WO2002005664A2 - Irradiated packaged moisturised tubular food casing - Google Patents

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-cm² 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 cm². 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 cm² 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 cm² 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%.