Dry sterilisation process

The term dry sterilisation process, DSP, denotes a dry aseptic sterilisation process. It is used for instance in the beverage industry during cold aseptic filling of beverages (juices, waters, UHT-milk, ...) into plastic bottles made from PET or HDPE, and also for some applications in the pharmaceutical industry.

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In cold aseptic filling the sterile or near-sterile product is filled into a bottle which has to be sterilised prior to bottling to avoid product contamination. Due to the heat-sensitive nature of the plastic material the sterilisation process must not heat the bottles. Therefore chemical sterilisation processes are used for this purpose. The Dry Sterilisation Process uses an aqueous solution of hydrogen peroxide (H₂O₂) with a concentration of 30...35% to achieve the germ-killing effect.

At first the bottles are placed into a sterilisation chamber. This chamber is designed to be a vacuum chamber and is evacuated by vacuum pumps down to the low vacuum range. A certain amount of aqueous solution of hydrogen peroxide is now delivered to an evaporator and abruptly evaporated. Driven only by the pressure difference between the hydrogen peroxide vapor inside the evaporator and the evacuated sterilisation chamber, the vapor flows through an appropriate piping into the sterilisation chamber. The vapor is strongly expanding when it enters the chamber, undercooled thereby and instantaneously condensing. The forming condensate layer is covering all surfaces inside the sterilisation chamber, all inner and outer bottle surfaces and all surfaces of the chamber itself.

The heat of vaporization, released by the phase change from gaseous to liquid, heats the forming condensate layer in such a way, that most of the hydrogen peroxide molecules are thermally dissociated thereby. The resulting free radicals, particularly the oxygen atoms, are immediately killing all the germs adhered to the surfaces already during the condensation. In contrast to other sterilisation processes the killing of the germs occurs instantaneously without any need for residence time.

The condensate layer is removed from the sterilisation chamber and all bottle surfaces immediately after the condensation. This is performed only by means of appropriate vacuum pumps which reduce the pressure inside the sterilisation chamber below 1 Torr. The condensate is rapidly re-evaporating when the decreasing chamber pressure reaches the condensates vapor pressure and the forming vapor is removed from the chamber by the vacuum pumps. This re-evaporation effects a total drying of the bottles and the surfaces inside of the sterilisation chamber and completely removes all hydrogen peroxide.

Prior to deloading of the bottles from the sterilisation chamber, the chamber is vented to ambient pressure with sterile air to avoid recontamination of the sterile bottles.

The complete process time amounts to 6 seconds. Using the common reference germs for hydrogen peroxide sterilisation processes, endospores of different strains of bacillus subtilis and bacillus stearothermophilus, the Dry Sterilisation Process easily achieves a germ reduction of 10⁶...10⁸ (log6...log8) in count reduction tests and also in end point tests.

The sterilised items leave the sterilisation chamber in a completely dry state. Only the surface temperature of the items is slightly increased by a few degrees (10°...15°) during the sterilisation process. Therefore, the process is particularly useful for the sterilisation of heat sensitive items like plastic bottles. It is also useful for applications which require a high germ reduction and short process times.

NB #1: Unfortunately it is common diction to say "the kill rate is log6" or "the germ reduction is log6", which strictly speaking is not only wrong but nonsensical. By saying this one means that the germ reduction is 6 orders of magnitude or the survival probability of each single germ is 10^-6. (This wrong diction originates from a misunderstanding of the mathematical expression log 10⁶ = 6)

NB #2: Strictly speaking it is also wrong to talk about single germs or the like. It's correct to use the item cfu or colony forming unit. The main problem is not inevitably the presence of germs (bacteria, spores, ...) but their ability of fast fissiparous, which gives an exponential increase of the number of the germs with time. If one tries to count "a number of germs" one has to, simply spoken, cultivate them on an agar plate, let them grow for a few days and count the macroscopic colonies which have formed. Each of these colonies is resulting from 1 cfu (= 1 "augmentable germ").

Example #1: One item which has to be sterilized carries a contamination of 10⁷ germs prior to sterilisation. The germ reduction capability of the sterilisation process is 6 orders of magnitude (=10⁶ or "log 6"), which means the survival probability of the germs is 10^-6. If such items are sterilised the average number of "surviving germs" or, correctly spoken, cfu's which are found on the items after sterilisation is: 10⁷ / 10⁶ = 10 or 10⁷ * 10^-6 = 10.

Example #2 (statistically equivalent to #1): A lot of items which have to be sterilized are carrying a contamination of 10 germs each prior to sterilisation. The germ reduction capability of the sterilisation process is 6 orders of magnitude (=10⁶ or "log 6"), which means the survival probability of the germs is 10^-6. If one sterilises a statistically significant number of these items the average number of cfu's which is found on the items after sterilisation is: 10 / 10⁶ = 10^-5 or 10 * 10^-6 = 10^-5. This means that, in average 1 cfu is found per 10⁵ = 100.000 items.