IRRADIATED PACKAGED MOISTURISED TUBULAR FOOD CASING

Abstract: An irradiated package comprising a mold and bacteria impermeable barrier encasing a tubular food casing which food casing comprises a cellulose film matrix and from about 9 to about 40 weight percent water. The package has been irradiated with sufficient radiation to prevent growth of microorganisms within the package for at least four weeks and insufficient radiation to reduce burst pressure of the casing by more than about 12 percent. The amount of radiation is usually from about 1 to about 20 kilograys and preferably from about 5 to about 10 kilograys. The amount of radiation is sufficient to reduce the number of microorganisms within the package by well over 99 % and is usually sufficient to reduce the number of microorganisms by over 99.9 % and is often sufficient to reduce the number of microorganisms by 99.99 %. Reduction in burst pressure due to radiation exposure is preferably less than 4 percent. Growth of microorgnaisms, especially mold, is prevented inside of the package for at least four, desirably at least six and preferably for at least twelve weeks. In the most desirable case, microorganism growth inside the package is prevented indefinitely.

Description: IRRADIATED PACKAGED MOISTURIZED TUBULAR FOOD CASING Pflcjc rpυn of the Inven ion This invention relates to packages for food casing in the form of "strands" or "reel stock". Food casing strands are shirred tubular film food casing. "Shirred" means radially folded so as to be compressed along the longitudinal axis of the tubular film. "Reel stock" is tubular food casing that is flattened and wound upon a reel. Such food casings are packaged for storage and shipment to food processors, e.g. meat packers making cylindrically shaped sausage product. Numerous types of packages for food casing have been used in the prior art. Such packages have included rigid cartons as well as net and film wrappings. Film wrappings have had certain advantages, e.g. they are light weight and usually provide a moisture barrier to prevent dehydration of moisturized food casing products. Food casings are often moisturized before shipment to a food packer customer because water is a natural plasticizer for certain tubular food casing products, especially food casings comprising cellulose film. Such casings may then be considered "ready to stuff and are sometimes referred to as "premoisturized". "Cellulose film" is intended to include any film product that is primarily cellulose that is either regenerated from a cellulose derivative, such as xanthate viscose, or precipitated from a solution of cellulose in a cellulose solvent, such as a mixture of tertiary amine oxide and water. "Cellulose film" is intended to include both unreinforced cellulose and cellulose reinforced with fibrous or particulate material. An especially suitable reinforcing material is cellulose fiber in the form of woven or felted mats. Fiber reinforced cellulose tubular food casing is referred to as "fibrous" food casing. Packaged moisturized cellulose tubular food casing does, however, present difficulties. In particular, moisturized cellulose presents an environment for growth of mold and other microorganisms. While small amounts of most such microorganisms are not harmful, they are nevertheless often unsightly and a few are infectious or produce toxins. Where it is not possible to control whether infectious or toxin producing microorganisms grow among or preferentially to essentially harmless microorganisms, it is necessary that growth of all such microorganisms in food products be restricted. It has been known that antimycotics, e.g. propylene glycol, propyl paracept or alkyl esters of p-hydroxybenzoic acid could be used to reduce mold growth, see e.g. U.S. Patents 4,867,204 and 4,664,861. Such chemical antimycotics are effective at preventing mold growth but create their own problems. In particular, chemical additives to food products or wrappings for food products are generally undesirable. The mere fact that they inhibit microbial growth shows that chemical ant microbials are biologically active and may thus have unknown undesirable effects upon humans. Further, antimycotics do not necessarily prevent the growth of non-mold microorganisms, such as bacteria, that may be less visible than mold. Such antimycotics may thus actually mask dangerous food spoilage. Further, the incorporation of an antimicrobial, especially antimycotics, into food casing may cause some of the antimicrobial to be transferred to the surface of encased food. This not only causes ingestion of the "chemical" antimycotic, but may also result in a misleading surface effect, i.e. prevention of mold growth at the surface of food product thus preserving appearance, while permitting dangerous spoilage to occur beneath the surface. It would therefore be desirable to avoid the use of chemical antimycotics and other chemical antimicrobial compounds to preserve packaged moisturized food casings. It has been known that high energy radiation could kill microorganisms. It has also been known that high energy radiation can adversely affect properties of cellulose, see e.g. abstracts of the following articles: Ueno et al., "Manufacture of Low DP (degree of polymerization) Pulps by Irradiation", Journal Kami Pa Gikyoshi, vol. 26, no. 4, pp 164-172 (1972); Ueno et al., "Manufacture of Low DP (degree of polymerization) Pulps by Irradiation", Journal Kami Pa Gikyoshi, vol. 25, no. 9, pp 465-474 (1971); Ueno et al., "Manufacture of Low DP (degree of polymerization) Pulps by Irradiation", Journal Kami Pa Gikyoshi, vol. 25, no. 5, pp 242-249 (1971); Imamura et al., "Manufacture of Low Degree of Polymerization Pulps by Irradiation", Journal Kami Pa Gikyoshi, vol. 25, no. 3, pp 121-127; Usmanov et al., "Change of Some Properties of Cellulose under Gamma-Irradiation and During Storage", Journal Vysokomolekulyarnye Soedineniya, Series A, vol. 22, no. 1, pp 77-82; Takacs et al., "Effect of Gamma-Irradiation on Cotton Cellulose"; Journal of Radiation Physics and Chemistry, nos. 5-6, pp 663-666, (Aug 1999); Gikyoshi, "Effect of Electron Beam Irradiation on Characteristics of Paper", Japanese Tappi Journal, vol. 49, no. 7, pp 1086-1097. A decrease in an important property of cellulose known as "degree of polymerization" (DP), believed to be related to film strength, is known to occur by exposure to high energy radiation. For example it has been reported that DP dropped almost 75%, i.e. from 1200 to 330, in cotton fibers with an exposure of only 15 kilograys (Takacs et al. above). The even higher exposure of 25 kilograys has been reported as being the usual sterilizing dose for papers. This knowledge has led persons skilled in the art to believe that suitable radiation sterilization of cellulose products could not be obtained without unacceptable loss of needed physical properties. It is known that at least some of the strength of cellulose containing papers can be maintained if dry and wet strength resins are used in conjunction with cellulose fibers. See abstract Takeo et al., "Effect of Irradiation on the Properties of Paper and Paperboard", Journal of the Japan Wood Society, vol. 36, no. 8, pp 665-671. Unfortunately, in the case of tubular food casing, the use of such strengthening resins are undesirable since they may add undesirable stiffness and add cost. Further, many resins, that might help maintain cellulose integrity and strength, are not suitable for contact with food. It should be understood that the manufacture and sale of food casings is a cost sensitive business. Irradiation for sterilization has typically been done by shipping product to be sterilized to a licensed facility where highly radioactive isotopes can be safely handled and stored. There have been only a few, widely dispersed, facilities suitable for this type of sterilization. After the product reaches the facility, the product is sterilized by irradiation. The sterilized product must then again be shipped from the facility. The shipping and sterilizing process steps add complexity to the packaging process and add significant cost to the final product. Another problem associated with shipment to a sterilization facility is that microorganisms may grow in transit. The use of radiation for sterilization of cellulosic tubular food casings has therefore been considered impractical, if not entirely impossible. Recently, smaller radiation facilities have been proposed and developed that can be set up next to a plant that has significant sterilization needs. Such smaller "on site "radiation sterilization facilities reduce shipping costs that cannot be easily absorbed in cost sensitive products. Further, such "on site" facilities could reduce the risk of growth of microorganisms in transit. This does not, however, help to overcome the basic problem of degradation of properties of cellulose by exposure to radiation. Brief Description of the Invention It has now been surprisingly discovered that despite the fact that it is known that cellulose is severely degraded by radiation, tubular cellulose film can be irradiated with sufficient radiation for sterilization without unacceptable loss of properties required for food packaging. It has been surprisingly found that the important burst pressure property of cellulose food casing is reduced by less than 12 percent at an exposure as high as 20 kilograys. It has even more surprisingly been discovered that effective sterilization of cellulose film food casing can be accomplished at exposures of less than 10 kilograys with a loss of burst pressure of less than 4 percent. In accordance with the invention, there is therefore provided an irradiated package comprising a mold and bacteria impermeable barrier encasing a tubular food casing which food casing comprises a cellulose film matrix and from about 9 to about 40 weight percent water. The package has been irradiated with sufficient radiation to prevent growth of microorganisms within the package for at least four weeks and insufficient radiation to reduce burst pressure of the casing by more than about 12 percent. The amount of radiation is usually from about 1 to about 20 kilograys and preferably from about 5 to about 10 kilograys. The amount of radiation is sufficient to reduce the number of microorganisms within the package by well over 99% and is usually sufficient to reduce the number of microorganisms by over 99.9% and is often sufficient to reduce the number of microorganisms by 99.99%. Reduction in burst pressure due to radiation exposure is preferably less than 4 percent. Growth of microorganisms, especially mold, is prevented inside of the package for at least four, desirably at least six and preferably for at least twelve weeks. In the most desirable case, microorganism growth inside the package is prevented indefinitely. Detailed Description of the Invention "Irradiated" means exposed to penetrating high energy radiation, e.g. gamma rays or x-rays. The amount of radiation used is sufficient radiation to prevent growth of microorganisms within the package for at least four weeks and preferably at least about six weeks and more preferably at least about twelve weeks, Insufficient radiation is used to reduce burst pressure of the casing by more than about 12 percent. Preferably insufficient radiation is used to reduce burst pressure by more than 4 percent. The irradiation used for sterilization is usually from about 1 to about 20 kilograys and preferably from about 2 to about 10 kilograys. "Package" means one or more tubular food casings completely and tightly encased by a mold and bacteria impermeable barrier. The barrier preferably comprises a polymer film. The film may be made of one or more laminated or eoextruded layers. At least one of the layers is preferably a high density moisture barrier, e.g. polyvinylidene chloride or nylon or an ultra high density polyolefin, e.g. a metallocene polymer. The "tubular food casing" comprises a cellulose film matrix and from about 9 to about 40 weight percent water. The cellulose film may be unreinforced or reinforced, especially with cellulose fibers in the form of a woven or felted mat. Such films are usually either made by extrusion of a viscose into the shape of a tubular film followed by regeneration or precipitation of cellulose from the viscose, or by extrusion of viscose into a fiber mat curved into the shape of a tube followed by regeneration or precipitation of the viscose. The food casing may, for example, be a shirred fibrous food casing comprising a tubular film wherein the film comprises a cellulose matrix reinforced with fibers or may be an unreinforced cellulose film. The viscose is a solution of derivatized or non-derivatized cellulose. The most common viscose is xanthate viscose which is a solution of xanthanated cellulose in aqueous alkali solution. In forming a tubular film the xanthate viscose is extruded to form a tube and the cellulose is regenerated from the xanthate using a salt such as sodium sulfate in an acid solution. More recently it has been proposed that a viscose of non-derivatized cellulose in an aqueous solution of tertiary amine oxide could be used to form tubular food casing. In that case, after extrusion, cellulose is precipitated from solution by washing the amine oxide from the cellulose. The invention also includes a method for making a package containing a tubular food casing strand comprising a cellulose matrix and from about 9 to about 40 weight percent water where the contents of the package are sterile and thus do not permit the growth of bacterial or mold microorganisms. The method comprises encasing the strand in a barrier layer impermeable to mold and bacteria and irradiating the package with sufficient radiation to prevent growth of microorganisms within the package for at least four weeks. Insufficient radiation is used to reduce burst pressure of the casing by more than about 12 percent and preferably by less than 4 percent. The radiation is usually from about 1 to about 20 kilograys and preferably from about 2 to about 10 kilograys to reduce the number of microorganisms by at least 99 percent, preferably by 99.9 percent and most preferably by 99.99 percent. Growth of mold is prevented for at least four weeks usually for at least six weeks, preferably for at least twelve weeks and most preferably is prevented indefinitely. The following examples serve to illustrate and not limit the present invention. The following examples are directed to mold growth since mold spores are considered some of the most difficult viable microorganism forms to destroy. It would be expected that most other microorganism would be destroyed at similar radiation doses or less. Food casing strands were tightly shrink wrapped with polyethylene plastic film and analyzed for mold growth and loss of burst strength after irradiation. The following examples illustrate that irradiation at levels as low as 1.3 kilograys to levels below 19.6 kilograys showed the positive effect of an at least 3.44 log reduction (>99.9%) in mold spores with little significant negative impact on xy burst results of the casing. Radiation levels of 19.6 kilograys (kGy) and above also eliminated mold (at least a 3.65 log reduction, about 99.99% or more), but xy results show that the casing is significantly weaker than the non-irradiated control. To eliminate mold with minimal damage to the casing it is recommended that the dose target from 7 to 15 kilograys, but the minimum dose could be as low as 1 kilogray. XY Burst Analysis Shirred fibrous casing was irradiated with gamma rays in strand lengths of approximately 12". Dosimeters were attached to the strands and the following doses were delivered: After the casing was irradiated, it was shipped from the irradiation facility for rewet xy analysis. The casing was deshirred in lengths of 24 inches and soaked for 30 minutes in tap water. After 30 minutes, the casing was loaded into a burst analyzer and a diameter versus pressure results were generated for each sample. Results and Discussion: XY Burst Analysis Pressure at Recommended Stuffing Diameter (RSD) (PfgRSD) - no significant change was noted between control and the irradiated test parts. Burst Pressure (BP) - decreased slightly (<4%) in the 1.3 kGy to 9.8 kGy range, decreased more (6%) at 12 kGy, and decreased significantly (12% and 18%) at 19.9 and 28.7 kGy respectively. Burst Diameter BD) - generally it was noted that as the radiation level increased the diameter decreased, but the change was not at a significant level. Residual $tretch — typically as the radiation level increased the residual stretch decreased, with the most significant decreases of 6.4% and 12% being at the 19.9 kGy and 29.5 kGy doses. Residual Strength - significant decreases of 20% and 29.5% were seen at the 19.9 kGy and 29.5 kGy radiation levels. Energy to Burst - generally the values decreased as compared to the control. The results of this study are summarised in Table 1. Mold Growth Analysis Fibrous casing was divided into seven groups of four approximately 12" strands. One group served as a negative control. The other groups were inoculated with approximately 1,000,000 spores of a mold composite. The mold composite consisted of Penicillium and Aspergillus species and a mold isolated from a commercial strand of fibrous casing. The number of viable spores were verified by plate count methods (PDA, 7 days, 25°C). For each shrink wrapped strand that was inoculated, five 9-cm 2 squares were drawn with a marking pen on the shrink wrap at 3 different positions of the strand: end (2" from end), end-center (4" from end) and center. The mold composite culture was injected under the shrink-wrap onto the casing squares via a syringe and a septum tape placed on the edge of the square on the shrink-wrap. After inoculation, one group served as a positive control to determine the innoculum level and five groups with four strands each were sent to SteriGenics for irradiation treatment by radioisotope. When the strands were returned, all samples were stored at 75°F in a 95% relative humidity chamber. Samples of the control and inoculated portions were analyzed initially (week 0) and after weeks 1, 2, 3 and 4. All strands were visually examined for the presence of mold. At each interval, 4 squares randomly selected from each of the following 3 positions were removed, placed into 100 ml of phosphate buffer, shaken for 2 minutes, and analyzed by plate count methods for mold. The following numbers noted the position of the square used from each strand. The method of analysis is outlined in the following table. Test Medium Incubation Time/Temperature/Atmosphere Mold Count Potato Dextrose Agar (PDA) 5 days/25°C/aerobic with antibiotics The reduction in counts of mold due to irradiation is viewed in terms of log reduction. A one-log reduction is the same as a 90% reduction, for example from 10,000 to 1,000 colony-forming units. Similarly a 2 log reduction is 99% or 10,000 to 100 colony forming units and 3 logs is 99.9% (10,000 to 10 colony forming units). The results of this study are summarized in Tables 2-8. As the data in Table 2 shows, detectable levels of mold spores were found on 3 of 16 samples tested suggesting that a low level of mold contamination was present at the processing facility when the samples were produced or in the testing facility. The data in Table 3 shows that the average laboratory inoculation level for the casing before irradiation was 5.65 log colony forming units per 9 cm 2 . There was a slight reduction in counts by week 4 of about 1 log. Visible mold appeared on 1 strand in the 4 weeks. The results for the irradiation are shown in Tables 4-8. As the data indicates, irradiation at all levels effected at least a 3.44 log reduction (>99.9%). At 2.6 kGy, 7/16 samples showed counts of mold. At 4.7 kGy, 5/16 exhibited mold. At 7.8 kGy, mold was not detected. At 19.6 kGy, 1 sample showed a count while at 27.6 kGy, mold was not detected. TABLE 1 Irradiated Fibrous XY Data TABLE 2 Noninocuiated, Nonirradiated Samples TABLE 3 Inoculated, Nonirradiated Samples TABLE 4 Inoculated, Irradiated Minimum Dose 2.6 kGy, Maximum Dose 2.7 kGy Samples Irradiation Run Numbers DVTS DEV-BATl 4900004, 4900002, 4900003 and 4900005 TABLE S Inoculated, Irradiated Minimum Dose 4.7 kGy, Maximum Dose 5.2 kGy Samples Irradiation Run Numbers DVTS DEV-BAT84900004, 4900002, 4900003 and 4900005 1A1JL.E 6 Inoculated, Irradiated Minimum Dose 7.8 kGy, Maximum Dose 7.9 kGy Samples Irradiation Run Numbers DVTS DEV-BAT44900004, 4900002, 4900003 and 4900005 TABLE 7 Inoculated, Irradiated Minimum Dose 19.7 kGy, Maximum Dose 21.5 kGy Samples Irradiation Run Numbers DVTS DEV-BATl 1 4900004, 4900006, 4900003 and 4900005 TABLE 8 Inoculated, Irradiated Minimum Dose 27.6 kGy, Maximum Dose 29.4 kGy Samples Irradiation Run Numbers DVTS DEV-BATl 1 4900004, 4900006, 4900003 and 4900005 The less than 100 plate counts, due to background contamination, are the limits of detection of the test employed and may indicate as much as a 100 percent kill rate making indefinite storage a possibility.

Claims: What is claimed is: 1. An irradiated package comprising a mold and bacteria impermeable barrier encasing a tubular food casing which food casing comprises a cellulose film matrix and from about 9 to about 40 weight percent water, wherein said package has been irradiated with sufficient radiation to prevent growth of microorganisms within the package for at least four weeks and insufficient radiation to reduce burst pressure of the casing by more than about 12 percent. 2. The package of Claim 1 wherein the irradiation is from about 1 to about 20 kilograys and growth of mold is prevented for at least six weeks. 3. The package of Claim 1 wherein the irradiation is from about 1 to about 10 kilograys and the reduction in burst pressure of the casing due to exposure to the radiation is less than about 4 percent. 4. The package of Claim 1 wherein sufficient radiation is used to prevent growth of mold and bacteria within the package for at least twelve weeks. 5. The package of Claim 1 wherein the food casing is a shirred fibrous food casing comprising a tubular film wherein the film comprises a cellulose matrix reinforced with fibers. 6. The package of Claim 5 wherein the cellulose is regenerated from xanthate viscose. 7. The package of Claim 5 wherein the cellulose is precipitated from a solution of cellulose in a solvent comprising tertiary amine oxide. 8. The package of Claim 1 wherein the tubular film is an unreinforced cellulose film. 9. The package of Claim 8 wherein the cellulose film is regenerated from xanthate viscose. 10. The package of Claim 8 wherein the cellulose film is precipitated from a solution of cellulose in a solvent comprising a tertiary amine oxide. 11. The package of Claim 1 wherein the contents of the package are free of sufficient chemical antimycotic to prevent mold growth for four weeks in the absence of the irradiation. 12. The package of Claim 1 wherein the contents of the package are free of sufficient chemical antimycotic to prevent mold growth for eight weeks in the absence of the irradiation. 13. The package of Claim 1 wherein the contents of the package contain insufficient mold spores to be detectable by shaking 9 cm 2 of the internal surface of the barrier layer in phosphate buffer and analyzing the buffer solution for mold spores by inoculation of potato-dextrose agar plates, containing antibacterial antibiotic, with the buffer solution to obtain plate counts. 14. The package of Claim 1 wherein sufficient radiation is used to reduce the number of microorganisms within the package by over 99.9%. 15. A method for a package containing a tubular food casing strand comprising a cellulose matrix and from about 9 to about 40 weight percent water where the contents of the package do not support the growth of bacterial or mold microorganisms, which method comprises encasing the strand in a barrier layer impermeable to mold and bacteria and irradiating the package with sufficient radiation to prevent growth of microorganisms within the package for at least four weeks and insufficient radiation to reduce burst pressure of the casing by more than about 12 percent. 16. The method of Claim 15 wherein the irradiation is from about 1 to about 20 kilograys and growth of mold is prevented for at least six weeks. 17. The method of Claim 15 wherein the irradiation is from about 1 to about 10 kilograys and the reduction in burst pressure of the casing due to exposure to the radiation is less than about 4 percent. 18. The method of Claim 15 wherein sufficient radiation is used to prevent growth of mold and bacteria within the package for at least twelve weeks. 19. The method of Claim 15 wherein the food casing is a shirred fibrous food casing comprising a tubular film of fibers embedded in a cellulose matrix. 20. The method of Claim 19 wherein the cellulose is regenerated from xanthate viscose. 21. The method of Claim 19 wherein the cellulose is precipitated from a solution of cellulose in a solvent comprising tertiary amine oxide. 22. The method of Claim 15 wherein the tubular film is an unreinforced cellulose film. 23. The method of Claim 22 wherein the cellulose film is regenerated from xanthate viscose. 24. The method of Claim 22 wherein the cellulose film is precipitated from a solution of cellulose in a solvent comprising a tertiary amine oxide. 25. The method of Claim 15 wherein the contents of the package are free of sufficient antimycotic to prevent mold growth for four weeks in the absence of the irradiation. 26. The method of Claim 15 wherein the contents of the package are free of sufficient antimycotic to prevent mold growth for eight weeks in the absence of the irradiation. 27. The method of Claim 15 wherein the contents of the package contain insufficient mold spores to be detectable by shaking 9 cm 2 of the internal surface of the barrier layer in phosphate buffer and analyzing the buffer solution for mold spores by inoculation of potato-dextrose agar plates, containing antibacterial antibiotic, with the buffer solution to obtain plate counts. 28. The method of Claim 15 wherein sufficient radiation is used to reduce the number of microorganisms within the package by over 99.9%.

Inventors: REEVES MARK D, REEVES, MARK, D.
Applicants: DEVRO PLC, TEEPAK INVESTMENTS INC, TEEPAK INVESTMENTS, INC.
Application year: 2001
Main classification: A23L3
IPC codes: A22C13, A61L2