NO168622B - PROCEDURE AND APPARATUS FOR ASEPTIC PACKAGING OF FOODS - Google Patents

Description

The present invention relates to a method and apparatus for aseptic packaging of foodstuffs by deactivating organisms, microbes or viruses using intense incoherent pulsed light.

Considerable technical efforts have been directed at extending the storage time of foodstuffs and other microbiologically labile products as well as at preserving these products against microbiological degradation. Such efforts have included both the treatment of the products and the development of packaging techniques for preservation.

There are special needs with regard to methods and devices for sterilizing or reducing the microbiological load on surfaces of or inside foodstuffs or other products, and which can be used to reduce or eliminate the need for chemical preservatives. Bakery products such as bread can e.g. pick up micro-organisms, such as mold spores, from the air after they are baked but before they are cooled sufficiently to be packaged. Any significant reheating of the bakery products would greatly dry out the products, and new methods of surface sterilization of such foods and their packaging would be desirable. In a similar way, such foodstuffs as fresh fish can only withstand a relatively limited storage time before being exposed to spoilage by microbes, which limits the distribution and marketing of fresh fish products. Methods and apparatus suitable for extending the shelf life of perishable foodstuffs such as fresh fish would have a very beneficial effect on the availability of fresh fish products to consumers.

Many products, such as e.g. certain fruit juices, are now treated with heat to produce the desired reduction in biological activity, under conditions which cause a reduced taste and body for the treated foods. Methods and apparatus for reducing or eliminating biological activity without such harmful heating would be desirable to produce better taste and fullness of flavor, which would increase consumer interest in the product and thus the market for products thus treated.

Considerable research and development efforts have recently been directed at aseptic packaging technology for packaging sterilized foodstuffs (including foodstuffs with a high and low acid content) in sterile packaging materials, thereby producing preserved foodstuffs with an extended shelf life. However, such methods and devices can have various disadvantages, such as the considerable use of chemical disinfectants which can leave residues of chemical compounds on the packaging material or the foodstuffs. New methods and apparatus for sterilizing food packaging materials and for aseptic packaging would be desirable.

The photobiological effects of light, including visible light (380-780 nm), near-ultraviolet light (300-380 nm) and far-ultraviolet light (190-300 nm) have been studied for many years, such as e.g. reported in Jagger, J., "Introduction to Research in Ultraviolet Photobiology", Prentice Hall, Inc., 1967, and attempts have been made to utilize light to sterilize food items or containers for such items. US-PS 2,072,417 describes irradiation of certain substances, e.g. milk, with active rays, such as e.g. UV rays.

US-PS 3,817,703 describes the sterilization of light-transmitting materials using pulsed laser light. US-PS 3,941,670 describes a method for sterilizing certain materials, including foodstuffs, by exposing the relevant material to laser radiation to thereby passivate microorganisms. However, these methods have various disadvantages, such as limited production capacity, limited efficiency, adverse effects on the food, inefficient energy conversion (electricity to light) and also economic disadvantages.

It is therefore an aim of the present invention to develop a method and an apparatus for reducing biological load on foodstuffs, by improved aseptic packaging of the foodstuffs.

The invention thus relates to a method for aseptic packaging of foodstuffs and which comprises process steps which involve producing an aseptic packaging material with a light-irradiated food contact surface intended to form at least part of the inside of a sealed food container.

On this background of known technique in principle, e.g. from US-PS 4,175,140 and 4,464,336, the method according to the invention has as a distinctive feature that said food contact surface is exposed to one or more polychromatic and incoherent light pulses with high intensity and each of which has a duration in the range from 0.001 to 100 milliseconds, whereby at least 70% of the pulses' electromagnetic energy is brought to lie in the wavelength range between 170 and 2600 nanometers and an energy density is produced on said food contact surface in the range from 0.01 to 50 joules per cm<2> in order to thereby create a sterilized food contact surface, whereupon a sterilized food is supplied for packaging and thereby brought into contact with the sterilized food contact surface, while microorganisms are excluded from contact with said food and said food contact surface, and the sterilized packaging material is sealed for to form a sealed aseptically packaged food product.

The invention also applies to an apparatus for aseptic packaging of foodstuffs and which comprises sterilization equipment to produce a sterilized foodstuff to be packaged as well as equipment for pulsed light irradiation, and whose distinctive feature according to the invention is that the irradiation equipment is arranged to apply a food contact surface to an aseptic packaging material at least one high-intensity polychromatic light pulse of a duration in the range from 0.001 to 100 milliseconds, and with at least 70% of the pulse's electromagnetic energy distributed in the wavelength range between 170 and 2600 nanometers and an energy density on the food contact surface in the range from 0.01 to 20 joule, as the apparatus further comprises equipment arranged to introduce a sterilized food item from said sterilization equipment onto the sterilized food contact surface produced by operation of said pulsed light irradiation equipment while micro-organisms are excluded from contact with the relevant food item and said sterilized food contact surface, as well as yr for sealing the sterilized packaging material to form a sealed food container.

The distinctive features of the invention will appear more clearly from the following detailed description with reference to the attached drawings, on which: Fig. 1 schematically shows an embodiment of an aseptic packaging machine which continuously forms and fills the continuous packaging film and sterilizes the film by means of incoherent light pulses with high intensity in order to produce aseptically packaged food products. Fig. 2 is a perspective sketch with partially cut away parts of an assembly for high intensity irradiation with incoherent pulsed light and sterilization of the aseptic packaging apparatus in fig. 1. Fig. 3 schematically shows another embodiment of a packaging system which is designed to carry out aseptic packaging of a sterilized food product in pre-formed containers which are sterilized by means of incoherent light pulses with high intensity. Fig. 4 shows schematically and seen from the side an embodiment of an aseptic packaging device which forms and sterilizes containers from rolls of thermoplastic material and covering material. Fig. 5a and 5b graphically show deactivation of E. coli and culture media by means of incoherent light pulses with high insensitivity. Fig. 6a and 6b graphically show deactivation of Bacillus subtilis (vegetative) on culture media by means of incoherent light pulses with high intensity. Fig. 7a and 7b graphically show deactivation of Bacillus subtilis (spores) on culture media by means of incoherent light pulses of high intensity. Fig. 8a and 8b graphically show deactivation of Staphylococcus aureus on culture media by means of incoherent light pulses of high intensity. Fig. 9 graphically shows deactivation of Saccharomyces cerevisiae on culture media by means of incoherent light pulses of high intensity. Fig. 10a and 10b graphically show the deactivation of Aspergillus niger in spore suspension on culture media of dextrose agar from potato with the addition of rose bengal, by means of incoherent light pulses with high intensity. Fig. 11 shows the spectral transmission curve for a glass filter made to remove far-UV from the light spectrum of a pulsed flashing lamp. Fig. 12 graphically shows the nominal output spectrum for the linear xenon flashing lamps in a lamp system, with 5% and 50% transmitted waves for the glass filter in fig. 11 indicated by dashed lines. Fig. 13 and 14 graphically show deactivation of E. coli using non-fat dry milk in the culture medium to thereby increase its relative absorption of incoherent light pulses with high intensity and from which UV light has been filtered out. Fig. 15 graphically shows the deactivation of E. coli by adding marker to the culture medium to increase its relative absorption of high intensity incoherent light pulses from which UV light has been filtered out.

The present invention is thus directed to a method and apparatus for deactivating living organisms, microbes or viruses using intense short pulses of incoherent, continuous broad-spectrum, polychromatic light. Application of intense pulses of incoherent polychromatic light with high intensity provides effective treatment at a high rate and with a high degree of efficiency, which entails many practical and economic advantages. The short duration of each pulse further allows, under certain conditions, localization of various lethal effects of the light pulses to a thin surface layer, such as the surface of packaging material.

Such short and powerful incoherent light pulses can be emitted from pulsed gas-filled flashing lamps, spark gaps, discharge apparatus or other pulsed incoherent light sources. Pulsed gas-filled flashers produce broadband light when an electrical current pulse is discharged through the flasher and ionises the gas and produces an intense burst of both continuous radiation and line emission over a wide spectral range. Such flashing lights usually contain inert gases such as xenon and krypton, due to their high efficiency when converting from electrical to optical energy. The use of other gases or gas mixtures as well as gas discharge systems is possible and may be desirable for special applications. Application of an intense pulse of broadband light in accordance with various aspects of the present invention is believed to produce varying extinction effects over a wavelength range where the product to be treated is capable of absorbing light energy, as opposed to the effect of a single line emission spectrum, from e.g. from a low-power germicidal lamp.

According to the invention, special spectral distributions for the pulsed incoherent light with high intensity can also be selected for special purposes, by choosing the operating characteristics of the pulsed light sources and/or by suitable filtering.

In this connection, it is e.g. desirable in certain aseptic packaging processes that the packaging material is treated with pulses that have a relatively high ultraviolet spectral content, thereby reducing the total light flux that is necessary to achieve annihilation or a predetermined reduction of the colony-forming units (CFU) that develop microorganisms. Far and near ultraviolet components of a high intensity incoherent light pulse can be used for efficient and economical deactivation of microorganisms, microbes or viruses by photochemical action to render them reproductively inactive on the surface of or within the area near the surface of solid food packaging material . Spectral distributions and light pulse intensities that utilize a photothermal mechanism, such as through photothermal chromophores inside the relevant microorganism, microbe or virus, or by photothermal adsorption on or near a surface of the object to be treated, are also taken into account here. Both mechanisms can be utilized in effective treatment methods with a high degree of effectiveness.

Preferably, a particular wavelength distribution will be chosen, which will give at least a reduction of initially present colony-forming units on the surface to be treated (or within the volume of a fluid medium to be affected) by a factor of at least 10 (a logarithmic unit reduction with base 10) and most preferably with a factor of one thousand (three logarithmic units reduction by base 10) when processing from at least two pulses of the intense light. Reduction of colony forming units by a factor of at least one million or more (six logarithmic unit reduction, base 10) and up to complete sterilization can be achieved according to the present invention. The desired intensity will be less for ultraviolet-rich light pulses, and will be higher for processes that utilize a considerable degree of surface heating for the inactivation of the organism in question.

Packaging materials to be sterilized may be exposed to intense, short pulses of ultraviolet-rich polychromatic incoherent light, which may possibly utilize surface heating to produce inactivation of microorganisms. In such methods, at least about 15% and preferably at least about 30% of the energy in the light pulses will be at wavelengths shorter than 300 nanometers, while at least about 5% and preferably at least about 10% of light pulse energies should be at wavelengths shorter than 250 nanometers. Such ultraviolet-rich pulses can often have relatively low total energy intensity, such as in the range from about 0.01 to about

15 joules per square centimeter, and typically from about 0.1 to about 3 joules per square centimeter. A single pulse of such ultraviolet-rich light over a wide spectral range can produce effective sterilization of a desired substrate, as well as can be absorbed by and damage with obliterating effect a wide range of different chromophoric groups of microbiological cell constituents over a wide spectral range.

Surfaces of packaging substrates can typically be exposed to 1 to about 20 pulses of incoherent light with high intensity and short duration, the use of at least 2 pulses being particularly preferable.

In accordance with various aspects of the present invention, particularly with respect to such methods where the ultraviolet component of the pulsed light flashes is suppressed or substantially eliminated, the intensity of the pulsed light should be sufficient to heat a surface layer of the relevant packaging material and with a thickness of less than 10 p, at least to about 50°C and preferably to about 75°C, and preferably to at least about 100°C. Such a very thin layer can be heated to a significantly higher temperature within a short time (eg higher than 150°C, such as in the range from 300 to 700°C) at the same time as one or more light pulses are applied. In this way, the heat can be limited to a very thin surface layer to thereby kill microorganisms on the surface without significantly raising the temperatures in the interior of the packaging material. In the context of the present invention, microorganisms are considered to be inactive if they are either killed or rendered reproductively inactive. During the space between successive pulses, the heat present in the surface layer of the relevant packaging material can be diverted to the interior and distributed there without significantly changing the product. The number of light pulses and their total energy can be limited in such a way that the measurable surface temperature of the material is kept below about 100°C for 10 sec. after the pulsed light, and preferably so that the increase in surface temperature due to the pulsed light is limited to less than 50°C for at least 10 sec. after treatment, and preferably to less than 15°C.

In certain embodiments, the treated packaging material may be substantially impermeable to the light to which it is exposed, so that very little light penetrates the material and essentially the entire amount of light (beyond what is reflected) is absorbed in a very thin surface layer, usually of thickness less than around 1 micrometer and up to 1 mm. Light penetrates a material surface in accordance with an exponential formula, namely: where I is the energy intensity of light that has reached a certain distance below the surface, R is the reflection coefficient of the surface, IQ is the intensity of the incident light on the surface, and a is the extinction coefficient which measures the opacity to light of the material used. The light that penetrates the material, but does not penetrate through it, is lost as heat in the material. At any point on the surface, the energy per surface unit (EqO which is recorded down to a depth d in the material indicated by the formula:

As soon as the heat is absorbed in the material by absorption of the light pulse, it begins to dissipate by thermal conduction, mainly in accordance with the well-known law of heat transfer, namely:

where Ec is the energy that is conducted in the material between two planes of surface area A and separated by a distance dx, k is the thermal conductivity of the medium in question, dT is the temperature difference between the two planes mentioned in degrees Kelvin, and t is the time in seconds involved in the heat conduction process.

In certain embodiments, the treated material will be pretreated with a suitable adsorption enhancing agent to achieve a suitable effective average extinction coefficient (a) over the desired spectral wavelength band to produce the desired energy absorption within a suitable depth.

When a beam of continuous light is absorbed in a material surface, the light is transferred to heat mainly in accordance with equation (2), the heated surface being the hottest and creating a temperature gradient in the material which leads to a heat flow down into the deeper layers of the material at a rate which is mainly given in accordance with equation (3). Finally, a stable state is created where the surface temperature is such that the same amount of heat flows down into the depth of the material as is supplied to the surface from the light beam. However, by applying incoherent light pulses of high intensity and a duration that is short in relation to the time constant of the thermal conductivity, the energy can be confined to the treated surface within a very short period of time, during which little or no heat conduction takes place, while a very thin surface layer is mainly heated instantly to a temperature which is much higher than the temperature which can be achieved in a stable state with the help of a continuously acting light beam with the same average effect.

According to certain aspects of the present invention, heating of a surface layer of relatively light-impermeable packaging material is achieved by means of light sources which are capable of producing pulses which can each emit energy densities between about 0.01 and about 50 joules per second. cm<2> and preferably between 1 and approx. 20 joules per cm<2> to the surface of the material in question during the duration of the pulse. Light pulses with an energy content between approx. 2 and approx. 20 joules per cm<2> (e.g. between 8 and 16 joules per cm<2>) can be easily and effectively applied to the food surface. Usually, the energy density of the light pulses that are applied to the product's surface is sufficient to produce the pulsed thermal treatment of a very thin surface layer. In order for the surface temperature to be able to rise before significant amounts of energy have been diverted inwards, this energy is preferably supplied in pulses with a duration in the range from about 0.001 to about 100 milliseconds, and preferably from about 0.1 to about 3 milliseconds. such as e.g. between 0.1 and 1 millisecond. The duration of a pulse is determined by the time that elapses between the rising light energy density of the pulse to half of its peak value and the time when the intensity has decreased to half of the peak value. The total amount of light energy that will be supplied to each product type depends on the properties of the relevant material, such as its extinction coefficient (or absorption) and the reflection coefficient of the material surface. For methods that utilize surface heating, the required heating for the packaging material in question will also depend to some extent on the type or types of surface organisms, microbes or viruses in question that must be destroyed.

For the treatment of foodstuffs or other treatment processes where it may be desirable to limit the application of UV light to the product, the supplied light can mainly be distributed over wavelengths in a range that extends through the visible and into the far and near ultraviolet as well as the near infrared, preferably at least around 80% of the light pulse's energy is distributed within the wavelength range between 270 and 2600 nanometers. In the case of certain special materials to be treated, the supplied light can be distributed mainly on such wavelengths that at least about 90% of the light energy is distributed on wavelengths between 300 and 2500 nanometers. Such light pulses can have at least around 10% of their light energy distributed within the near UV wavelengths, namely between 300 and 400 nanometers. However, visible and infrared light are also very effective when it comes to producing the desired heating of the surface. If desired, part or substantially all of the light energy can be cut off at a predetermined cut-off frequency or a special wavelength band can be omitted, e.g. by filtering, from the pulsed light spectrum. Such filtering can be carried out by means of solid filters, such as UV-absorbing glass filters, or by liquid filters of the kind obtained with a static or flowing liquid envelope around a flashing lamp with unwanted spectral components. This liquid jacket may contain suitable organic or inorganic absorbents, such as inorganic salts which absorb the wavelengths to be removed. A solution of copper sulphate in water and used as a cooling medium in a flash lamp casing (eg 13 grams of CUSO4 per liter of water) provides an effective UV filter for the far ultraviolet. Absorption spectra for solid filter materials, liquids and solutions of organic and inorganic materials are well known and can be selected as desired.

It has been found that short light pulses of high intensity and within the far and near UV range can very effectively deactivate vegetative forms and spore forms of microorganisms using thermal and/or photochemical means. The use of short intense light pulses has been found to allow a significant reduction in the product's processing time and a significantly increased production rate. However, pulsed visible and infrared light is effective when it comes to achieving the desired effect in strongly absorbing media by means of surface heating. The ability to inactivate organisms, microbes and viruses on surfaces using broad-spectrum light makes it possible to inactivate microorganisms (eg microbes or viruses) more effectively by applying incoherent broad-spectrum light pulses through translucent packaging materials, such as glass or clear plastic material , some of which may tend to absorb certain ultraviolet wavelengths.

A plurality of closely consecutive pulses of intense light, and in certain cases only a single pulse, will substantially reduce the population of microorganisms, usually by more than about one order of magnitude (base 10) and preferably by at least two orders of magnitude. Higher levels of reduction (including complete sterilization) can be achieved at appropriate energy levels and number of treatment pulses. Usually between 1 and approx. 50 light pulses to treat packaging material effectively on the surface and preferably between 1 and approx. 20 pulses. It is highly desirable that a number of at least 2 light pulses with high intensity are applied.

In methods and devices which are aimed at sterilizing packaging materials for foodstuffs, fluids such as air or water, or possibly medical equipment, light pulses with high energy and which contain a considerable proportion of ultraviolet radiation can preferably be used. If the surface being sterilized is on a food container, the surface will usually be exposed to between 1 and approx. 20 light pulses, typically from 1 to 3 pulses, to ensure sufficient inactivation of microorganisms. A higher number of pulses, such as e.g. between 5 and 20 pulses, can however be used at lower power levels and/or to achieve increased deactivation.

The space between the pulses with high light intensity and which are applied to a packaging material should be sufficiently long that some of the heat can be dissipated from the surface layer, but still sufficiently short that the different pulses have an overall effect. The time between the pulses applied to the surface to be treated should preferably be between 0.001 sec. and approx. 30 sec.

(e.g. 0.1 to 5 sec.), and preferably less than about 2 sec. in commercial applications. When the pulses are emitted from a single flashing lamp (or a lamp assembly of several lamps that are lit at the same time), the maximum repetition rate is determined based on practical considerations for the cooling parameters of the individual lamps, which will usually give a repetition rate in the range from less than 1 to approximately 1000 times per Sec. The effective repetition rate can, however, be increased by utilizing several flashing lights that are lit in sequence, as well as by creating relative movement between the flashing light and the surface being treated.

Incoherent pulsed light of sufficient intensity as well as of suitable duration and wavelength distribution can be obtained from a flashing lamp system. One such suitable flashing lamp system is sold by Maxwell Laboratories, Inc. under the trademark "Flashblast". A special design model, namely the Flashblast model FB-300, consists of a power supply with direct current and which charges energy-storing capacitors, a switch to control the discharge of the capacitors, a trigger circuit to trigger the switch at pre-programmed time intervals (automatic mode) or when a contact button pressed by the operator (manual mode), a set of high-voltage coaxial cables carrying the discharge pulses from the capacitor and switch assembly, as well as from one to four flashing lamps mounted in metal reflectors to direct the emitted light from the lamps.

In order to increase the effectiveness of the treatment with pulsed incoherent light of high intensity, especially for translucent, reflective or relatively non-absorbing substrates, a suitable absorption enhancing agent can be applied to the surface of the packaging substrate.

In accordance with various aspects of such methods, an absorption promoting agent may first be applied to the surface to be treated. The agent may be applied in any suitable manner, such as by spraying or coating the surface with a powder containing the agent in question or by applying the agent dissolved in a liquid, such as an aqueous or non-aqueous solution of the agent, which may then applied by spraying, coating or immersing the substrate to be treated, or by spraying the agent on the surface of the relevant packaging material.

Suitable absorption promoting agents should have a high optical absorption coefficient at the desired spectral wavelengths within the spectral range of the high intensity light pulses used in the treatment. Although the agent can mostly be completely removed from the product by treatment, it should ideally be an edible material which is known to be safe and which can be easily applied to the relevant packaging surfaces which are to be brought into contact with the surface of food products.

It is desirable that the agent in question can be selectively absorbed into the surfaces of living cells, so that the amount of the agent used can be reduced to a minimum or its effect concentrated. Indicator agents, such as dyes which are photon-sensitive, pH-sensitive or which are sensitive to oxidation potential, can be utilized to the advantage of the treatment, so that the photon absorption of the agent can be varied as part of the treatment process. Such indicator dyes can be useful for special packaging films or treatment processes where the dye's absorption is increased or decreased during treatment with pulsed light. Absorption-promoting agents can preferably be chosen in such a way that they evaporate without decomposing or have benign decomposition products. Examples of such agents include dyes for foodstuffs, medicines or cosmetic products, such as carotene, red dye no. 3, lemon green, black cherry or mixtures thereof.

The different natural dyes and natural food coloring agents can be suitably used, as well as different natural oils or food oils. Mixtures of two or more components with different absorption maxima can, if desired, be used to increase the optical absorption over the desired spectrum.

As the present invention has now been described in its generality, the invention will in the following be described in more detail with reference to special embodiments shown in the figures as well as various examples. In this connection, it is in fig. 1 shows an apparatus 10 for aseptic packaging and where a roll 102 of ordinary flexible aseptic packaging material by means of a series of rollers 104 is led in accordance with usual practice to a solution of an optionally used absorption promoting agent, as previously described, in a dipping vessel 106. The packaging material can typically comprise a layered structure of one or more inner coatings and sealing layers, a metal foil, e.g. of aluminium, a lamination layer or paper layer and an outer layer, all in accordance with normal practice.

Excess of the applied solution of absorption-promoting agent can be removed with the help of rollers 110, while the relevant film is then formed into an elongated sealed tube with the help of a longitudinal sealing device 112. Depending on whether an overlap seal or a butt seal is desired, a strip 108 is applied to one edge of the packaging material to reinforce the longitudinal sealing seam, as well as to prevent the product from coming into contact with the edge of the film 102.

An important aspect of the aseptic packaging apparatus 10 is the product fill and flasher assembly 200, which is

shown in more detail in fig. 2. The shown assembly 200 comprises an outer carrier tube 202 with applied one or more flashing lights 204 distributed around and along the tube 202 in such a way that upon pulse release the entire inside of the sealed packaging material tube will be exposed to intense incoherent light pulses of short duration . Several different arrangements of flashing lights along the carrier tube 202 are possible in practice, the essential feature being that the entire inside of the packaging material tube is exposed to the pulsed light. Inside the carrier tube 202 is a tube 206 for sterile food product and an electric flashing lamp cable 208, and possibly lamp coolant lines 210 can be placed between the tubes 202 and 206. In addition, sterile air can be supplied under pressure from a suitable source (not shown) and is introduced into the closed pipe. Sterile air can be produced using many different techniques (filtration, combustion), including the use of intense coherent light pulses, as described here. In operation, the elongated, closed film tube, which is sealed transversely by means of a suitable transverse sealing device 114, has been filled with a predetermined quantity of mainly sterile food product 212. The sterilized food product can be produced by short-term high temperature treatment or other processes. The elongated sealed film tube is advanced a package length while the flasher assembly is pulsed a number of times to repeatedly sterilize the entire interior of the tube adjacent to the food product 212. Sterile air 220 flows out of the carrier tube 202 and is passed over the flasher assembly to cool the flashers and to remove from the longitudinally sealed film tube any flaking products produced by the flashing lamp discharge as well as to prevent contaminants from settling on the treated area. After the sealing in the transverse direction, the different packages can be separated from each other to form individual consumer packages 116.

The present method can also be used in other types of aseptic packaging systems, such as those that use pre-formed product containers. In this connection, it is in fig. 3 shows an aseptic packaging device 30. The device 30 utilizes preformed product containers 302 which are introduced into the sterilization zone 304 of the device 30. Optionally, a solution of absorption-promoting agent, as previously described, can be injected into the containers 302 using the nebulization device 306. The containers then pass in turn through several treatment stations 308 where inverted U-shaped flashing lights, linear flashing lights, bulb-shaped flashing lights and/or flashing lights of other designs are passed over or into the container openings, with the flashing lamps sending out at least one pulse per container 302. The treatment stations are then withdrawn and the containers are advanced one station, the process being repeated in such a way that the entire inside of each of the containers is exposed to a plurality of intense incoherent light pulses as it is advanced along the treatment stations. A device for cleaning with sterile air can also be used to remove any burnt-on device from the interior of the containers and to prevent contaminants from settling in the treated containers, as well as to cool down the flashing lights. A suitable stationary battery of flashers may also be provided to treat the exterior and edge surfaces of the containers after their passage through the flasher processing zone, if desired. The sterilized containers then pass through the filling station 312, where a pre-treated food product is introduced into the containers, which are then sealed at the top by means of a sterile lid.

A laminar, sterile air curtain may be arranged over the entire aseptic packaging apparatus 30 for the purpose of preventing infection of the packaging units. The sterile air may be generated from a gas sterilization apparatus 350 which includes an air blowing fan 352, which drives air through the filter 354 to a pulsed light treatment zone 356 and which contains a battery of high power xenon flash lamps 358 which is surrounded by a reflective housing 360. Air is continuously driven through the zone 356 at a flow rate consistent with the pulse rate of the lamps 358, which ensures that all air is exposed to multiple polychromatic, incoherent light pulses of high intensity, as previously described, as the air passes through the zone 356. It is desirable that the light pulses are rich in ultraviolet radiation (e.g. with at least 15% of the light energy at wavelengths shorter than 300 nanometers) and that they have an energy density of at least 0.5 joules per square centimeter everywhere in the treatment zone through which all air passes. The pulse duration can typically be in the range from about 0.1 to 3 milliseconds. The lamp and reflector group provides multi-directional, mainly uniform illumination of the air and other flowing gas, so that a dust particle or a bacteria-colony-forming unit is treated from all sides and is not self-shielded. This multidirectional treatment is an important feature of the system 350. Other arrangements for fluid treatment may possibly be utilized in place of the air treatment system 350.

In fig. 4 shows a further embodiment of an aseptic packaging apparatus 40 which comprises two coils 402 and 404 for packaging material made of plastic, namely one for the finished packaging body and one for the packaging lid. The material for the container body may optionally be passed through a bath 406 of absorption-promoting agent, as previously described. The packaging material 402 can then be passed through a suction and drying section to remove excess absorbent solution. The packaging material is then exposed to intense incoherent light pulses from a series 408 of flashing lights along the direction of movement of the packaging material. The packaging material 402 can then be hot-formed into suitable containers in the forming apparatus 410, which are then filled with aseptically prepared food in the filling station 412. The lid material can likewise optionally be passed through a bath 414 of absorption-promoting agent, and exposed to a number of powerful incoherent light pulses from the flashing lamp array 416, as well as used to close the filled and shaped containers. The device as a whole can be kept under a sterile air blanket.

When treating with light of high intensity and short duration to achieve sterilization of packaging materials, the entire broad radiation spectrum from the flashing lamp will usually be used, including the near and far ultraviolet radiation components of the spectrum, so that relatively low amounts of energy can be used. Even at very high organism densities (up to 1 x IO<6> to 1 x IO<8> CFU/cm<a>), e.g. one or two light flashes at an energy density of 1.5 J/cm<2> per light flashes result in sterilization both with regard to spores and vegetative bacteria as well as viruses.

When the flash lamp spectra have at least about 10% of their energy at wavelengths less than about 300 nanometers, as is preferred, or when the product itself is sufficiently absorbent to provide the desired spectral interaction, the bath of absorption enhancing agent in the previously described aseptic packaging apparatus may be eliminated, if desired. For operation at a high processing speed and high power densities, it may be desirable to cool the flashing lamps using a possible water tap. For translucent packaging or other packaging materials, it may also be desirable to arrange a row of lamps on the outside of the elongated closed film, so that only the product and the tube for counter-flowing sterile air need to be introduced into the film tube.

Having described the present invention in principle, various aspects of the invention will now be described in more detail by means of the following special examples. These examples will show the effectiveness of the invention in reducing or eliminating microorganisms on various bacteriological media and packaging materials. In certain examples, microorganisms were intentionally placed on the surfaces to be treated, while in other examples different materials were treated to remove naturally occurring microorganisms. It should be noted that the deliberate application of very high densities of microorganisms to certain surfaces provides a high degree of self-shielding for the microorganisms. This self-shielding involves an increase in the intensity and/or number of light pulses required to achieve a given degree of reduction of colony-forming units beyond what is required to achieve the same degree of reduction for a lower density of present colony-forming units. These examples, which use high microorganism densities, therefore constitute a dramatic demonstration of the efficiency achieved by treatment with pulsed light.

EXAMPLE 1

In order to show the effectiveness of intense, pulsed and incoherent flashes of light in the aseptic packaging and preservation of foodstuffs, a series of tests were carried out where cultures of different microorganisms, which were representative of naturally occurring food spoilage microorganisms, were applied to the surface of a culture medium. The inoculated culture medium was then exposed to intense, incoherent and pulsed light under different experimental conditions. The light pulses were emitted from the light pulse generator system FB-300 Flashblast from Maxwell Laboratories, Inc. This light pulse system has a rectilinear xenon flashing lamp in a reflective housing, and through this lamp, under the control of a high current switch, a 745 microfarad capacitor battery is discharged which can be recharged to 2600 volts and emit a maximum energy of 2500 joules. The xenon flasher has a slightly UV-permeable quartz glass dome with a 7 mm bore and an arc length of about 23 cm and was filled with xenon at a pressure of 450 torr.

The spacious reflector housing for the lamp in the pulsed light system used in the following examples was designed for no more than 25% luminous flux variation over a sample area of

2.54 x 10.16 cm.

Cultures of Aspergillus niger and Saccharomyces cerevisiae were grown on potato dextrose agar (PDA) (pH 5.6, not further acidified with tartaric acid) at 25°C. Asgergillus niger mold spore suspensions were collected in 0.1% sterile Tergitol 7 (an-ionic), and treatment blots were performed on PDA containing 0.05% rose bengal (acid red 94: tetraiodotetra-chlorofluorescein, Na salt) to prevent mycelial spread. Saccharomyces cerevisiae and all the other microbial cultures, except Aspergillus niger, were treated on tryptic soy agar (DIFCO). Vegetative cultures of bacillus subtilis were grown in 15 ml tryptic soy broth as an unshaken flat culture in a 10 x 100 mm petri dish at 35°C. Other bacterial cultures were grown as 10 ml deep-water culture at 35°C. The spore suspension of Bacillus subtilis was a purified spore polylation containing more than 95% spores and was stored in distilled water at 4°C and diluted for plating immediately before processing. Fungal cultures were incubated at 25°C while the bacterial cultures were incubated at 35°C.

The stock cultures, namely the spore suspensions of Bacillus subtilis and Aspergillus niger, were serially diluted before treatment in sterile tryptic soy broth of a 1/10 strength.

Drops of 25 microliters of undiluted stock suspension as well

1 x 10-<1> through 1 x 10~<6> dilutions of the stock suspension were spot plated in two rows along the midrib of a Petri dish containing approx. 15 ml tryptic soy agar. The approximate width of the pattern in two dots was slightly less than 2.5 cm (one inch). The drops were air-dried at 35°C and then exposed to intense and incoherent pulsed light.

After exposure and incubation on plates, the growth pattern was examined on exposed plates as well as on control plates. The growth pattern on the treated plates was recorded for comparison and to determine the degree of inactivation in exponent factors of 10 (each reduction by an exponent of ten will be referred to as a "log"). Treatments that produced no survivors were denoted as X log where " denotes no survivors and where X denotes the analytical limits of the trial.

The results of the deactivation tests are shown in fig. 5 to 10. Deactivation using an unfiltered spectrum was usually very strong, with complete sterilization observed

host in many cases. Even with very low energy flux on

0.1 J/cm<2> or less, several log deactivation was observed. When far UV irradiation was essentially eliminated from the pulsed light spectrum using a 6.4 mm (1/4 inch) thick glass filter (Pyrex), the luminous flux and number of light flashes had to be increased substantially to achieve a similar deactivation effect. The spectral transmission curve for UV glass filter is shown in fig. 11. The nominal spectral output for a linear xenon flashing lamp in the FB-300 system is shown, together with the wavelengths for 5% and 50% transmission (shown by each dashed line) for the glass filter, in fig. 11, indicating a steep transmission cutoff at about 300 nm for light pulses through the glass filter. At energy fluxes of 8 to 12 J/cm<2>, typically ten light flashes were used to achieve a deactivation of more than one log. At these energy levels, sterilization was achieved after a greater number of light flashes (from 15 to 30 depending on the nature of the sample). A threshold effect will also be apparent from the experimental data shown in fig. 5 to 15, where a small increase (less than 20%) in the treatment intensity produced a dramatic increase in the achieved deactivation.

EXAMPLE 2

As previously stated, the use of incoherent light pulses with a significant content of far-ultraviolet light will significantly reduce the energy flux in the light pulses which is necessary to achieve a predetermined degree of lethal effect when treating food products or packaging material substrates. In the absence of significant content of far UV in the light pulses, absorption in the substrate (and/or the microorganisms) in the near UV, visible and near infrared wavelength range is important to achieve high deactivation of microorganisms.

In experiments of the kind indicated in example 1, where E. coli was treated with and without a glass filter with one, two and four flashes of light at 1.5, 4.1, 8 and 12 J/cm<2>, e.g. e.g. no surviving organisms were observed on any of the plates treated with the full light spectrum, while no effect was observed with the filter in place at these treatment levels. However, it was noted that the bacteriological medium used was substantially translucent to the intense incoherent irradiation. A 5 mm thick layer of tryptic soy agar in the plastic petri dishes used thus only attenuated the light by about 35% and the majority of this attenuation took place at wavelengths less than 300 nm, which only represented about 15% of the total incident radiation. It was thus determined that when the treatment spectrum was filtered using a quarter inch pyrex glass filter to remove wavelengths less than 300 nm from the incident light on the specimen, the light absorption on the surface of the bacteriological medium was insufficient to produce efficient coupling of the light energy to the treated surface.

To demonstrate the effect of media absorptivity (and surface properties) on deactivation by pulsed light with the ultraviolet pyrex light filter in place, an E. coli culture was inoculated onto a 45 mm diameter white millipore filter with 0.65 micrometer pore diameter, as well as on similar filters that had been colored black with marker.

Inoculated filters were treated in a sterile petri dish and then placed with the bacterial side up on the surface of a tryptic soy agar plate. Bacteria seeded on a white millipore filter showed greater than 6 log deactivation after four light flashes at 12 J/cm<2>. In comparison, essentially no deactivation was observed using bacteria inoculated directly onto tryptic soy agar and exposed to four light flashes at 12 J/cm<2>. Similarly, bacteria on a black millipore filter required only one light flash at 5 J/cm<2> to produce inactivation greater than 6 log.

A further demonstration of the effect of the medium in question's ability to absorb pulsed light for deactivation with the quarter-inch pyrex glass filter in place, a series of experiments were carried out in which no fat dry milk or ink was added to the bacteriological medium to thereby increase its relative absorption capacity against the intensely incoherent pulse light. The results obtained are shown in fig. 13 to 15 indicating the inactivation of E. coli on media containing different concentrations of non-fat dry milk (NFDM) or ink when treated with 5, 10, 15 or 20 light flashes at 4 J/cm<2> incident irradiation with the pyrex glass filter in place .

The mechanism of deactivation using pulsed light with wavelengths greater than 300 nm is believed to be different from that observed using spectral output with a broad far-ultraviolet spectrum. Deactivation using the full strobe spectrum is believed to be similar to the effects of far ultraviolet in terms of dose and deactivation kinetics, while deactivation using only spectral output greater than 300 nm is believed to reveal a threshold relationship between dose and deactivation. This mechanism is then considered to be produced by flash heating of the surface of the medium and varies with the ability of the uppermost medium layer to absorb (ie interact with) the incident spectral flux during the short flash period.

When intense and incoherent pulsed light is used, two mechanisms are thus believed to influence the inactivation of microorganisms. These are respectively a photochemical and a photothermal mechanism. Both mechanisms exist through effective treatment processes.

EXAMPLE 3

A number of experiments were conducted to determine the effectiveness of intense and incoherent pulsed light in sterilizing the packaging material. The lid and container material used by a leading food processing company in the USA for packaging a dessert pudding product was chosen as the test substrate. The stock material for manufacturing the container comprised a plastic material/polyethylene laminate with an oxygen barrier of polyethylene/ethylvinyl alcohol. The starting material for the lid was an aluminium/polyethylene laminate. Small pieces of each material were sterilized by autoclaving, hydrogen peroxide (3% or 10%) and heat, or possibly bactericidal ultraviolet light. The results thus obtained were not influenced by which presterilization technique was used. A polyethylene surface of each sample of each material was then uniformly inoculated with staphylococcus aureus in saline, Bacillus cereus spores in water or Aspergillus niger spores in a 0.1% tergitol 7 and then air dried at 35°C. The inoculum concentration was between 100 and 1000 CFU per square centimeter surface area. Grafted pieces of the cap and container material were then pulsed with intense incoherent light using the FB-300 system. A single pulse of length 1.5 milliseconds. (full width between half maximum values) was given at the total determined light energy per square centimeter. Both the treated samples and the control samples of the lid and container material were incubated individually in sterile tryptic soy broth at 35°C for samples inoculated with Staphylococcus and Bacillus and at room temperature (21°C) for the Aspergillus samples. The results obtained are shown in table 6 where a + value indicates that growth was detected and a - value indicates that no growth was determined after one week of incubation.

These results show the ability of intense and incoherent pulsed light to sterilize common packaging materials in the form of polyethylene laminate.

The use of intense and incoherent pulsed light for the deactivation of organisms has been found to have many advantages compared to the use of conventional continuously operating light sources or laser light sources operating in a continuous or pulsed mode. These advantages of the inactivation of microorganisms by the use of intense and incoherent pulsed light sources include the achievement of high energy fluxes, the use of wide spectral ranges, possibilities for spectral tuning by varying the operating conditions and/or by filtering, high efficiency when converting electrical energy into light energy . High product throughput is also made possible by the use of strongly pulsed light sources, as well as economical operation.

Claims (8)

1. Method for aseptic packaging of foodstuffs and which comprises process steps which involve producing an aseptic packaging material with a light-irradiated food contact surface intended to form at least part of the inside of a sealed food container, characterized in that said food contact surface is exposed to one or more polychromatic and incoherent light pulses with high intensity and each of which has a duration in the range from 0.001 to 100 milliseconds, with at least 70% of the pulses' electromagnetic energy being brought to lie in the wavelength range between 170 and 2600 nanometers and an energy density is produced on said food contact surface in the range from 0.01 to 50 joules per cm<J> to thereby create a sterilized food contact surface, whereupon a sterilized food item is supplied for packaging and thereby brought into contact with the sterilized food contact surface, while microorganisms are excluded from contact with said food item and said food contact surface, and the sterilized packaging material is sealed to form a sealed aseptically packaged food product. 2. Method as stated in claim 1, characterized in that light pulses are used with such an energy distribution that at least approx. 10% of their energy lies in the wavelength range from 170 to 300 nanometers. 3. Method as stated in claim 1, characterized in that a previously designed food container is used as aseptic packaging material which is sealed by applying and sealing a sterile lid to the container. 4. Method as stated in claim 1, characterized in that a flexible material is used as packaging material which is formed into a tube by longitudinally joining the opposite side edges of the packaging material, and where a sealed aseptically packaged food product is formed by transverse sealing of the longitudinal edge-joined packaging tube. 5. Method as stated in claim 1, characterized in that the aseptic packaging material is brought into contact with an absorption-promoting agent before the exposure to the light pulses. 6.. Method as stated in claim 2, characterized in that light pulses are used with an energy density at said food contact surface in the range from 0.01 to 15 joules per cm<J>. 7. Method as stated in claim 1, characterized in that each of the light pulses is set to a duration in the range from 0.5 to 3 milliseconds. 8. Apparatus for aseptic packaging of foodstuffs and which includes sterilization equipment to produce a sterilized foodstuff to be packaged as well as equipment for pulsed light irradiation, characterized in that the irradiation equipment is arranged to apply to a food contact surface on an aseptic packaging material at least one high-intensity polychromatic light pulse of a duration in the range from 0.001 to 100 milliseconds, and with at least 70% of the pulse's electromagnetic energy distributed in the wavelength range between 170 and 2600 nanometers as well as a energy density on the food contact surface in the range from 0.01 to 20 joules, the device further comprising equipment arranged to introduce a sterilized food from said sterilization equipment onto the sterilized food contact surface produced by operation of said pulsed light irradiation equipment while microorganisms are excluded from contact with the food in question and said sterilized food contact surface, as well as equipment for sealing the sterilized packaging material to form a sealed food container.