Dry Sterilisation Process
Encyclopedia
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, etc.) into plastic bottles made from PET
or HDPE
, and also for some applications in the pharmaceutical industry.
with a concentration of 30 to 35% to achieve the germ-killing effect.
by vacuum pump
s 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.
s 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...15K) 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 #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 = 100000 items.
Sterilization (microbiology)
Sterilization is a term referring to any process that eliminates or kills all forms of microbial life, including transmissible agents present on a surface, contained in a fluid, in medication, or in a compound such as biological culture media...
process. It is used for instance in the beverage industry during cold aseptic filling of beverages (juices, waters, UHT-milk, etc.) into plastic bottles made from PET
Polyethylene terephthalate
Polyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...
or HDPE
Polyethylene
Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons...
, and also for some applications in the pharmaceutical industry.
Use
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)Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
with a concentration of 30 to 35% to achieve the germ-killing effect.
Procedure
At first the bottles are placed into a sterilisation chamber. This chamber is designed to be a vacuum chamber and is evacuatedVacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
by vacuum pump
Vacuum pump
A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...
s down to the low vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
range. A certain amount of aqueous solution of hydrogen peroxide is now delivered to an evaporator and abruptly evaporated
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
. Driven only by the pressure difference between the hydrogen peroxide vapor
Vapor
A vapor or vapour is a substance in the gas phase at a temperature lower than its critical point....
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
Condensation in aerosol dynamics
Condensation can be summarized as a phase transition from a gas to a liquid as vapor condenses on a pre-existing surface, the exact opposite of the transition from liquid to vapor which occurs in evaporation. Both condensation and evaporation are happening all the time; atmospheric conditions...
. 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
Standard enthalpy change of vaporization
The enthalpy of vaporization, , also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas at a given pressure .It is often measured at the normal boiling point of a substance; although tabulated values are usually...
, released by the phase change
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
from gaseous to liquid, heats the forming condensate layer in such a way, that most of the hydrogen peroxide molecules are thermally dissociated
Homolysis
In general it means breakdown to equal pieces There are separate meanings for the word in chemistry and biology.-Homolysis in chemistry:...
thereby. The resulting free radicals
Radical (chemistry)
Radicals are atoms, molecules, or ions with unpaired electrons on an open shell configuration. Free radicals may have positive, negative, or zero charge...
, 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
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
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.
Results
The complete process time amounts to 6 seconds. Using the common reference germs for hydrogen peroxide sterilisation processes, endosporeEndospore
An endospore is a dormant, tough, and temporarily non-reproductive structure produced by certain bacteria from the Firmicute phylum. The name "endospore" is suggestive of a spore or seed-like form , but it is not a true spore . It is a stripped-down, dormant form to which the bacterium can reduce...
s of different strains of bacillus subtilis
Bacillus subtilis
Bacillus subtilis, known also as the hay bacillus or grass bacillus, is a Gram-positive, catalase-positive bacterium commonly found in soil. A member of the genus Bacillus, B. subtilis is rod-shaped, and has the ability to form a tough, protective endospore, allowing the organism to tolerate...
and bacillus stearothermophilus
Bacillus stearothermophilus
Bacillus stearothermophilus is a rod-shaped, Gram-positive bacterium and a member of the division Firmicutes. The bacteria is a thermophile and is widely distributed in soil, hot springs, ocean sediment, and is a cause of spoilage in food products. It will grow within a temperature range of 30-75...
, 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...15K) 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.
Examples
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 = 100000 items.