as heat to reduce the number of organisms to one-tenth of the initial value, i.e.,... 

Index Entries Green fluorescent protein thermal stability decimal reduction time three-phase partitioning fluorescence intensity acetate phosphate. 

The term thermal stability (also thermostability) refers to the resistance of a protein to adverse intrinsic and extrinsic environmental influences, i.e., the thermal characteristic of the protein to remain steady against the dena-turation of its molecular integrity and inactivation of its biologic activity on facing high temperatures or other deleterious agents (6). One of the most important indices to measure protein stability is the decimal reduction time, or D-value, the time required to reduce 90% of the initial protein concentration exposed to the reference temperature. The D-value was used... 

Time t is defined as the decimal reduction time or D value. Therefore... 

The bacterial species to be used in a BI must be selected carefully, as it must be non-pathogenic and should possess above-average resistance to the particular sterilization process. Resistance is adjudged from the spore destruction curve obtained upon exposure to the sterilization process recommended BI spores and their decimal reduction times (D-values ... 

Inactivation, regarding thermal treatment, is based on the assumption that death of microorganisms versus time is linear in a semi logarithmic graph. Thus, inactivation by HP usually uses the kinetic concepts of thermal treatment D (decimal reduction time time in minutes required to inactivate... 

FIGURE 4.1 First-order reaction of the type A. B. (a) Concentration relative to the original one as a function of time (t) over relaxation time (r). (b) Example of log concentration versus time tan a — k log e — 0.434C. D is the decimal reduction time. 

Ingallinera et al. (2005) compared total pectinesterase activity of Sicilian blood oranges (Sanguinello, Moro, and Tarocco) with the blonde Navel cultivar, checking enzyme stability with various pasteurization time and temperature (70-8 5 °C) conditions in order to optimize the heat treatment and increase the shelf life of the pasteurized juice. To do this they stored the juices at 4, 15, and 25°C for times ranging between 10 minutes and 50 days. Decimal reduction time and temperature (D and z) and the kinetic constant k) were established to optimize and increase the shelf life of the pasteurized juiee. Finally, a heat treatment (85°C x 3 minutes) of both microbiological and enzymatic efficacy was developed that does not compromise anthocyanin stability. 

FIGURE 33.1 Dynamics of bacterial cell density (log x cfu/mL) at different periods (after 30, 60, 90, and 120 min) after incubation of microcapsules containing bacteria cells in alginate (AG) matrix (1%, 1.5%, and 2%) in simulated gastric juice (pH 2). The decimal reduction time values (D ) (min) are represented for each variant. 

Figure 5.6. Alternative concepts for D> quantification of temperature effects (a) decimal reduction time, D o (see Equ. 5.8), and (b) Z value (see Equ. 5.9).

An additional alternative to the Arrhenius law for correlation of T dependencies is given by the concept of decimal reduction time, or the value, ... 

Decimal reduction time (DRT also called D-value) The length of time needed to kill 90% of the organisms in a given population at a specified temperature. 

Sometimes microbiologists use the term decimal reduction time D, which is the time in min during which the original number of viable microbes is reduced by 1/10. Substituting into Eq. (9.12-4),... 

At constant temperature T, Eq. (9.12-9) becomes (9.12-3). Since fc is a function of temperature, the decimal reduction time D, which is related to k by Eq. (9.12-6), is also a function of temperature. Hence, D is often written as Dt to show that it is temperature-dependent. 

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