NITROGEN ATMOSPHERES

Abstract: A nitrogen-based atmosphere which comprises a small amount of oxygen (0.05 to 10 % by volume) and trace amounts of ozone (1 to 500 vpm) finds application as an inerting gas with an inhibiting effect on spoilage mechanisms in the storage and processing of foodstuffs, beverages, chemicals and pharmaceuticals. The atmosphere is conveniently provided by ozonizing (in G) a dry oxygen-depleted air fraction from a membrane separator (S2).

Description: NITROGEN ATMOSPHERES This invention relates to nitrogen atmospheres, in particular to nitrogen in admixture with ozone. Because of its low level of chemical reactivity nitrogen is widely employed in inert atmospheres, for example in protective or purging atmospheres in the food, chemical and petrochemical industrie;-;. Typical applications include blanketing or purging of storage vessels such as grain silos, oil storage tanks and hazardous waste containers. Other important applications arise with foodstuffs ar.di beverages, including for example modified atmosphere packaging (MAP) to prolong the shelf life of a food product. In several fields it has however been increasingly found that while nitrogen atmospheres are effective in controlling the primary problems such as a risk of chemical explosion or the spoilage of foods by oxidation they are less effective in controlling more insidious risks such as the build-up of mould in a grain silo or the growth of anaerobic bacteria in packaged foods. These other risks have therefore hitherto been reduced by other forms of treatment. In the example of grain silos the mould growth has been prevented by the addition of insecticides. In the fresh vegetable and salads food industry there is common use of sterilizing techniques using relatively high concentrations of chlorine in water to eliminate the bacteria which give rise to food spoilage. Such sterilization is however not popular with the consumer since it tends to affect the taste of the food. Moreover the relatively high volumes that are required to achieve complete sterilization of food or other products may permit residual chlorine to escape into the environment. We have now found that the addition of trace amounts of ozone to nitrogen-based atmospheres creates a treatment medium which is highly effective in storage atmospheres, both for general inerting and in inhibiting spoilage processes such as the growth of bacteria. Thus according to the present invention there is provided a nitrogen-based atmosphere which comprises, by volume, 1 to 500 vpm ozone, 0.05 to 10% oxygen and at least 90% of a nitrogen-containing inert gas. The invention also provides a process which comprises contacting the material with a nitrogen-based atmosphere containing, by volume, 1 to 500 ppm ozone, 0.05 to 10% oxygen and at least 90% of a nitrogen-containing inert gas. The invention further provides apparatus which comprises means for supplying a nitrogen-based atmosphere containing, by volume, 1 to 500 vpm ozone, 0.05 to 10% oxygen and at least 90% of a nitrogen-containing inert gas. The atmospheres, processes and apparatus according to the invention can advantageously be used for a wide variety of applications and products, including foods and beverages, chemical, medical and and pharmaceutical products. They are especially suitable for blanketing and purging of storage vessels such as grain silos and for use in vegetable or fruit stores (e.g. for apples or bananas). Surprisingly it has been found that volumes of ozone well below the levels required for full sterilization achieve the required effects, in particular in the inhibition of spoilage mechanisms. Thus in modified atmosphere packaging the atmospheres of the invention substantially increase the shelf life of the product while not adversely affecting its taste and flavour. Additionally since the breakdown product of the ozone is simply oxygen there is no harmful residual product to be released into the ambient atmosphere. The invention is beneficial in permitting the use of low oxygen atmospheres, which would otherwise tend to encourage anaerobic activity, in situations where anaerobic activity is to be strictly prevented. The reason for the effectiveness of the low proportions of ozone according to the invention appears to be that the ozone volumes, while small, have a highly disruptive effect on spoilage mechanisms. Typical of these mechanisms are rapid bacterial or mould growth on a material surface, cell autolysis through enzyme action and premat " re ripening through the spontaneous generation of ripeι._ng hormones. In many instances the spoilage is displayed as a discolouration of the product or the generation of unpleasant odours. According to the invention the mechanisms causing these effects are disrupted or substantially prevented. We have found that even if the ozone volumes are insufficient to achieve complete destruction of bacteria, enzymes or mould-producing organisms and even if they do not penetrate into the interior of a solid material, they display a considerable inhibiting effect, especially at the contact surface(s). Since much of the spoilage tends to occur at or near the surface the ozone is thus applied to portions of the material at which it has its most significant effect. Another benefit is that the ozone reacts with ripening hormones such as ethylene at the very surfaces which emit such hormones and thus inhibits the ripening process. This benefit is especially marked in the case of banana storage, in which removal of the ethylene generated by the initially-ripening bananas in a store prevents this ethylene from ripening the other bananas and thus permits all the bananas in the store to be offered to the consumer in a "ready to ripen" condition. Similar advantages to the above are obtained in employing the atmospheres to sparge liquids, in particular to sparge beverages such as water, soft drinks and alcoholic drinks (carbonated or non-carbonated) . It is generally preferred to conduct the sparging prior to any carbonation of the liquid and prior to the introduction of ingredients such as flavouring concentrates. In the case of liquid sparging the benefits of the invention appear to result from the intimate contact between the ozone and the liquid throughout a high proportion of the liquid volume. In the medical and pharmaceutical fields, the atmospheres find application in maintaining sterile conditions achieved in a preceding full sterilisation step. For example the packaging of wound dressings, prepared under sterile conditions, can be achieved under the low-dosage ozone atmospheres of the invention without loss of sterility. A side benefit of the use of atmospheres according to the invention in many of its applications is the suppression of unpleasant odours. For example in the bulk storage of organic materials the avoidance of associated odours from anaerobic activity provides a more acceptable ambient atmosphere in the vicinity of the store. As indicated above, the invention is particularly well suited to the provision of nitrogen/ozone mixtures for modified atmosphere packaging. The MAP package is provided with a closure film, typically a laminated film, comprising such polymers as cellulose, polypropylene, polyvinyli ene dichloride and polyamines. The film encloses and seals in the protective atmosphere and is generally required to maintain substantially the same atmosphere through the life of the product. In some instances the film is chosen to allow sonie permeation of gases, such as carbon dioxide or oxygen, in order to maintain the preferred proportions of constituent gases. The particular gas compositions and the closure films are selected to suit the type of food product. They are generally characterized by a low oxygen content, possibly together f'ith a small content of carbon dioxide. In some instances the film is chosen to allow a small amount of permeation of gases, such as carbon dioxide or oxygen, with the objective of maintaining the preferred proportions of constituent gases within the package. Within a sealed film envelope there exists a risk of undue growth of anaerobic bacteria in the oxyge '-depleted atmosphere, leading to rapid spoilage of che packed product. The invention guards against such spoilage by substantially delaying the onset of any significant anaero c activity. The invention is especially suitable for MAP storage of foodstuffs with a high surface area, for example beansprouts, chopped salads produce, mushrooms and cabbage. It is also well suited to packaging of fish, which tends to be susceptible to anaerobic spoilage. The presence of the ozone is also beneficial in ensuring that the surfaces of the closure film and containers for the foodstuff are maintained in a hygienic condition. The absence of moisture has also been found to be advantageous when using atmospheres according to the invention, especially for food storage. Accordingly it is preferred in many embodiments of the invention that the atmosphere contains substantially no water vapour (e.g. less than 500 vpm) . A significant catalytic effect is displayed by water vapour in many chemical and biological procedures, including spoilage mechanisms. In order to ensure a hygienically clean product it has hitherto been customary to include a step of washing with chlorinated water, often immediately prior to packaging. The washing however leaves a residual moisture content which is difficult to remove and which increases the rate of spoilage within the package. Since the atmosphere of the invention has an inherent sterilising effect it is possible in many instances to employ a preparation procedure which avoids the use of chlorinated water altogether but still gives a hygienically clean product. In general it is preferred that the substantially all of the inert gas content of the inhibiting atmosphere is nitrogen (possibly together with argon and other rare gases), but there are some applications (especially in modified atmosphere packaging) in which up to 50% of the nitrogen can be replaced by another gas, such as carbon dioxide, which is substantially inert under the conditions of use. Compared with the relative nitrogen:oxygen volumes in air the atmospheres of the invention should be of depleted oxygen content, specifically an oxygen content of not more than 10% by volume. This volume is below the combustible limit for almost all combustible materials, and thus generally ensures the inert character of the atmosphere. The complete absence of oxygen is not required and in many instances, again particularly in modified atmosphere packaging, must be avoided. Indeed, the complete absence of oxygen encourages undue growth of anaerobic bacteria, which are mostly harmful in their effects on the materials to be treated, and would increase the volume of ozone required to maintain the desired conditions. In general the lower limit of oxygen content is 0.05% by volume. For many foodstuffs, especially vegetables, the oxygen content is preferably in the range 2.0 to 6.0% by volume. Another key factor in delaying spoilage mechanisms is the storage or treatment temperature. For foodstuffs this is advantageously in the range 1 to 7°C, often preferably about 2 to 4°C. It is therefore desirable in many applications to employ a combination of the atmospheres of the invention and the preferred low temperatures for the material concerned. Freezing is generally not required when employing atmospheres according to the invention. Its absence makes for a "fresh" product with more consumer appeal, since there is no risk of frost damage to the structure of the product, and reduces the amount of energy required for the storage. In terms of ozone content it is found that the desired inhibiting effect is obtained with ozone levels in the atmosphere of 1 to 500 vpm by volume. The generally preferred limits for ozone content are 2 to 200 vpm, most preferably 5 to 100 vpm. A further key factor in treatment of materials with a gas having a sterilising effect is the duration of contact between the gas and the material. In general the required period of treatment is inversely proportional to the concentration of the sterilising component. Storage atmospheres can therefore employ low ozone concentrations, typically 1 to 10 vpm, whereas treatments involving short contact times require ozone concentrations towards the upper end of the above limits discussed above. According to the invention the ozone is produced by passing an oxygen-containing stream through an ozonizer. The ozonizer applies an electrical charge which converts some of the oxygen into ozone. Although it may be possible to adjust the electrical input to the ozonizer, and thereby to adjust the ozone content of the product stream, in general it is preferred for the purposes of the invention to operate the ozonizer at a fixed electrical setting and to adjust the volume and content of the product stream by adjusting other parameters such as the proportion of a feed stream to be passed through the ozonizer or the oxygen content of the feed stream. According to the invention it is also preferred to use an oxygen-depleted feed stream to the ozonizer, i.e. a feed stream containing not more than 10% by volume of oxygen. This gives the advantage of producing a product stream which is already low in ozone content and which can be further diluted if required. Hitherto the objective in operating an ozonizer has been to maximise the output of ozone and thus to employ an oxygen-enriched feedstream wherever possible. Thus in one embodiment the invention provides a process for inhibiting bacterial growth in a medium, which comprises ozonizing a feed stream containing nitrogen and not more than 10% oxygen by volume to form a nitrogen-based atmosphere containing, by volume, 1 to 500 vpm ozone, 0.05 to 10% oxygen and at least 90% of a nitrogen-containing inert gas and contacting the medium with the said nitrogen-based atmosphere . In a further embodiment of the invention the gas mixture to be ozonized is part of a larger gas stream which is first divided into a portion to be ozonized and a portion which by-passes the ozonizer. The two portions are reσombined downsteam of the ozonizer. By appropriate selec ion of the respective volumes of portions it is possible to give precise control of the ozone content in the combined stream without adjustment of the ozonizer which can thus be operated throughout at its most effective setting. Further control of the ozone output from a given ozonizer can be provided by the proportion of oxygen in the feed stream. Thus in a typical example with a gas feed volume of 3 3.8 Nm /h to the ozonizer an oxygen content of 2.0% (the balance being nitrogen) gave a product with 26ppm ozone, an oxygen content of 3.5% gave a product with 31ppm ozone and an oxygen content of 5.0% gave a product with 36ppm ozone The nitrogen-containing gas to be ozonized will generally most conveniently be a nitrogen-enriched stream from an air separator. For many duties, including modified atmospheres for food packaging, the air separator is most conveniently a membrane separator. This can produce a nitrogen stream with an oxygen content up to 10% by volume which can be used directly as the feed .trea to the ozonizer. Alternatively it is possible to oz nize the oxygen-rich stream from the membrane separator and to dilute the thus-ozonized product with inert gas to the required proportions. A membrane-generated nitrogen stream has the further advantage that it delivers a moisture-free feed stream to the ozonizer, which thus provides directly the preferred moisture-free atmosphere of the invention. A further advantage of atmospheres according to the invention when produced from a dry feed stream is that they have little or no content of nitrogen oxides (NO ) , thereby assisting in meeting the environmental standards for N NOOx lleevveellss in working atmospheres and in contact with foodstuffs. If desired, the feed stream to the ozonizer can be pre-cooled so that the atmosphere is obtained directly at a preferred low temperature. For additional flexibility of production the system can include one or both of a feed reservoir upstream of the ozonizer and a product reservoir downstream of the ozonizer. Several different versions of apparatus according to the invention are illustrated in the accompanying drawings, Figures 1 to 5, which show schematic views of several forms of apparatus for introducing a small and controlled ozone concentration into a nitrogen/oxygen mixture. The illustrated versions are intended for use in a food packaging line. In each of the illustrated versions a compressor C (Cl, C2, etc) supplies compressed air, via a water separator/filter unit W to an air separator S, from which an oxygen-containing stream is fed to an ozonizer G powered by an electrical supply E. In the version shown in Figure 1 the feed stream is passed through a membrane drier Dl to ensure a dry air feed to a membrane air separation unit SI. Flow line Fl conveys nitrogen-enriched non-permeate gas having a higher nitrogen:oxygen ratio than is required for the packaging gas. Flow line F2 conveys an oxygen-i ch dry permeate gas stream through the ozonizer Gl. The stream leaving the ozonizer Gl has contents of oxygen and ozone higher than required for the packaging gas and is therefore diluted by recombination with the gas stream from line Fl. Adjustment of the volume and ozone content of the packaging gas is provided by a control valve VI in line Fl, which adjusts the flow rate through the separator SI, and a control valve V2 which permits venting of part of the oxygen-rich stream in line F2. In the version shown in Figure 2, the oxygen-rich stream from the separator S2 is simply vented through outlet T2. Part of the nitrogen-enriched non-permeate gas is passed via line F4 through the ozonizer G2 and remainder is passed via line F3 directly to the product supply point P2. The nitrogen:oxygen ratio in the non-permeate output of the separator S2 is set at the desired final ratio by adjustment of control valve V3, and the respective proprtions of the said output passing through lines F3 and F4 are adjusted by control valve V . The figure 2 version offers a particularly simple configuration of process and apparatus for achieving the desired atmosphere and thus represents a particularly preferred version of process and apparatus according to the invention. The apparatus shown in Figures 1 and 2 is best suited to applications in which the nitrogen/ozone product volume matches the demand flowrate of the packaging line. In many instances however, this match is not possible. Figures 3 and 4 therefore illustrate apparatus in which the ozone generation is substantially separate from the main gas sup ly. In the apparatus shown in both Figures 3 and 4 a nitrogen-rich stream from an air separator S3 is fed to a buffer reservoir Rl. This stream has a nitrogen:oxygen ratio above that required for the packaging duty. The Figure 3 version also has a separate, smaller, ozone generating system with a membrane air separator S4 supplying an oxygen-rich stream to the ozonizer G3. A membrane drier D3 is provided upstream of the separator S4 and a recycle line F6 returns nitrogen-rich gas to the drier D3. The ozonized output is conveyed via line F8 for blending with the main stream drawn from the reservoir Rl. The flow rate through the product outlet point P3 is adjusted by control valve V5 in line F7. The Figure 4 version similarly has a separate, smaller, ozone generating system with a membrane air separator S5 supplying an oxygen rich stream to the ozonizer G4 and again has a membrane drier D4 upstream of the separator S5. In this version however the nitrogen-rich non-permeate is simply vented through control valve V8. The ozonized output is conveyed via line F10 for blending with the main stream drawn from the reservoir Rl. The flow through the product outlet point P4 is adjusted by control valve V7 in line F9 and the ozone supply is adjusted by the control valve V8. The Figure 3 version produces a dry and highly oxygen-enriched feed gas for the ozonizer G3. The Figure 3 version produces dry but moderately oxygen—enriched feed gas for the ozonizer G4. The version shown in Figure 5 has components C6, W, S4, V9 and R2 generally similar to those Figures 3 and 4 for producing a nitrogen-rich gas stream. In the Figure 4 version however the supply to the ozonizer G5 is also drawn from the reservoir R2, the proportion of stored gas passing through the ozonizer being controlled by valve V10. Like those in Figures 3 and 4 this system is suitable for use where the gas output does not match the pattern of the packaging requirements. Examples The invention is further described in the following non-limiting examples of the use of a nitrogen/oxygen/ozone mixture in processing and packaging of fresh produce. Example 1. A batch of newly-picked lettuces was stored overnight in a refrigerator at 2°C, removed from storage and chopped into strands of typically 35 mm x 0.4 mm. The chopped lettuc, was then placed in polystyrene packaging trays, in batches weighing 250 g per tray, and put into a packaging chamber containing a controlled atmosphere. A packaging atmosphere was produced in apparatus of the type illustrated in Figure 2 above and had a composition, by volume, of carbon dioxide 10.0%, oxygen 5.0%, ozone 25 vpm, water vapour nil, balance nitrogen. Each tray was then covered by a transparent film of extruded polyvinylidene dichloride and the film was heat sealed to the tray. Ten such sealed packages were placed in a refrigerator maintained at 2°C. Each package was examined once daily for signs of spoilage of the lettuce, in particular for browning of the leaves. No spoilage signs were observed until day 12 when eight of the packages were found to have a brown discolouration at some of the edges of the chopped leaves. Control Example. As a control a further ten packages were stored in an ambient atmosphere at a temperature of 2°C and also examined once daily. Brown discolouration of the leaf edges was detected in seven of the packages on the fifth day and in the remaining three packages on the sixth day. The treatment according to the example represented more than a doubling of the shelf life of the product. Example 2. A batch of newly-picked lettuces was stored overnight in a refrigerator at 2°C, removed from storage and chopped into pieces typically 50 mm x 50 mm after removal of damaged outer leaves. The chopped pieces were exposed to a pre-cooled atmosphere at 2° to 4°C containing, by volume, carbon dioxide 10%, oxygen 5%, ozone 5 vpm, water vapour nil, balance nitrogen, in a tunnel conveyor with the product and gas running countercurrent for a treatment time of 5 minutes. The treated pieces of lettuce were then placed in polystyrene packaging trays in batches weighing 50 gm per tray and put into a packaging chamber containing a controlled atmosphere of composition the same as used in Example 1. The trays were then each covered by the polyvinylidene dichloride film, sealed and stored in the same manner as described in Example 1. Each package was examined once daily for signs of spoilage of the lettuce, in particular for browning of the leaves. No spoilage signs were observed until day 17 when 6 of the packages were found to have some browning at the edges of some chopped leaves.

Claims: CLAIMS 1. A nitrogen-based atmosphere which comprises, by volume, 1 to 500 vpm ozone, 0.05 to 10% oxygen and at least 90% of a nitrogen-containing inert gas. 2. A nitrogen-based atmosphere as claimed in claim 1, which comprises 2 to 200 ppm ozone. 3. A nitrogen-based atmosphere as claimed in claim 2, which comprises 5 to 100 ppm ozone. 4. A nitrogen-based atmosphere as claimed in any preceding claim, which comprises 2.0 to 6.0% oxygen, by volume. 5. A nitrogen-based atmosphere as claimed in any preceding claim, which comprises less than 500 vpm water vapour. 6. A nitrogen-based atmosphere as claimed in any preceding claim, in which the inert gas is a mixture of nitrogen and up to 50% by volume (relative to the nitrogen) of carbon dioxide. 7. A process for preparing a nitrogen-based atmosphere which comprises, by volume, 1 to 500 ppm ozone, 0.05 to 10% oxygen and at least 90% of a nitrogen-containing inert gas, wherein a gas mixture containing oxygen and at least 90% by volume of nitrogen is passed through an ozonizer. 8. A process as claimed in claim 7, wherein the gas mixture to be ozonized is part of a larger gas stream which is first divided into the portion to be ozonized and a portion which by-passes the ozonizer and the two portions are recombined downsteam of the ozonizer. 9. A process as claimed in claim 7 or claim 8, wherein the gas mixture to be ozonized is produced in a membrane air-separation unit. 10. A process as claimed in any of claims 7 to 9, wherein the gas mixture to be ozonized contains not more than 10% by volume of oxygen. 11. A process as claimed in any of claims 7 to 10, wherein the gas mixture to be ozonized is substantially free from water vapour. 12. A process as claimed in any of claims 7 to 11, wherein the gas mixture to be ozonized is pre-cooled. 13. A process as claimed in any of claims 7 to 12, wherein the ozone output from the ozonizer is controlled by the proportion of oxygen in the gas mixture to be ozonized. 14. Apparatus which comprises means for supplying a nitrogen-based atmosphere containing, by volume, 1 to 500 vpm ozone, 0.05 to 10% oxygen and at least 90% of a nitrogen-containing inert gas and which comprises an air separator and an ozonizer. 15. Apparatus as claimed in claim 11, in which the air separator is a membrane unit.

Inventors: PAGE JOHN KENNETH RURIK, PAGE, JOHN, KENNETH, RURIK
Applicants: CALOR GAS LTD, CALOR GAS LIMITED
Application year: 1991
Main classification: A23L3
IPC codes: A23L3, A61L2, A61L9, B01D53, C01B13, C01B21