For radiation inactivation, the d-value is stated in the terms absorbed dose (kGy).ĭ-value is the time required to kill 90% of the spores or vegetative cells of a given microorganism at a specific temperature in a specific medium. Thus, with data for heat inactivation of microbes the temp is shown D121 ℃. The D-value and K are specific for each set of microorganisms and each sterilization process. N t= surviving population after an exposure time The D-value may be estimated graphically see graph or mathematically from the equation The D-value is the exposure time required for the number of survivors to change by a factor of 10 or the time required to achieve a decrease of one log cycle in the survivor curve, in other words the temperature or radiation dosed required to reduced the initial population by 90%.
It is often more convenient to use the D-value as a measure of rate of microbial inactivation. For example, a hypothetical organism is reduced by 90% after exposure to temperatures of 300F for 2 minutes, Thus the D-value would be written as D 300F = 2 minutes. When referring to D values it is proper to give the temperature as a subscript to the D. The population number has been reduced by one decimal place in the counting scheme. Thus, after an organism is reduced by 1 D, only 10% of the original organisms remain. The decimal reduction time is dependent on the temperature, the type of microorganism and the composition of the medium containing the microorganism. Decimal reduction time (D-value):- The D-value, which denotes the decimal reduction time, is the time required at a specific temperature and under specified conditions to reduce a microbial population by one decimal. This value is obtained by keeping temperature constant and measuring the time necessary to kill the number of cells specified. Thermal Death Time (TDT):- Thermal death time is the amount of time that is necessary to kill a specific number of microbes at a specific temperature. UHT Aseptic Technology (Ultra High Temperature Sterilization):- A sterilization process is defined as a UHT (Ultra High Temperature) process, if the product is heat-treated in a continuous flow at a temperature of not less than 135℃ for a very short time, aseptically packaged in sterile containers, and has undergone minimum chemical, physical and Organoleptic changes in relation to the severity of the heat treatment required for sterilization. The probability of finding a non-sterile unit (PNSU = Probability of Non-Sterile Unit) must therefore be lower than 10 -6. More properly, in the pharmaceutical industry, in order to define a unit as sterile we must be able to certify, on a statistical basis related to the conditions of preparation and sterilization of that specific product and of that specific batch, that less than one unit in a million is exposed to the risk of not being sterile. It is therefore agreed that the product is acceptable as sterile when the probability of finding a non-sterile unit in a sterilized batch entails a risk which is lower than the other risks associated with the use of the product itself. increasing time and/or temperature) usually entails a decay of the qualities of the product and certainly increases process costs. Making the characteristics of the sterilization treatment more drastic (i.e. Therefore, only a treatment of infinite duration provides the absolute certainty that the entire microbial population has been destroyed and that the system is sterile. The destruction of a microbial population subjected to a sterilization process follows a logarithmic progression. In other words, they are absolute terms: an object or system is either “sterile” or “non-sterile”. Therefore the terms “sterile” and “sterilization”, in a strictly biological sense, describe the absence or destruction of all viable micro-organisms. Sterilization :- Any physical or chemical process which destroys all life forms, with special regard to microorganisms (including bacteria and sporogenous forms), and inactivates viruses.
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Sterile :- Free from viable micro-organisms.
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