METHOD FOR STERILIZATION OF OBJECTS

FIELD OF THE INVENTION

The present invention relates to a method for sterilization of an object or objects at atmospheric pressure, the method comprising the step of encapsulating the object or objects to be sterilized in a sterilization vessel. The sterilization vessel also encapsulates a predetermined amount of atmospheric air and a predetermined amount of a inert gas, such as helium, neon, argon, krypton, xenon, nitrogen etc.. The method further comprises the step of exposing the object or objects to an electric field in order to ignite a plasma in the sterilization vessel. The present invention further relate to an apparatus for carrying out the invention.

BACKGROUND OF THE INVENTION

Various methods and techniques for sterilization of objects have been suggested in the patent literature over the recent years.

EP 0 863 772 (and corresponding US patent US 6,007,770) relates to a closed system and an associated method for sterilizing objects under atmospheric pressure by converting oxygen present in the atmospheric air surrounding the object to be sterilized by ozone. According to EP 0 863 772 the object to be sterilized is positioned in a vessel, such as a plastic bottle, said vessel also containing a sufficient amount of atmospheric air. By placing the vessel between two electrodes (with a high voltage potential therebetween) arranged in a closed sterilization chamber, and applying a sufficient high voltage level to the electrodes the oxygen present in the atmospheric air inside the vessel is converted to ozone which sterilizes the object in the vessel.

JP 2004/173704 relates to a method and a system for, in an open system, sterilization of objects under atmospheric pressure. It is in JP 2004/173704 mentioned that a gas mixture of argon and oxygen has previously been used for

sterilization of objects. The presence of argon is reported to stabilize the generation of plasma.

It is a disadvantage of the method disclosed in EP 0 863 772 that in order to ignite a plasma sufficient to sterilize the object or objects positioned inside the vessel a relatively high voltage (up to 25 kV) must be applied to the high voltage electrodes. Since the sterilization process of EP 0 863 772 purely relies on the conversion of oxygen to ozone extremely high local field densities are generated inside the vessel. Such high local field densities typically create sparks which may easily cause damage to the vessel or container carrying the object or objects to be sterilized.

It is an object of the present invention to provide a method for sterilization of an object or objects at atmospheric pressure at low local field density levels in order to avoid damage of a vessel or container carrying the object or objects to be sterilized.

SUMMARY OF THE INVENTION

To comply with the above mentioned object the present invention relates, in a first aspect, to a method for sterilization of an object or objects at atmospheric pressure, the method comprising the steps of

- encapsulating the object or objects to be sterilized in a sterilization vessel, said sterilization vessel also encapsulating a predetermined amount of atmospheric air and a predetermined amount of a inert gas, and

- exposing at least part of the interior of the sterilization vessel to an electric field in order to ignite a plasma in the sterilization vessel.

The sterilization vessel may in principle be any container capable of encapsulating the object/objects and the predetermined amounts of atmospheric air and inert gas. Thus, the sterilization vessel may be a bottle-shaped plastic container having essentially rigid outer sidewalls and a lid attached thereto, or a

bag-like sterilization vessel having flexible outer sidewalls. Such flexible outer sidewalls may be made of a flexible foil, such as a flexible polymeric foil.

The predetermined amounts of atmospheric air and inert gas are provided during packing of the object/objects in the sterilization vessel. After packaging the mixture of atmospheric air and inert gas remains essentially constant during a period of at least some weeks. Thus, after packaging the object or objects positioned in the sterilization vessels may be sterilised by igniting a plasma in the vessel at any time during a period of at least some weeks.

The predetermined amount of the inert gas may be larger than 80%, such as larger than 85%, such as larger than 90%, such as larger than 95% or such as larger than 98% of the total amount of gas in the sterilization vessel. The inert may be helium, neon, argon, krypton, xenon or nitrogen or any combination thereof. The amount of atmospheric air may be smaller than 20% of the total amount of gas in the sterilization vessel.

It is an advantage of the present invention that the presence of the inert gas stabilises the generated plasma, and, at the same time, lowers the threshold voltage (or electric field) at which a plasma is ignited. The combined effect of lowering the threshold voltage (or electric field) at which a plasma inside the sterilization vessel is ignited, and stabilizing an already ignited plasma significantly lowers the risk of damaging the sterilization vessel. The reason for this being that sparks are unlikely to be generated when the sterilization vessel is placed under the influence of the electric field which preferably is an alternating electric field. The electric field may have a strength of at least 5 kV/cm, such as 10 kV/cm, such as 15 kV/cm.

The step of exposing at least part of the interior of the sterilization vessel to an electric field may comprise the step of positioning the sterilization vessel between the two electrodes, and applying a predetermined voltage difference to the two electrodes. A first of the two electrodes may be adapted to receive the predetermined voltage level, whereas a second of the two electrodes may be electrically connected to a fixed potential level, such as ground.

The predetermined voltage applied to the at least one electrode may be a continuous AC voltage, or it may comprise a number of pulses having a predetermined duty cycle. The duty cycle of the applied pulses may have a duration which is linked to the frequency of the applied AC voltage. The reason for this being that the "on time" equals at least one period of the AC voltage. Thus, the total "on time" is an integer number multiplied with the period of the AC voltage. The same applies to the "off time" in that the "off time" equals an integer number (ranging from one to infinite) multiplied with the period of the AC voltage. Thus, if an 40 kHz AC voltage is applied the on and off times can be chosen to 25 μs, 50 μs, 75μs. For most application "on times" of 25 μs or 50 μs are appropriate, whereas the "off times" may be varied in order to reach or match a predetermined average power level. The duty cycle ("on-time" plus "off- time") may be in the range 25-500 μs, such as 25-400 μs, such as 25-300 μs, such as 25-200 μs.

Preferably, the sterilization method is performed at room temperature. However, the temperature of the object or objects to be sterilized may be significantly lower, such as in the case of frozen food items. By room temperature is meant that the sterilization method is performed at around 20 ⁰ C. Sterilization may also be performed at lower temperatures. For example when meat is to be sterilized the temperature may be below the denaturation temperature of proteins.

Efficient cooling of the sterilization vessel in order to reduce damage of the vessel may be required. One suitable method for efficient cooling may be a method where the sterilization vessel is in tight contact with an insulator which may be attached to the electrodes. In order to achieve such tight contact the vessel and the insulator may be mechanically biased towards each other with a predetermined force.

In a second aspect, the present invention relates to an apparatus for sterilization of an object or objects encapsulated in a sterilization vessel, the sterilization apparatus comprising

- a sterilization vessel adapted to encapsulate the object or objects to be sterilized, said sterilization vessel further being adapted to encapsulate a predetermined amount of atmospheric air and a predetermined amount of a inert gas, and

- means for generating an electric field sufficient to ignite a plasma inside the sterilization vessel.

The means for generating the electric field may comprise a first and a second electrode, said first and second electrodes being arranged to receive the sterilization vessel therebetween. The first and second electrodes may be substantially plane electrodes arranged in a substantially parallel configuration thereby forming a capacitor-like arrangement where the sterilization vessel may be positioned between the capacitor plates. However, it should be noted that the shapes of the first and second electrodes may differ from substantially plane electrodes.

The first electrode may be adapted to receive a high voltage of some kVs. The voltage level strongly depends on the object or objects to be sterilized, and the environment into which the object or objects is contained. The second electrode may be electrically connected to a fixed potential level, such as for example ground.

The apparatus according to the present invention may further comprise first and second insulator means. The first insulator means may be positioned between the first electrode and the sterilization vessel, whereas the second insulator means may be positioned between the second electrode and the sterilization vessel. By positioning the first and second insulator means in such a way unintentionally generated sparks may be prevented from damaging the sterilization vessel.

Preferably, the shape of the first and second insulator means match the shape of the first and second electrodes, respectively. Thus, the first insulator means may follow the surface contour of the first electrode, whereas the second insulator

may follow the surface contour of the second electrode. The first and second insulators may be manufactured of a polymeric foil material attached to the respective electrodes. The foil material may be a non-conducting material, such as thermoplastic and thermoset polymers, in particular polypropylene, polyester, fluoropolymer, polyimide, polyamide, PEEK, capton, mica, having a thickness smaller than 500 μm. Alternatively, the first and second insulators may be made of ceramics (e.g. oxides, nitrides, e.g. AI ₂ O ₃ ) or glasses.

In the apparatus according to the present invention the first electrode may be moveably arranged so that its position may be adjusted to match the shape and the dimensions of the sterilization vessel encapsulating the object or objects to be sterilised. Thus, the position of the first electrode may be varied from vessel to vessel in order to ensure optimal sterilization performance with varying sterilization vessel dimensions. For example, in case of a foil-based sterilization vessel encapsulating a piece of meat the sterilization vessel will essentially take the shape of the meat to be sterilized. In case different sizes of meant are to be sterilized the space or volume between the first and second electrodes must be variable in order to ensure optimal sterilization performance.

In case of a moveably arranged first electrode the sterilization of an object or objects in a sterilization vessel may be performed in the following way:

The first electrode is moved away from the second electrode thereby creating the necessary space for easy positioning of a sterilization vessel on the second electrode, or on an insulator means covering at least part of second electrodes. In this way the second electrode, or the insulator means covering at least part of the second electrode, forms a bottom part of a virtual sterilization chamber. Optionally, the sterilization vessel may be positioned on a conveyor belt which brings the sterilization vessel to an optimal position relative to the second electrode. In this situation the second insulator means may be omitted because the conveyor belt itself may be configured to act or function as an insulator means. The second electrode may be positioned below the conveyor belt, whereas the moveably arranged first electrode is positioned above the conveyor belt.

When the sterilization vessel is correctly positioned the moveably arranged first electrode, which may be regarded as a top part of the virtual sterilization chamber, is moved closer to the second electrode so that the required electric field may be generated between the first and second electrodes. The first insulator means may be arranged to follow the movements of the first electrode. This may be achieved by attaching the first insulator means to the first electrode. The first electrode, and optionally the first insulator means, may be moved relative to the second electrode by electric, hydraulic, pneumatic or similar means.

As previously stated the optimal position of the first electrode relative to the second electrode depends on the dimensions of the sterilization vessel, which in case of a sterilization vessel made of a flexible foil, depends on the shape of the object to be sterilized. Thus, the optimal position of the first electrode relative to the second electrode may vary of sample to sample, where the term sample should be interpreted as a sterilization vessel including an object or objects to be sterilised. In order to obtain optimal sterilization conditions for every sample appropriate sensor means may be provided. Such sensor means may for example include one or more force sensors adapted to measure a reaction force generated by the first electrode abutting the sample. The optimal position of the first electrode relative to the second electrode may be defined by a predetermined reaction force or a predetermined range of reaction forces. Thus, when the moveably arranged first electrode abut the sample and an associated reaction force is measured by appropriate sensor means the optimal position may be reached when the measured reaction force equals a predetermined reaction value, or falls within a predetermined range of reaction values.

In a third aspect, the present invention relates to a method for encapsulating an object or objects to be sterilized in a sterilization vessel, the method comprising the steps of

- providing the object or objects into the sterilization vessel through an opening in said sterilization vessel,

- providing a predetermined amount of atmospheric air and a predetermined amount of an inert gas into the sterilization vessel, and

- sealing the sterilization vessel thereby encapsulating the object or objects, the predetermined amount of atmospheric air and the predetermined amount of the inert gas in the sterilization vessel.

According to the third aspect of the present invention, the predetermined amounts of atmospheric air and inert gas are provided during packing of the object/objects in the sterilization vessel. After packaging the mixture of atmospheric air and inert gas remains essentially constant during a period of at least some weeks. Thus, after packaging the object or objects positioned in the sterilization vessels may be sterilised by igniting a plasma in the vessel at any time during a period of at least some weeks.

The predetermined amount of the inert gas may be larger than 80%, such as larger than 85%, such as larger than 90%, such as larger than 95% or such as larger than 98% of the total amount of gas in the sterilization vessel. The inert may be helium, neon, argon, krypton, xenon or nitrogen or any combination thereof. The amount of atmospheric air may be smaller than 20% of the total amount of gas in the sterilization vessel.

The sterilization vessel may in principle be any container capable of encapsulating the object/objects and the predetermined amounts of atmospheric air and inert gas. Thus, the sterilization vessel may be a bottle-shaped plastic container having essentially rigid outer sidewalls and a lid attached thereto, or a bag-like sterilization vessel having flexible outer sidewalls. Such flexible outer sidewalls may be made of a flexible foil, such as a flexible polymeric foil.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further details with reference to the accompanying figure, where Fig. 1 shows a set-up suitable for carrying out the invention.

While the invention is susceptible to various modifications and alternative forms, a specific embodiment has been shown by way of example in the drawing and will be described in details herein. It should be understood, however, that the invention is not intended to be limited to the particular form disclosed. Rather, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

In its broadest aspect the present invention relates to a method and an apparatus for sterilizing encapsulated objects under the influence of an ignited plasma within a sterilization vessel.

Fig. 1 shows an example of an apparatus for carrying out the invention. As depicted in Fig. 1 encapsulated food items have been placed on a moving conveyor belt. The conveyor belt moves the food items from left to right at a speed allowing mass sterilization. Thus, the food item to the very left has not been sterilized whereas the food item to the very right has been sterilized. The food item depicted in the middle is undergoing sterilization in that a plasma has been generated in the sterilization vessel as indicated in Fig. 1.

Each of the food items is encapsulated in a sealed vessel (sterilization vessel) which may have rigid or flexible outer boundaries. Preferably, the sealed vessels are bag-like sealed vessels made of a flexible foil-like material, such as thermoplastic and thermoset polymers, in particular polypropylene, polyester, fluoropolymer, polyimide, polyamide, PEEK, capton, mica.

As previously stated the sealed vessel contains predetermined amounts of atmospheric air and inert gas. These predetermined amounts are provided into the sealed vessels during packing of the food items to be sterilized. After packaging the mixture of atmospheric air and inert gas remains essentially constant during a period of at least some weeks. Thus, after packaging the food items may be sterilised by igniting a plasma in the vessel at any time during a period of at least some weeks.

As mentioned previously, the predetermined amount of the inert gas may be larger than 80%, such as larger than 85%, such as larger than 90%, such as larger than 95% or such as larger than 98% of the total amount of gas in the sterilization vessel. The inert may be helium, neon, argon, krypton, xenon, or nitrogen or any combination thereof. The amount of atmospheric air may be smaller than 20% of the total amount of gas in the sterilization vessel.

The required plasma is ignited by positioned the mixture of inert gas atmospheric air in an electric field. This electric field is generated between the two electrodes depicted in Fig. 1. As depicted in Fig. 1 one of these electrodes is positioned below the conveyor belt whereas the other electrode is positioned above the conveyor belt. The bottom electrode is electrically connected to ground whereas the upper electrode is arranged to be connected to the high voltage potential in order to establish an electric field between the two electrodes. To ensure proper sterilization of food items an alternating electric field of at least 5 kV/cm, such as 10 kV/cm, such as 15 kV/cm must be applied. The AC voltages generating the electric field can be applied continuously or it can be applied in pulses along the lines set out previously. Thus, pulses having a duration (on-time) of 25 μs or 50 μs are applicable. The "off times" are typically varied in order to match an average power level.

As depicted in Fig. 1 the upper electrode is moveably arranged along a direction perpendicular to the direction of movement of the conveyor belt. The mechanism (not shown) adapted to move the upper electrode up and down can be of electric, pneumatic or hydraulic nature. Thus, the vertical positioned of the upper electrode may be adjusted to match a specific height of a given sealed vessel or a series of sealed vessels. An automatic control mechanism may be implemented to ensure optimal positioning of the upper electrode prior to igniting the plasma in the sealed vessel. Such automatic control mechanism typically involves a position and/or pressure sensor and a controller in the form of a PID-regulator.

By positioning a dielectric barrier in the form of an insulator between the upper electrode and the sealed vessel unintentionally generated sparks is prevented

from damaging the sealed vessel. The insulator may be manufactured of a polymeric foil material attached to the upper electrode. The foil material must be a non-conducting material, thermoplastic and thermoset polymers, in particular polypropylene, polyester, fluoropolymer, polyimide, polyamide, PEEK, capton, mica, having a thickness smaller than 500 μm. Alternatively, the insulator may be made of ceramics (oxides, nitrides) or glasses.