Classifications

• • A—HUMAN NECESSITIES
  • A22—BUTCHERING; MEAT TREATMENT; PROCESSING POULTRY OR FISH
  • A22C—PROCESSING MEAT, POULTRY, OR FISH
  • A22C13/00—Sausage casings
  • A22C13/0013—Chemical composition of synthetic sausage casings
• • A—HUMAN NECESSITIES
  • A22—BUTCHERING; MEAT TREATMENT; PROCESSING POULTRY OR FISH
  • A22C—PROCESSING MEAT, POULTRY, OR FISH
  • A22C13/00—Sausage casings
  • A22C2013/0063—Sausage casings containing polyamide, e.g. nylon, aramide
• • A—HUMAN NECESSITIES
  • A22—BUTCHERING; MEAT TREATMENT; PROCESSING POULTRY OR FISH
  • A22C—PROCESSING MEAT, POULTRY, OR FISH
  • A22C13/00—Sausage casings
  • A22C2013/0083—Sausage casings biaxially oriented

Definitions

• the invention relates to single- or multilayer synthetic heat-shrinkable sausage casings that one can fill without preliminary humidification and virtually without stretching and make as a result taut package, for instance sausages. Additionally the invention relates to a method for production of such casings.
• Synthetic casings for sausage products such as sausages, frankfurters, hams, liver pastes enclosed in a casing and so on are well-known from the prior art.
• Such film casings are practical in use and allow making packed products attractive for consumers.
• Such casings allow making taut sausages without wrinkles.
• Such taut sausages do not only have attractive appearance but also guarantee an absence of liquid, fat or gel between the casing and sausage meat (so called "purge” or "cook out”).
• the first of them is based on using of phenomenon of heat shrinkability in making of sausages in plastic casings. It consists in filling of heat-shrinkable plastic casing without significant expanding with sausage meat and subsequent cooking or scalding of the sausages. Sausage meat expands during the cooking and stretches the casing. At the beginning of subsequent cooling (that begins from the surface of a sausage) the new geometry of the sausage casing is fixed because it gains low temperature while sausage meat remains still hot and expanded. Therefore after complete cooling of the sausage when sausage meat decreases in its volume the sausage casing covers it not-tautly and even can form wrinkles.
• casings are made by their short-time heating usually by means of treatment with hot water or steam, which results in thermal shrinkage of the casing.
• the casings can be made of various resins such as polyvinylidene chloride (hereinafter referred to as PVDC), polyolefins: polypropylenes (PP) and polyethylenes (PE) of various grades and types, polyesters such as polyethylene terephthalate, polybutylene terephthalate and others, and also polyamides.
• PVDC polyvinylidene chloride
• PP polypropylenes
• PE polyethylenes
• polyesters such as polyethylene terephthalate, polybutylene terephthalate and others
• polyamides also polyamides.
• European patent application Ns 0 974 452 (published 2000.01.26) disclosed a stretched (drawn) multilayer film casing having desirable heat-shrinkability level and decreased heat-shrinkage stress value.
• the casing comprised a pair of outer layers comprising polyolefin resins of identical species, and a gas-barrier intermediate layer comprising a polyamide resin.
• the film casing was made according to a usual process for oriented films with thermofixation step carried out at a low temperature by hot (60-98°C) water or steam. Resulted casing had heat-shrinkage stress maximum of 2 MPa at 50°C and hot-water shrinkability at 90 °C of 5 - 20% in each of machine (longitudinal) direction and transverse direction.
• Such casing can be filled with sausage meat automatically or manually. Resulted sausage was cooked according to a usual cooking process, cooled and then put into hot water. In the inventor's opinion, limited heat-shrinkage stress was needed to avoid violation of packing integrity (rupture of casing or its sealed seams, etc.) during cooking of sausages.
• Second approach is based on elastic properties of plastic casings.
• a range of suitable materials is shorter.
• resins as thermoplastic polyurethanes for their production, or a less known spatiality block-copolymers characterized with elastic properties (thermoplastic elastomers).
• thermoplastic polyurethanes for their production
• thermal polyplastic elastomers characterized with elastic properties
• thermoplastic elastomers are very expensive and difficult in processing. Therefore the most widespread raw materials for making elastic sausage casings are aliphatic polyamides. They are used both as a basic raw material of single-layer casings and as a raw material for one of layers (usually thickest) in multilayer casings.
• Such casings commonly have good barrier properties towards moisture and oxygen, what gives a possibility to store food products packed therein for quite long time.
• polyamide 6 or PA 6 polycaprolactam
• polyamide 66 or PA 66 polyhexamethyleneadipamide
• other aliphatic polyamides in non-oriented state for instance, in a form of cast films
• the following process is used to provide them with elastic properties: extrusion of a cast tubular film (or primary tube) from single- or multilayer extrudate; orientation drawing of the cast tubular film resulting in secondary tube; most possible complete thermofixation of the secondary tube resulting in ready-to-use sausage casing.
• Typical dry film based on common commercial aliphatic polyamides is too stiff (its Young's modulus or elastic modulus is about 400-500 MPa (40-50 kgf/mm 2 ) in both directions and cannot be stretched to desirable level by filling with sausage meat using common stuffing equipment. Therefore before stuffing, in other words, before filling with sausage meat under excess pressure that stretchs the casing, it is humidified by soaking in water. After this step a value of its Young's modulus drops to about 200-250 MPa in both directions.
• this value of the Young's modulus can on one hand provide the possibility of stretching the casing under pressure of sausage meat to desirable level and, on the other hand, to prevent deformation of hung up sausages under their own weight.
• a stuffing machine stuffing tube and clipping unit
• This stretching is generally enough to provide tough consistence to a sausage.
• casing is stretched unequally in the machine and transverse directions that results in a deformation of a printed image applied onto this casing.
• stuffing and following operations are followed by alternating-sign deformations (decompression-compression) that weakens adhesion between casing surface and printing ink layer and can lead to partial delamination of printed image.
• German patent application J ⁇ S 41 28 081 Al disclosed a tubular sausage casing consisting of at least three layers.
• An external moisture-barrier layer was made of olefinic (co)polymers.
• a core gas-barrier layer was made of ethylene- vinyl alcohol copolymer (EVOH) or (co)polyamides such as poly (m-xylyleneadipamide) hereinafter referred to as polyamide (or nylon) MXD6 or poly(hexamethyleneisophthal- amide/terephthalamide) hereinafter referred to as polyamide (or nylon) 6I/6T.
• An internal food-contact layer was made of polyamide resin.
• the casing was moistened in the step of primary tube formation whet it was cooled by cold water both from inside and outside and then was wound on a core.
• moisture soaked an internal polyamide-based layer can be kept for a long time but if the casing was shirred and supplied to sausage-maker in a form of shirred "sticks" it should be packed into additional a moisture-impermeable package otherwise it would dry up. Consumers should stuff this casing in common manner, i.e. with its stretching.
• Russian Patent N2 2189146 (published 2002.09.20) disclosed a casing comprising a blended layer made of polyamide and polyamide-block-polyether. Said block-copolymer has a significant elasticity and can impart these properties to its blends with polyamide. Therefore such casing had reduced Young's modulus and could be stuffed under relatively low pressure of sausage meat and even manually.
• the casing according to this invention had a level of heat shrinkage value typical for conventional polyamide casings or a little bit more. It was produced according to usual processing scheme for single- or multilayer polyamide-based casings making.
• the invention description disclosed neither a typical degree of extension of the invented casing that takes place as a result of its stuffing nor concrete values of sausage meat pressure.
• Heat-shrinkable multilayer tubular casing according to this invention had a core layer mainly comprising polyolefin and two peripheral layers mainly comprising polyamide and optionally additional layers from ethylene - vinyl acetate copolymer (EVA) and from EVOH.
• EVA ethylene - vinyl acetate copolymer
• German Patent DE JNb 28 50 182 (published 1980.06.04) stated that single-layer polyamide casing based on PA 6 or made by orientation drawing and thermofixation at 120°C can be filled with sausage meat with no preliminary moistening and generally without applying pressure (manually). It also stated that in this case it is possible to prepare tough sausages enclosed in non-wrinkled casing only due to its heat shrinkability (illustrated by no concrete value). However, in description of their later patent (US Pat. N° 4 897 295, published 1990.01.30) the same inventors acknowledged that sausage products with satisfactory appearance cannot be made by using the casing according to DE 28 50 182 without its moistening and stretching in its elastic area.
• the casing was filled with sausage meat at filling pressures of 0.3-0.6 bar, that are typical for stuffing of moistened polyamide casings, that definitely implies, taking in the account elasticity of the casing, its stretching under stuffing conditions.
• polyamide of the casing gained addition elasticity and tough sausages covered by non-wrinkle casing were prepared.
• the casing according to this invention was made using appropriate thermofixation (120°C for 3-5 minutes) but nevertheless had as inventors stated shrinking properties (also illustrated by no concrete value).
• casing is fraught with risk of undesirable deformation of sausages under their own weight because moisture begins to penetrate into polyamide resin of the casing and reduce mechanical modulus of the casing directly after its stuffing.
• casing cannot be used as a film for filling by sausage meat by means of abovementioned packing machines.
• a method of production of polyamide-based tubular casing most analogous to the invented method was disclosed in US Pat. JN 4 560 520 (published 1985.12.24).
• a single-layer polyamide film was made by means of a method, comprising the steps of extruding a primary tube of polyamide and simultaneously multi- axially drawing of the primary tube and subjecting the extruded and multi-axially drawn primary tube to a controlled shrinkage by at least 15% and at most 40% in the transverse and the longitudinal direction in accordance to its transverse and longitudinal dimension after drawing, at temperatures ranging from 90° C to 150°, thereby thermally fixing the primary tube.
• casing had zero shrinkage both at 40 and 80 °C. Therefore it is impossible to prepare a tough non-wrinkled sausage in such casing unless it has been filled with sausage meat with appropriate stretching.
• an object of the present invention is providing of a sausage casing that does not have above-described drawbacks, the casing, which:
• - is suitable for being coated with printed image by means of flexography or other high-performance printing technique
• a single- or multilayer synthetic heat- shrinkable sausage casing comprising at least one polyamide layer made of polyamide resin (or, in other words, polyamide composition) based on at least one aliphatic (co)polyamide, selected from polyamide 6, polyamide 66, copolyamide 6/66 or their blend, wherein said sausage casing:
• the polyamide layer comprises from 60 to 100% of at least one aliphatic (co)polyamide preferably selected from a group including polyamide 6, polyamide 66, copolyamide 6/66.
• the polyamide layer can comprise from 0 to 20% of at least one half- aromatic (co)polyamide selected from the group, including copolyamide 6I/6T, polyamide MXD6, or their blends.
• the polyamide layer apart from polyamide composition can comprise from 0 to 20% of at least one resin selected from the group, comprising polyesters, olefin homopolymers, olefin copolymers, ionomers, or their blend.
• the synthetic casing can be made in a form of tube, half-tube (prepared from flattened tubular film by cutting along one bent line), or sheet (prepared from flattened tubular film by cutting along both bent lines).
• the casing according to the present invention can be made by a method, including consecutive steps of single- or multilayer extrudate extrusion, resulting in formation of a primary tube; orientation drawing of said primary tube, resulting in preparation of a oriented tubular film; thermofixation of said oriented tubular film resulting in preparation of an oriented thermofixed tubular film, wherein said oriented thermofixed tubular film is further subjected to an additional treatment during which said oriented thermofixed tubular film having temperature 60-140°C is drawn by 3-8% in the machine direction and 4-10% in the transverse direction, then cooled, while it is in the drawn (strained) state, to the temperature that is not higher than glass transition temperature of the polyamide composition of the polyamide layer, then is flattened and wound up on a core.
• Thermofixation is carried out by heating of the oriented tubular film, being inflated by air in bubble form, up to 130-160°C, preferably up to 135-140°C and its exposure to such temperature lasts for 3-5 seconds.
• the casing is subjected to shrinkage by no more than 10%, preferably by 5-10% both in the machine and transverse direction.
• thermofixed oriented tubular casing is drawn by 4-8% in machine direction and by 5-10% in transverse direction.
• the drawing during the said additional treatment is preferably performed at 70-130°C and the casing in the drawn (strained) state is cooled to the temperature not higher than glass transition temperature of polyamide layer matter and preferably not higher than temperature ranging between 45 and 57°C, depending on the composition of the polyamide layer.
• the additional treatment of the oriented tubular film can be performed both in the same manufacturing line as steps for production of the oriented tubular film and off-line in a separate apparatus.
• the resulted sausage casing can be used per se, or be cut along one or both bent lines of flattened tube resulting in making of correspondently one half-tube or two extensive flat films (sheets).
• the tubular casing can be made in a form of shirred stick.
• Manufacturing process of sausage product in the casing according to the present invention consists in filling of the casing with sausage meat virtually under zero pressure (increment of its diameter herein can be no more than 1.5%) resulting in row sausage product, its moisture-heat treatment and following cooling. There is no need in preliminary moistening of the casing and additional heat treatment of the resulted sausage.
• the sausage product made in the invented casing according to above making process is tough and non- wrinkled sausage.
• ready-to-be-filled casing or “sausage casing in the ready-to-be- filled state” mean sausage casing that can be filled with sausage meat directly after its pulling out of its package with no additional operation. Said package can be impermeable for moisture or another factor depending on concrete structure and form of the invented casing.
• the casing according to the present invention in the ready-to-be-filled state has Young's modulus typical for non-moistened and not-plasticized polyamide-based casings. In other words it is characterized with significant stiffness, which is absolutely untypical for polyamide-based casings in the instant of their filling. Such casings being preliminarily moistened usually have Young's modulus that is 1.5-2.5 times lower.
• the casing can keep its dimensional characteristic at least being packed into a moisture-impermeable package made, for example, of polyethylene film, which can provide constancy of air humidity inside of the package at temperature changes during its transportation.
• dimensional stability can be estimate by so called “free shrinkage” measured after exposure of test casing to air to the temperature of 25°C and RH of 60% for 5 days. Under these conditions its free shrinkage does not exceed 2% and preferably 1% both in the machine and transverse direction.
• Such humidity conditions can be created in an exsiccator onto bottom of which suitable crystalline salt, for instance, sodium bromide (NaBr) or its saturate aqueous solution is put.
• shrinkage value is about 8-15% in the machine direction and 9-18% in the transverse direction, a tension stresses of casing on sausage are approximately equal to those, developed by cooking of sausage in conventional polyamide-based casing, which was not additionally treated according to the invented procedure, but was stretched by sausage meat during stuffing in usual manner (i.e. by 10- 12%). If the shrinkage values exceed upper values of abovementioned range, it can cause excess tension of the casing connected with risk of its rupture or clip sliding off from sausage.
• hot shrinkage value of the invented casing is inside above range and is preferably 9-12% in the machine direction and 10-15% in the transverse direction.
• the invented casing has a special feature, namely, significant fraction of the cooking time shrink force developed therein continues to take affect after cooling of sausages.
• the indispensable condition for activation of this residual shrink force is a significant humidification of the polyamide layer that can be rapidly achieved by sausage cooking (boiling or scalding).
• Such specific shrink behavior of the invented casing is a result of its additional treatment according to the present invention and gives possibility to prepare taut sausages enclosed therein without the additional heat treatment of the sausages after their cooling.
• This residual shrink force can be activated at lower temperatures, while other part of shrink force, caused by changes in crystalline phase of polyamide occurring at higher temperatures, is inactive.
• activation of the residual shrink force can be achieved by moistening of the invented casing at temperatures from 20 to 50 °C, and inside of above range (20-50 °C) the value of equilibrious (maximum at a given temperature) shrinkage does not depend on temperature of water, wherein the casing is immerged, but depends on exposure time.
• activation of above-mentioned part of shrink force, caused by changes in crystalline phase of polyamide takes place.
• said humidification of the polyamide layer at temperatures around room temperature (18-28 °C) takes too much time, therefore the measurement of the shrink properties is reasonable to be carried out at higher temperatures.
• the casing has external (or all) layers made of polyamide composition its equilibrious shrinkage can be measured at 25°C for 1-2 hours. But if it has at least one polyamide layer, enclosed by layers made of moisture-barrier resins, it requires either very long time of soaking of the casing at room temperature or the same result can be achieved at 40°C for 2 hours, as it will be shown bellow (see Table 3). In the general case it is reasonable to carry out this test at 40°C for 2 hours.
• This type of shrinkage hereafter will be denoted as «wet shrinkage)).
• wet shrinkage values of invented casing are correspondent to these figures, and preferably are 4-10% in the machine direction and 5-10% in the transverse direction.
• the invented casing can be both single-layer and multilayer.
• the casing structure is chosen according to its form, supposed application, storage and transportation conditions and also supposed shelf-life of food product packed therein.
• the invented casing in any embodiment comprises at least, one polyamide layer, wherein total percentage of aliphatic (co)polyamides is not less than 60%.
• nylons conventional commercially available polyamide resins (nylons) can be used.
• the nylons are not characterized with such specific features as, for example, high elasticity in non-oriented (cast) state.
• nylons being raw materials for sausage casing certainly should be resistant to prolonged action of hot water, i.e. they should not melt, resolve, disperse or lose their mechanical strength in hot water.
• Preferable (co)polyamides used for invented casing are nylon 6, nylon 66 or nylon 6/66, which are components of said layer in percentage of 60 to 100%.
• Semiaromatic (co)polyamides such as copolyamide 6I/6T, polyamide MXD6 in percentage of 0-20% can also be used therein.
• Said polyamide layer except (co)polyamides can comprise up to 20% of other polymeric resins such as polyesters like, for instance, polyethylene terephthalate (PET), polyethylene terephthalate modified by glycols (PETG), polybutylene terephthalate (PBT), olefin (co)polymers, like, for instance, polyethylenes, polypropylenes, ethylene/propylene (co)polymers (EPR), ionomers etc, or their blends.
• PET polyethylene terephthalate
• PET polyethylene terephthalate modified by glycols
• PBT polybutylene terephthalate
• olefin (co)polymers like, for instance, polyethylenes, polypropylenes, ethylene/propylene (co)polymers (EPR), ionomers etc, or their blends.
• the invented casing can include more than one polyamide layer, in this case total thickness of the polyamide layers should be generally at least 40% and preferably at least 45% based on total thickness of the casing. At the same time, total thickness of the casing should be from 18 to 120 ⁇ .
• the casing can have one or more layers, preferably comprising (co)polyamides other than polyamide components of above polyamide layer, including polyamide 9, polyamide 10, polyamide 610, polyamide 12, polyamide 612, copolyamide 6/66/10, copolyamide 6/66/12, olefin (co)polymers, olefin (co)polymers, grafted by maleic anhydride, polyvinylidene chloride, ethylene/vinyl alcohol copolymers, thermoplastic polyurethanes.
• These layers can also comprise blends of above listed resins selected from both same and different polymeric types.
• Polyamide components of above polyamide layer (layers): polyamide 6, polyamide 66, copolyamide 6/66, copolyamide 6I/6T and polyamide MXD6 can also be the components of these additional layers in percentage of no more than 40%, for example, as their blend with EVOH.
• the use of rigid polymeric resins like aromatic polyesters, for example, PET as main raw materials for such separate layers is not desirable in the context of the invention objects, because said resins would impart to the casing stiffness, which cannot be eliminated by humidification.
• the casing in tubular form can include food-contact layer, made of any thermoplastic resin, approved by its hygienic characteristics for direct contact with food.
• this layer should mainly consist of (co)polyamide or linear thermoplastic polyurethane, because such resins provide adhesion of the casing to meat emulsion that is strong enough.
• this layer is mostly made of (co)polyamide resin. If material of the layer has no inherent adhesion to sausage meat, this property can be imparted to it by means of known technique, for example, by corona discharge treatment.
• the food-contact layer is made of any sealable resin approved for direct contact with food, that can be sealed by certain technique at certain machine, providing seam that is strong enough and resistant to the conditions of sausage cooking. It is preferable that said sealable resin should have melting temperature in the range of 101-160°C.
• Suitable resins comprise olefin (co)polymers, ionomers, and also thermoplastic polyurethanes or polyamides, for example copolyamide 6/66/12, sold by EMS Co (Switzerland) under trademarks Grilon CF6N or Grilon CF7N.
• this layer is made of ionomer, EVA, metallocene polyethylene (linear low density polyethylene, hereafter LLDPE, linear very low density polyethylene, hereafter LVLDPE, etc), ethylene/(meth)acrylic acid copolymer or PVDC or their blends.
• EVA metallocene polyethylene
• LLDPE linear low density polyethylene
• LVLDPE linear very low density polyethylene
• PVDC ethylene/(meth)acrylic acid copolymer
• the preferable method of such treatment is the corona treatment technique. If the film or half-tube is intended to be sealed with an overlap during formation of the package, an air-contacting layer should be also made of sealable resin.
• the polyamide layer (layers) is preferably insulated from environment air by layers made of moisture barrier resins. If the casing is supplied to a consumer as a tubular film wound in roll form, only one external (air-contacting) moisture-impermeable layer is enough. If the tubular casing is supplied to a consumer as a shirred stick and air has free access inside the tube, at least the thickest polyamide layer should preferably be surrounded by two moisture- impermeable layers.
• polyamide layer(s) semiaromatic (co)polyamides such as copolyamide 6I/6T and/or polyamide MXD6, increasing glass transition temperature, and also polyolefins, ionomers, or polyesters like PET, PETG and PBT can be added therein.
• ethylene/propylene copolymer comprising at least 80 mol. % of propylene units, low density polyethylene (LDPE), high density polyethylene (HDPE), metallocene LLDPE, metallocene LVLDPE, EVA, and also products of maleic anhydride grafting onto said resins, PVDC and blends of all above polymers are preferably used.
• any layer or a few layers of the casing according to the present invention can additionally comprise from 0 to 10% of at least one low molecular weight additive, selected from a group, comprising colorants, pigments, fillers, lubricants, processing aids, foaming additives, slipping additives, matting additives.
• at least one low molecular weight additive selected from a group, comprising colorants, pigments, fillers, lubricants, processing aids, foaming additives, slipping additives, matting additives.
• the casing according to the present invention can be used for packing of sausage products with various shelf-life.
• a casing for short-time storage can be single-layer.
• a casing for more prolonged storage should prevent drying-induced loss of weight of the food content and protect it from atmospheric oxygen penetration, causing its spoilage (fat rancidity, etc). Therefore such casing comprises layers made of resins having oxygen-barrier properties.
• Polyamide resins such as polyamide 6 or polyamide 66 have enough high oxygen- barrier properties; however their oxygen permeability strongly depends on their humidity. Therefore it is preferable that during storage of a sausage product, at least thickest (main) of polyamide layers is relatively dry. It can be achieved if the main of polyamide layer is in contact with environing air and insulated from humidity of sausage meat by layer of moisture-impermeable resin. However, in this case, the condition of polyamide layer insulation from environing air, quite desirable for casing storage, is not achieved.
• the main polyamide layer at the same time is insulated both from environmental air and humid sausage meat by layers made of moisture-impermeable resin, but in such a way that water vapor transition rate (WVTR) of the first of said layers (between polyamide and air) is greater than of a second one (between polyamide and meat).
• WVTR water vapor transition rate
• the said moisture-impermeable layers are made of resins having similar values of moisture permeability coefficient, the last condition can be achieved by proper choice of their thickness, i.e. total thickness of moisture- impermeable layers placed between polyamide and air is far less than the thickness of moisture-impermeable layers placed between polyamide and meat.
• the polyamide layer mainly made of aliphatic polyamide, such as polyamide 6 or polyamide 66, by adding therein up to 20% of semiaromatic (co)polyamide 6I/6T or polyamide MXD6.
• the oxygen penetration protection can be provided by additional layers from such oxygen-barrier resins as PVDC or EVOH. Oxygen permeability of PVDC layer does not depend on its humidity; therefore a position of this layer in casing structure does not matter. If it is desirable that both polyamide-based layer and PVDC layer simultaneously act as oxygen barrier, moisture impermeable PVDC layer should be placed closer to sausage meat, than the polyamide layer.
• EVOH resins are moisture- sensitive too, and the dependence on humidity of their oxygen-barrier properties is even stronger than that of polyamide resins. Therefore the EVOH layer in the filled sausage casing is preferably placed as close to air as possible, but it should not be external.
• adhesive layers from olefin (co)polymers grafted by maleic anhydride can be used.
• the best interlayer adhesion is achieved if the type of olefin (co)polymer for moisture-barrier layer and type of backbone of grafted resin for layer providing adhesion between said barrier layer and other layer (polyamide, EVOH, etc) are the same.
• Such adhesive resins can be either the only component of the separate adhesive layer or one of components of mixed layer, also comprising relative to adhesive resins olefinic polymer and having both barrier and adhesive function.
• the casing can be performed in the form of tube, half-tube, or extensive flat film (sheet). It depends on whether it is intended to be stuffed by means of stuffing equipment (tube) or to be filled in a packing machine (half-tube or sheet).
• a printed image can be applied, and a tubular casing can also be shirred.
• the printed image can be applied onto invented casing by means of flexography or other high-performance printing technique.
• PA - polyamide composition comprising mainly polyamide 6, polyamide 66 copolyamide 6/66 or their blend and, optionally, (co)polyamide 6I/6T and/or MXD6; PO - polyolefin (PP, CPE, LDPE, HDPE, LLDPE, LVLDPE), EVA, ethylene/(meth)acrylic acid copolymer, ionomer or their blend;
• a - adhesive resin preferably, polyolefin, grafted by maleic anhydride
• a Layer PO 1 (PO 1 +A) is preferably far thicker than a layer P02 (P02+A).
• PA - polyamide composition comprising mainly polyamide 6, polyamide 66 or copolyamide 6/66 or their blend and, optionally, (co)polyamide 6I/6T and/or MXD6; PO - polyolefin (PP, CPE, LDPE, HDPE, LLDPE, LVLDPE), EVA, ethylene/(meth)acrylic acid copolymer, ionomer or their blend;
• a - adhesive resin preferably, polyolefin, grafted by maleic anhydride
• a Layer POl is preferably thicker than a layer P02 (P02+A, PVDC2).
• PVDC layer is selected by such way that it is characterized with WVTR far lower than WVTR of PO (PO+A) layer.
• the first three steps of the method for production of the casing according to the present invention including extrusion of single- or multilayer extrudate as a primary tube, the orientation drawing of the primary tube resulting in production of an oriented tubular film and thermofixation of the oriented tubular film tube resulting in a production of a thermofixed oriented tubular film disclosed above are typical for production method of conventional sausage casings, and the method is usually limited by aforesaid.
• Conventionally used following additional operations for applying a printed image and/or shirring can only change appearance or shape of the casings, but not their properties.
• a cast "workpiece" of the tubular casing is formed by the extrusion.
• extrudate formed by means of a circular die hole is cooled as fast as possible (quenched) to be transformed into so called primary tube.
• Water is preferably used as cooling medium. Such technique prevents nylon crystallization and facilitates following orientation drawing.
• This step can be performed in a convection tunnel oven, infrared tunnel oven or a bath with hot water.
• the biaxial drawing of tubular casing is performed by means of inflating of primary tube by air, which remains entrapped therein by two pairs of driving nip rollers, to transform it into a bubble (secondary tube or secondary bubble) and its simultaneous drawing in the machine direction by means of said two pairs of nip rollers, due to a difference in rotation speeds of the rollers in each pair: rollers of second (in the direction of the film motion or downstream) pair rotate faster than of first one.
• This method is also called "entrapped bubble technique”.
• the primary tube is drawn 2-3 times (according to ratio of the rollers rotation speeds) and its diameter of is usually increased 3.5-4 times.
• the draw operation results in a tubular film, having the poly amide layer in unstable oriented state, characterized with presence of a plurality of small and imperfect crystallites tending to recrystallization.
• Thermofixation of the oriented tubular film is preferably performed by exposure of this film as an air-inflated bubble (tertiary tube or tertiary bubble) to temperatures of 130- 160°C, and preferably of 135-140°C for 3-5 seconds.
• a convection oven or infrared heater are preferably used for the heating of the bubble.
• Thermofixation by means of treatment of hot water or steam known from prior art is not acceptable, because in the context of the invention objects the polyamide layer should be dry after thermofixation.
• thermofixation the oriented tubular film is shrinked by up to 10% and preferably by 5 to 10%, both in the transverse direction and in the machine direction due to selected excess pressure in the bubble and speed ratio of pairs of nip rollers at the beginning and at the end of the bubble.
• thermofixation temperature is equal to or lower than 120°C
• the resulting casing is characterized by inferior dimensional stability and high values of heat shrinkage.
• the proper thermofixation leads to a formation of stable crystalline phase of polyamide. Its stability is manifested by absence of tendency to recrystallize either spontaneously under storage conditions or under the action of moderate tensile deformations for instance of those, that take place at the casing filling.
• the flattened tubular casing is cooled to a room temperature or a little bit higher and wound on a core.
• the casing prepared by means of said first three stages is generally characterized by virtually zero "free” shrinkage during the storage at a room temperature, "wet" shrinkage value of about 0 to 1.5 in both directions and "hot” shrinkage value of about 5 to 8% in both directions.
• thermofixation stage the tubular casing with temperature, exceeding glass transition temperature of polyamide composition of the above polyamide layer, is drawn by 4-10% in the transverse direction and by 3-8% in the machine direction, preferably by 5-10% in the transverse direction and by 4-8% in the machine direction, with following cooling in the drawn (strained) state to temperature below glass transition temperature of polyamide composition.
• the range of glass transition temperatures of polyamides 6 and 66 (dry), mainly composing polyamide composition is the temperature area between approximately 47 and 60°C (Nylon Plastics Handbook ed. by Melvin I. Cohan, Cincinnati: Hanser/Gardner Publication, Inc, 1995, CTp. 147).
• the glass transition point (temperature) is usually understood as the lowest value of this range.
• Addition of semiaromatic (co)polyamides 6I/6T and/or MXD6 into polyamide composition can result in an increasing of glass transition temperature of polyamide composition.
• adding of 20% copolyamide 6I/6T (trademark Selar® PA 3426, from DuPont) into polyamide 6 increases glass transition temperature of the latter by 10°C (see, for example, a brochure «DuPontTM Selar® PA3426 Blends With Nylon 6» - URL:
• a value of drawing temperature (above the glass transition temperature of polyamide composition) generally does not matter and can be selected mainly according to economic considerations.
• the drawing temperature is in the range 60-140°C, but preferably 70 to 130°C.
• polyamide composition comprises no semiaromatic (co)polyamides such as 6I/6T and/or MXD6
• the drawn tube is preferably cooled to temperature not higher than 47°C, preferably to temperature 45°C, but if said composition comprises 20% of polyamide 6I/6T, the tube can be cooled to temperature not higher than 57°C, preferably to temperature 55°C.
• the final cooling of the casing to room temperature can be performed after flattening of the tube, for example, by means of a chilling roller.
• the biaxial draw of the casing is performed by means of its simultaneous inflating by air in bubble and drawing in the machine direction by means of two pairs of driving nip rollers by the same manner that has been disclosed above for stage of the orientational draw.
• This step of the production process can be performed both in the same manufacturing line as steps for production of the oriented tubular film and in a separate apparatus.
• This apparatus comprises sequentially arranged unwinding roller, a heater, two pairs of driving nip rollers, a cooling device and a winding roller.
• Such apparatus can compose a manufacturing line together with shirring machine, operating at constant rate of casing feed.
• a tunnel convection oven or a tunnel infrared oven can be used as the heater.
• the heater can be set between the unwinding roller and the first pair of the nip rollers to heat a flattened tube, or first and second pair of the nip rollers to heat the bubble.
• the cooling of the inflated bubble is preferably performed with stream of cold air by means of a ring- shaped air-blower. If the step of the additional drawing is performed in-line with forgoing steps for production of conventional tubular casing, there is no need in additional heater, because the casing is still hot enough after thermofixation step, but other units should be set.
• the additional stage according to the present invention takes short time, because the additional bubble has a length not exceeding 3 meters, even if it is built-in into high- performance production line.
• the tubular casing has an external (air-contact) polyamide layer and a printed image is intended to be applied upon it by using printing inks dissolved in lower alcohols or water. If the casing has been additionally drawn, such procedure can cause its local shrinkage and distorting of its regular cylindrical shape. In this case the application of the printed image preferably foregoes the additional drawing step and a design of this image should be developed with a glance to the preprogrammed draw- inducted distortions.
• tubular form of the casing it can be supplied either as rolls or as shirred sticks. In the last case it can be produced from a wound flattened tube (roll) into shirred sticks by means of well-known technique of shirring. If an ultimate consumer needs a film intended to be formed, filled with sausage meat and sealed by means of SSF packing machines of horizontal or vertical type, the initial tubular casing according to the present invention can be performed into a half-tube or an extensive flat film by cutting along one or both bent lines of flattened tube correspondently. In the last case two rolls of the flat film are produced. Such films with applied printed image can be made by both applying the image onto these films per se and by applying the image onto the invented tubular casing and its following cutting.
• the extensive flat film can be produced out of the tubular casing, having twice smaller thickness, by bonding of two halves of flattened tubular casing by suitable glue comprising no solvent and require no heat to be applied, for instance, UV-curable glue.
• suitable glue comprising no solvent and require no heat to be applied, for instance, UV-curable glue.
• Such operation can be performed by means of an apparatus comprising sequentially arranged unwinding roller, two pairs of driving nip rollers, UV-lamp and a winding roller, wherein UV-curable glue is loaded into a bubble between the pairs of driving nip rollers and subjected to curing in a thin layer between the halves of flattened tubular casing.
• the casing can be stretched at stuffing so far as it is allowed by stuffing equipment without damage to the end product quality and without excess deterioration of the equipment. Nevertheless the invented casing is preferably filled with no stretching.
• the sausages can be slightly flabby by touch, except the case when the main polyamide layer directly contacts with sausage meat and the contacts with humid medium for very short time causes shrinkage of the casing, its tension and compression of its content.
• the final cooked sausage product enclosed into the invented casing in any embodiment is a tough stuffed log (stick). Filling of the casing by means of packing machine. In the case of the filling of the casing by means of packing machine, the use of casing with maximum "wet" shrinkage is preferable.
• Filling is performed according to usual processing scheme.
• Cooking of the sausage product In both cases of filling of the casing the cooking of the sausage product is performed according to usual processing scheme. Scalding, boiling and even retorting (autoclaving ) processes can be used. During these water and heat treatments water penetrates into polyamide layer and, from this moment the forces tensing the casing around the sausage meat begin to act.
• Cooling of the sausage product should be performed in such a way, that the polyamide layer remains humid for all cooling time period. Generally, cold water sprinkling or showering are the preferable methods of cooling. If the main polyamide layer of multilayer casing according to the present invention is a core layer, one may use cooling by cold air stream with caution, but if this layer is food-contacting, the cooling by cold air can be used with no doubt.
• Sausage products can be put into a refrigerator right after said cooling.
• Such sausage product is a tough log without wrinkles. Its following use depends on barrier properties of the casing which are determinated by its composition and structure.
• sausage product enclosed in the casing comprising food-contacting polyamide layer without additional oxygen barrier layers is preferable to use for following fabrication of either sausage slices enclosed into a package made of a barrier film, preferably into barrier vacuum bag.
• Other types of the product can be stored in a refrigerator for a time period approved for a certain type of the casing by Medical authorities basing on hygienic regulations.
• the casing according to this example was a tubular 5-layer casing having diameter of about 65 mm, wall thickness of about 42-45 ⁇ and sequence of layers (from meat to air - LTR):
• PAl was a blend of 60 % PA 6 (trademark Ultramid B4), 35% PA 66 (trademark Ultramid A4) (both polyamides from BASF GmbH, Germany) and 5% PA 6I/6T (trademark Selar® PA 3426 from DuPont Co, USA);
• PE1 was LDPE (trademark 15803-020 from Kazanorgsintez, Russia);
• PA2 was blend of 55 % PA 6 (trademark Ultramid B4), 40% PA 66 (trademark Ultramid A4) and 5% ionomer (trademark Surlin 1652 from DuPont Co, USA).
• This casing was manufactured according to first three steps of above method for production thereof.
• Granulated resins were loaded into hoppers of five extruders of multilayer extrusion apparatus, according to above sequence of layers. Said resins were melted in these extruders and their melts were transferred into extruding head where melted sequence of 5 layers was formed. Then this 5-layered extrudate was extruded trough a circular die hole as a melted tubular workpiece into a bath with cold water. The workpiece was chilled and solidified transforming into a multilayer primary tube, having diameter of about 20 mm and thickness of about 320 ⁇ .
• the primary tube was transferred by a set of pairs of driving nip rollers into the tubular IR-heater, where it was heated to a temperature of 80°C, and then it was biaxially oriented by a factor of 2.5 in the machine direction and 3.7 in the transverse direction by the described above inflating/drawing technique.
• thermofixation of the resulted oriented film was performed by heating of air-inflated tertiary bubble at temperature of 135-140°C for 3-5 sec by means of tubular IR-heater (of tunnel type).
• thermofixation the diameter of the tertiary bubble was maintained constant of 66-67 mm, but it was allowed to shrink in the machine direction by a factor of 1.10 due to corresponding adjustment of rotation speeds of the rollers in said two pairs limiting bubble.
• the resulted tubular casing was flattened, cooled to temperature of 25°C and wound on a core as a roll. Characteristics of the casing are shown in Table 3. After the casing sampling, the roll was packed into polyethylene bag for following transportation to a shirring section.
• the casing according to this example was made by treatment of the casing, produced according to comparative example 1 , at a separate apparatus. A printed image by inks containing ethanol as solvent was applied upon the casing, produced according to comparative example 1. Then a roll of this casing was set in the unwinding roller of an apparatus with total length of 1.5 m comprising except said roller sequentially arranged an IR-heater equipped with flat IR-radiators, first pair of driving nip rollers, a cooling ring- shaped air-blower, second pair of driving nip rollers, chilling roller and a winding roller.
• the casing was passed through the space between IR-radiators, both pairs of the rollers (unclenched) and the ring of the air-blower; then the first pair was clenched; the tubular casing was inflated to diameter of 72-73 mm; the second pair was clenched; the flattened tubular casing was passed through the chilling roller and its end was bonded on a core set in the winding roller. After that, compressed air was supplied to the blower and rotation of all driving rollers was switched. Rotation speed ratio between the rollers of second and first pair was 1.04; rotation speeds of the unwinding roller and rollers of the first pair on the one hand, and winding roller and rollers of the second pair on the other hand were equal.
• the temperature of the inflated bubble in an area between the first pair and the beginning of a cooling zone was 70 to 80°C and 45°C after this zone.
• the flattened tube had temperature of 25°C.
• the last piece of the casing with a length of 1.5 m was cut and thrown away.
• the treated casing has diameter of 71.5 mm (flattened tube width was 112-1 12.5 mm) and thickness of about 35 ⁇ . Characteristics of the casing are shown in Table 3. After sampling of the casing, the roll was packed into polyethylene bag for following transportation to a shirring section.
• the casing according to this example was a tubular 7-layer casing having diameter of about 65 mm, wall thickness of about 42-45 ⁇ and sequence of layers (from meat to air - LTR):
• PE2 was blend of H3 80% PE1 (according to example 1) and 20% of brown pigment concentrate with percentage of the pigment of about 48% (trademark 1340 Brown from A. Schulman Inc., Germany),
• PE3 was LVLDPE (trademark AFFINITY PL 1845G from Dow Chemical Co.).
• the casing was fabricated in the same manner as in Comparative Example 1, except that 7-layered head was used for forming of the multilayer extrudate and during thermofixation at 135-140°C the casing was allowed to shrink not by 10% only lengthwise, but by 5% in relation to dimensional parameters of the secondary tube, both in the transverse direction, and in the machine direction. Resulted casing had thickness of about 40.5 ⁇ and diameter of 68-68.5 mm (flattened tube width was 107-107.5 MM).
• one roll was packed into polyethylene bag and then was transported to a printing section in order to apply an image by UV-curable printing inks onto the casing.
• One more roll was packed into polyethylene bag and then was transported to the warehouse. Characteristics of the casing are shown in Table 3.
• the casing according to this example was made in the same manufacturing line as that which was used for producing of casings made by comparative examples 1 and 2, but characterized with additional assembly of units placed in the end of the line directly before chilling roller, wherein said assembly comprised sequentially arranged a zone for free running of the flattened tubular film, first pair of driving nip rollers, two cooling ring- shaped air-blowers and second pair of driving nip rollers.
• Such reconstruction resulted in increasing of the line length by 3 meters.
• the start of the producing process for this casing was identical to that of comparative example 1, but the finish of thermofixation stage and leaving the second of pairs of driving nip rollers, limiting the tertiary bubble, the flattened tubular film was passed through the free running zone and both pairs of driving nip rollers; and this pair was clenched. Then the tubular film was inflated to diameter of about 73 mm, said second pair was clenched and after that the flattened tubular casing was passed through the chilling roller and its end was bonded on a core set in the winding roller. Compressed air was supplied to the blowers and the casing began to wind on a core.
• Rotation speed ratio between the rollers of second and first additional pair was 1.05; temperature of the film before entering into the first pair was 105 °C, 60°C after the first blower, 48°C after the second one and 27 °C after passing through the chilling roller.
• Resulted casing had a thickness of about 35 ⁇ and diameter 71.5 mm (flattened tube width was 112-1 12.5 mm).
• one roll was packed into polyethylene bag and then was transported to a printing section in order to apply an image by UV-curable printing inks onto the casing.
• One more roll was packed into polyethylene bag and then was transported to the warehouse. Characteristics of the casing are shown in Table 3.
• the casing has sequence of layers identical to that of Example 2.
• the casing was fabricated in the same manner as in Comparative Example 1 , except that thermofixation was performed at 120°C for 2 seconds.
• the resulted casing had the same diameter and the same thickness, that the casing by comparative example 1.
• the casing has sequence of layers identical to that of Example 1. It was fabricated in the same manner as in Comparative Example 1 , except that to thermofixation at 90°C for 2 seconds was subjected a flattened tube. In the machine direction the casing was allowed to shrink by 10%. The resulted casing had a width of flattened tube of 98-99 mm and wall thickness of about 47 ⁇ . After sampling of the casing, one roll was packed into polyethylene bag and then was transported to a printing section in order to apply an image by UV-curable printing inks onto the casing. Characteristics of the casing are shown in Table 3.
• the roll of the casing according to comparative example 3 was manifestly compressed due to tension of the casing, but ink layer was also not damaged.
• the roll of the casing according to comparative example 4 was strongly compressed especially in the central part of its cylindrical surface. Inspection after unwinding of the roll showed that the ink layer was partly transferred onto opposite half of the flattened tube contacted with it.
• the casings according to examples 1 and 2, and also to comparative examples 1 , 2 and 3, bearing printed image and shirred were transported to a sausage-making factory wherein they were filled with sausage meat in the manner that diameters of the resulted sausages exceeded the nominal ones by no more than 0.5%.
• the filling was performed by means of stuffing line, comprising the stuffer HANDTMANN VF618 from Albert Handtmann Maschinenfabrik GmbH & Co. KG and automatic clipping device FCA-3463 from Poly-Clip System GmbH & Co. KG.
• sausages were transferred into a cooking chamber, wherein they were subjected to steam treatment (steam temperature was about 100°C) until the temperature of the central area of a sausage achieved 70°C that took about 40 minutes.
• steam temperature was about 100°C
• a part of the sausages enclosed in casings according to examples 1 and 2 was cooled by cold air stream.
• Resulted sausages cooled by water were taut if only they were enclosed in casings according to the present invention. In the case of cooling by air, only sausages enclosed in casings according to present example 2 were taut. In all other cases sausages were flabby and wrinkled. Imitation of filling by means of packing machine
• Outer surfaces of the tubular casings according to example 2 and comparative examples 2 and 3 were treated by corona discharge until surface energy of 80 dyne/cm was achieved. Then pieces of the casings with length of about 40 cm each were cut along one of bent lines of flattened tubes and resulted half-tubes and were rearranged in such manner that its surfaces comprising polyolefin layer were faced to each other and butt- sealed by means of contact welding with seam of 2 mm width. After that a casing film on one butt end of each resulted turned inside out tube was pleated and fixed by a clip from thick aluminum wire. Then the tube was manually filled with sausage meat and fixed by a clip from second butt end.

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