Dry sterilization process
The term Dry Sterilization Process ( DSP ) describes a specific dry antiseptic sterilization process that is used, for example, in the food industry for cold aseptic filling of beverages (juices, water, UHT milk , ...) in plastic bottles made of PET or HDPE as well as in the pharmaceutical industry .
With cold aseptic filling, a sterile or low-germ product is filled into bottles without being reheated to preserve it. The bottles must therefore be sterilized prior to filling in order to avoid recontamination of the sterile product in this way. Due to the temperature sensitivity of the plastic materials, the sterilization process must not heat the bottles and chemical sterilization processes are therefore used. The Dry Sterilization Process uses an aqueous hydrogen peroxide solution with a concentration of 30 to 35 percent to kill germs.
The bottles to be sterilized are first placed in a sterilization chamber. The sterilization chamber is designed as a vacuum vessel and is evacuated into the rough vacuum range by means of vacuum pumps . Now a measured amount of hydrogen peroxide solution is evaporated in an evaporator . By connecting the evaporation device to the evacuated sterilization chamber, the steam flows into the evacuation device due to the pressure difference between the evacuation device and the evacuated sterilization chamber. The steam expands strongly as it flows into the evacuated sterilization chamber, is supercooled and condenses instantly. The resulting film of condensate covers all surfaces inside the sterilization chamber, all bottle surfaces, inside and outside, as well as all inner surfaces of the sterilization chamber.
The evaporation heat released during the condensation heats the resulting condensate layer in such a way that the hydrogen peroxide contained therein largely dissociates . This creates radicals , in particular atomic oxygen, which kill the microorganisms adhering to the surfaces in fractions of a second during the condensation process. Compared to other sterilization methods, germs are killed immediately and no exposure time or holding time is necessary.
Immediately after the end of the condensation process, steam and condensate are removed from the sterilization chamber by reducing the pressure to below 1 hPa using vacuum pumps. The condensate film on the surface evaporates as soon as the falling pressure in the sterilization chamber falls below the steam pressure of the condensate. The re-evaporation of the condensate causes the surfaces to dry and the hydrogen peroxide to be completely removed.
To remove the sterilized bottles, the evacuated sterilization chamber must be flooded to atmospheric pressure. For this purpose, sterile air is used in order to avoid re-contamination of the now sterile bottles by unsterile air.
The process takes six seconds. Measured by the conventionally used in the hydrogen peroxide process reference germs, endospores of different strains of subtilis Bacillus and Bacillus stearothermophilus , reaches the Dry Sterilization Process both Count Reduction tests as well as in End Point tests kill rates of 10 6 to 10 8 ( "log6" to " log8 ”), or germ reduction to below 10 −6 to 10 −8 of the initial germ count .
The sterilized items leave the sterilization chamber in a completely dry state. Only the outermost surface layer of the objects is slightly heated during the sterilization process (by about 10 to 15 K). The method is well suited for all applications in which thermolabile objects have to be sterilized, in which very high kill rates are required and which require short processing times.
Performance evaluation
To describe the performance of a sterilization process, the terms "the germ reduction is log6", "the germ killing is log6" or "the killing rate is log6" are used synonymously, although this is not mathematically correct. These common expressions result from incorrect use of the mathematical expression log 10 6 = 6.
The terms kill rate, germ reduction and survival probability are statements of probability in the statistical sense. They can be illustrated by the following two examples, which are statistically equivalent:
Example 1: In an experiment, an object contaminated with 10 7 germs is sterilized using a process that achieves a germ reduction of 6 orders of magnitude (10 6 or “log6”), i.e. H. where the probability of survival for the germs is 10 −6 . Averaged over a statistically significant number of such attempts survive then 10 7 /10 6 = 10 or 10 7 * 10 -6 = 10 nuclei per object.
Example 2: In an experiment, objects with 10 germs each are sterilized using a process that achieves a germ reduction of 6 orders of magnitude (10 6 or “log6”), ie. H. where the probability of survival for the germs is 10 −6 . If a statistically significant number of such objects is sterilized, the number of germs surviving on the objects is 10/10 6 = 10 −5 or 10 * 10 −6 = 10 −5 , which means that one surviving germ per 10 5 = 100,000 objects is encountered.
Strictly speaking, it is also wrong to speak of surviving germs. The main problem is not necessarily the presence of some germs, but rather their ability to have short cell division cycles, which leads to an exponential increase in the number of germs over time. If a large number of germs are present, the amount of metabolic products produced by them, some of which are highly toxic, can lead to various kinds of symptoms of poisoning. The real problem is therefore germs capable of reproduction . In order to determine their number, after a sterilization process, for example, the surfaces of the sterilized object are washed off with a buffer solution and all germs in this solution are cultivated. If the buffer solution contained germs capable of reproducing, these form macroscopic colonies on the nutrient medium after a few days due to their exponential reproduction. Each of these colonies now comes - to put it simply - from a germ capable of reproducing. The correct technical term for this is colony-forming unit .