Synthetic Food Casings Comprising Polyvinyl Alcohols

Abstract

The invention is directed to synthetic food casings made from specific polyvinyl alcohol grades containing carbonyl groups, preferably in admixture with other polymers, as well as processes for using such synthetic food casings.

Claims

1. A synthetic food casing having at least one layer comprising a polyvinyl alcohol with equal to or more than 0.01 mol % and equal to or less than 2.0 mol % of a terminal carbonyl group.

2. The synthetic food casing according to claim 1 wherein the terminal carbonyl group is a terminal ketone group.

3. The synthetic food casing according to claim 1 wherein the terminal carbonyl group is a terminal methyl ketone group.

4. The synthetic food casing according to claim 1 wherein the terminal carbonyl group is introduced into the polyvinyl alcohol by using a chain transfer agent during the polymerization reaction.

5. The synthetic food casing according to claim 1 wherein the at least one layer further comprises a further thermoplastic resin selected from the group consisting of thermoplastic starch, thermoplastic polyvinylpyrrolidone, polyamide, block copolymers of polyamide and polyether, block copolymers of polyurethane and polyether, block copolymers of polyester and polyether, and blends thereof, preferably the at least one layer further comprises polyamide.

6. The synthetic food casing according to claim 1 wherein the synthetic food casing has a permeability to water vapor and smoke from 300 g/m.sup.2.Math.day to 1500 g/m.sup.2.Math.day at 50 p.m film thickness when measured according to DIN53122-1 at 40° C. and 95% RH.

7. The synthetic food casing according to claim 1 wherein the polyvinyl alcohol further comprises a plasticizer selected from the group consisting of glycerin, 1,2-propanediol, trimethylolpropane, di-trimethylolpropane, pentaerythritol, di-pentaerythritol, sorbitol, xylitol, inositol, mannitol, maltitol, triethanolamine, polyethylene glycol with a molecular weight from 200 to 10000 Da and mixtures thereof, preferably further comprising glycerin.

8. The synthetic food casing according to claim 1 wherein the polyvinyl alcohol comprises from 0.001 to 2.0 wt % sodium acetate.

9. A process to dry, cure or smoke foodstuff using a synthetic food casing according to claim 1.

10. The process according to claim 9 wherein the foodstuff is meat or cheese.

11. (canceled)

12. Foodstuff obtained by the process according to claim 9 or 10.

13. A mixture comprising a polyvinyl alcohol with equal to or more than 0.01 mol % and equal to or less than 2.0 mol % of a terminal carbonyl group and a polyamide.

14. The mixture according to claim 13 wherein the carbonyl group is a terminal methyl ketone group.

15. The mixture according to claim 13, wherein the polyamide is selected from a (co)polyamide selected from the group comprising polyamide 6, polyamide 66, polyamide 46, polyamide 6/12, copolyamide 6/66, copolyamide 46/6 and mixtures thereof.

Description

EXAMPLES

[0034] Measurement of Degree of Hydrolysis

[0035] Degree of hydrolysis indicates percentage of vinyl acetate unit saponified to vinyl alcohol unit and it is calculated using the following equation. EV means Ester Value which is the number of mg KOH needed to neutralize the acid released from the ester by saponification in 1 g of substance and it is measured according to EN ISO 3681.

Degree of hydrolysis=100×(100−0.1535×EV)/(100−0.0749×EV)

[0036] Viscosity:

[0037] For the measurements, 4 wt % solutions in deionized water were prepared. The measurements were performed at 20° C. in a falling ball viscometer according to DIN 53 015.

[0038] Measurement of Degree of Polymerization

[0039] The degree of polymerization is measured according to JIS K6727. Specifically, it may be calculated by the equation below from the limiting viscosity [η] (unit: L/g) measured in water at 30° C. after re-saponification and purification of the PVOH.

Degree of polymerization=([η]×10000/8.29).sup.(1/0.62)

[0040] Measurement NaOAc Content

[0041] PVOH is dissolved in distilled water to provide solutions having concentration of 4, 8 and 20 wt % and adjusted to a temperature of 20-25° C. The conductivity cell (for example type “cond 315i”, commercialized by WTW) is likewise adjusted to a temperature of 20-25° C. and washed with distilled water. The cell is immersed into the solution and after 5-10 seconds, the conductivity of the solution is read.

[0042] The sodium acetate content is calculated as follows:

NaOAc content on %=conductivity (4% solution)×0.002875

NaOAc content on %=conductivity (8% solution)×0.001687

NaOAc content on %=conductivity (20% solution)×0.001063.

[0043] Measurement of Yellowness Index

[0044] The yellowness index of the polyvinyl alcohol was determined from a 1 wt % aqueous solution in 33 mm quartz cell with a spectrophotometer in transmission mode according to ASTM E313 with illuminant C and the CIE 1931 standard colorimetric observer (2°).

[0045] Measurement of Terminal Carbonyl Group Content Exemplified for Terminal Methyl Ketone Groups

[0046] The terminal methyl ketone group content was determined by .sup.1H NMR spectroscopy (600 MHz) of the polymer dissolved in DMSO-d6 at 80° C. The spectrum was referenced to the solvent signal. The methyl ketone content was calculated from the integral of the methyl group protons adjacent to the ketone group with reference to the polymer backbone methylene protons to obtain the methyl ketone group content with respect to the vinyl alcohol and vinyl acetate repeating units.

Methyl ketone CH.sub.3CO=integral(2.218−2.197 ppm)+integral(2.197−2.175 ppm)+integral(2.120−2.080 ppm)

Acetate CH.sub.3COO=integral(2.010−1.910 ppm)

Sodium acetate CH.sub.3COONa=integral(1.660−1.650 ppm)

Backbone CH.sub.2=(integral(2.35 ppm−0.00 ppm)−CH.sub.3CO—CH.sub.3COO—CH.sub.3COONa)/2

Methyl ketone end-group content in mol %=CH.sub.3CO/3/CH.sub.2*100

[0047] For other terminal carbonyl groups, the skilled person is aware of the suitable proton signal to be used for the respective measurement. For example in case of a terminal aldehyde, the proton signal of the aldehyde C(O)H proton would be used.

[0048] Measurement of Water Vapor Transmission Rate (WVTR)

[0049] WVTR was measured according to DIN 53122-1 in a climate controlled cabinet at 40° C. and 95% relative humidity. The film samples were sealed to the measurement cup containing silica gel as a desiccant with paraffin wax. The area of the film measured was 28.27 cm.sup.2. The film samples were conditioned at 40° C. and 95% relative humidity before the measurement was started. Before the measurement the film thickness of each sample was measured at several points and the WVTR results at the end were normalized to a film thickness of 50 μm.

[0050] Investigation of Film Turbidity Before and After Exposure to Hot Water

[0051] Film samples were placed in deionized water at 80° C. for 2 h. After drying the film samples at ambient conditions the turbidity was determined and compared to untreated film samples. The turbidity is classified by the following categories in order of increasing turbidity: 1 (very little turbidity, i.e. comparable to pure polyamide film), 2 (minor degree of turbidity can be recognized, but the film is still relatively clear), 3 (slightly turbid), 4 (significant turbidity), 5 (film is white and transparency is significantly deteriorated).

[0052] Preparation of PVOH Compositions

[0053] PVOH compositions were prepared by feeding all components to a Leistritz ZSE 27 HP twin screw extruder with a screw diameter of 27 mm and a L/D ratio of 44 as described in WO 03/020823 A1. The length of a zone was 4D, thus the extruder has 11 zones. Feeding of the raw materials was performed in zone 1 and 2. Vacuum was applied in zone 10 to remove volatiles. The extruder screw profile was configured with conveying, kneading and mixing elements in a way to homogeneously melt and mix all components with little shear according to common knowledge known to those skilled to the art. Solids were added to the extruder in zone 1. Liquids were added in zone 2 with a feeding pump. Extrusion was carried out at a screw speed of 200 rpm and a throughput of 20 kg/h. The temperature profile of the extruder from the feeding section to the die was 50/150/175/200/3x210/3x205/200° C. Strands with a diameter of 3 mm were extruded, which were cooled by air cooling. After cooling the strands were cut to pellets by a strand cutter. The pellets were packaged in moisture tight aluminum bags to protect them from humidity until further use.

[0054] Blown Film Extrusion

[0055] Blown films were prepared on a Dr. Collin blown film line consisting of a E30P extruder (30 mm screw diameter, compression ratio 3.34, L/D 25), a 50 mm spiral mandrel distributor die and a BL180/400 blown film unit. For extrusion of PVOH/polyamide blends the PVOH pellets were mixed with polyamide 6/66 pellets (UBE Nylon 5033B, which was dried at 120° C. for 12h before use). The temperature profile of the extruder from the feeding section to the die was 50/80/215/225/5×215/205/195° C. Melt temperature was around 215° C. and extrusion was carried out at a screw speed of 60 rpm, a haul-off speed of 4.5 m/min and a blow up ratio of 5.

Example 1

[0056] A 6-liter reaction vessel equipped with a stirrer, nitrogen inlet, additive inlet and initiator inlet is charged with 1200 g of vinyl acetate, and 90 g of acetaldehyde. After raising the temperature to 60° C., the system was purged with nitrogen by nitrogen bubbling for 30 minutes. The temperature in the reaction vessel was adjusted to 60° C., and 1.5 g of 2,2′-azobis(isobutyronitrile) was added to initiate polymerization. The polymerization temperature was maintained at 60° C. during the polymerization. After 4 hours, when the conversion reached 60%, the polymerization was stopped by cooling to room temperature. Next, unreacted vinyl acetate was removed under reduced pressure and methanol was added to obtain a methanol solution of polyvinyl acetate (hereinafter sometimes abbreviated as PVAc). The concentration of the PVAc methanol solution was adjusted to 30 wt %, and a methanol NaOH solution (10 wt % concentration) was added so that the alkali molar ratio (number of moles of NaOH/number of moles of vinyl ester units in PVAc) was 0.0088. The reaction mixture was heated to 40° C. and the saponification was carried out for 30 minutes. Afterwards the obtained vinyl alcohol copolymer was washed with methanol and dried. By the above operation, a vinyl alcohol copolymer having a polymerization degree of 750, a saponification degree of 72 mol %, a sodium acetate content of 0.93 wt %, a yellowness index of 3 and a methylketone end-group content of 0.1 mol % was obtained.

[0057] The resulting polymer was pelletized by processing it with a twin screw extruder following the general procedure as described above.

[0058] Nylon 6/66 pellets were mixed with 25 wt % of the resulting PVOH pellets and extruded to a blown film with a thickness of 30 μm as described above. The film has a WVTR (standardized to a film of 50 μm thickness) of 450 g/m.sup.2.Math.day and turbidity was rated as 1. After exposure to hot water as described above, the turbidity of the film was rated 2.

Example 2

[0059] PVOH was prepared as described in Example 1 and compounded with 10% of glycerin in a twin screw extruder following the general procedure described above.

[0060] Nylon 6/66 pellets were mixed with 25 wt % of the resulting PVOH pellets and extruded to a blown film with a thickness of 30 μm. The film has a WVTR (50 μm film thickness) of 670 g/m.sup.2.Math.day and turbidity was rated as 1. After exposure to hot water film turbidity is rated 3. Thus, the resulting film exhibits similar WVTR as a film made with a similar PVOH grade without ketone end groups and a much better transparency after exposure to hot water.

Comparative Example 1

[0061] Pure Nylon 6/66 was extruded to a blown film with a thickness of 30-40 μm. The film has a WVTR(50 μm film thickness) of 270 g/m.sup.2.Math.day and turbidity was rated as 1. After exposure to hot water the film turbidity is still rated 1.

Comparative Example 2

[0062] Nylon 6/66 pellets were mixed with 25 wt % of the commercially available thermoplastic PVOH MOWIFLEX™ C30 (available from Kuraray Europe GmbH) and extruded to a blown film with a thickness of 30 μm. The film has a WVTR(50 μm film thickness) of 630 g/m.sup.2.Math.day and turbidity was rated as 1. After exposure to hot water film turbidity is rated 5.

Comparative Example 3

[0063] A commercially available PVOH grade (KURARAY POVAL 5-74), which does not contain ketone end-groups, with a degree of polymerization of 600, a degree of hydrolysis of 73.6% and a sodium acetate content of 0.75 wt % was pelletized by processing it with a twin screw extruder following the general procedure as described above.

[0064] Nylon 6/66 pellets were mixed with 25 wt % of the resulting PVOH pellets and extruded to a blown film with a thickness of 30 μm. The film has a WVTR (50 μm film thickness) of 460 g/m.sup.2.Math.day and turbidity was rated as 1. After exposure to hot water film turbidity is rated 5.

Comparative Example 4

[0065] A commercially available PVOH grade (KURARAY POVAL 5-74), which does not contain ketone end-groups, with a degree of polymerization of 600, a degree of hydrolysis of 73.6% and a sodium acetate content of 0.75 wt % was compounded with 10% glycerin by processing it with a twin screw extruder following the general procedure as described above.

[0066] Nylon 6/66 pellets were mixed with 25 wt % of the resulting PVOH pellets and extruded to a blown film with a thickness of 30 μm. The film has a WVTR (50 μm film thickness) of 590 g/m.sup.2.Math.day and turbidity was rated as 1. After exposure to hot water film turbidity is rated 4.

[0067] Results

[0068] Table 1 shows the turbidity ratings of the examples according to the invention and the comparative results:

TABLE-US-00001 Turbidity Turbidity rating before rating after hot water hot water WVTR Example treatment treatment (g/m.sup.2 .Math. day) Example 1 1 2 450 Example 2 1 3 670 Comparative 1 1 270 Example 1 Comparative 1 5 630 Example 2 Comparative 1 5 460 Example 3 Comparative 1 4 590 Example 4

[0069] Conclusion from the Experiments

[0070] The experiments show that the films according to the invention show similar WVTR (water vapour transition rate) as the films made with PVOH grades without carbonyl groups. However, the inventive films have substantially less turbidity after exposure to hot water as compared to the films made with conventional PVOH groups (comparative examples 2 to 4). Comparative example 1, which is a synthetic food casing made from polyamide without the addition of polyvinyl alcohol, displays good turbidity. However, as expected, the WVTR is not sufficient for many applications in the food industry.