Title of Invention: MULTILAYERED COEXTRUDED THERMOPLASTIC FOOD CASING Abstract: Provided is a multilayered coextruded thermoplastic food casing comprising a porous inner layer capable of absorbing, retaining, desorbing and transferring at least one functional additive to food encased in said casing; a layer having a barrier effect for water vapor; and an adhesive layer, wherein the porosity of said porous inner layer is generated by (co)extruding a composition comprising a polymer and a supercritical pore-forming agent, and a method for producing said multilayered coextruded thermoplastic food casing. Detailed Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a multi-layered coextruded thermoplastic food casing comprising at least one porous inner material free of any liquid pore-forming agent, especially any non-supercritical liquid pore-forming agent, a method for producing said multi-layered coextruded thermoplastic food casing and use of said multi-layered coextruded thermoplastic food casing as a sausage casing, which food casings are able to transfer functional food additives such as flavours or colours onto the enclosed foodstuff, especially in the production of meat, cheese or fish products. BACKGROUND ART OF THE INVENTION In the production of meat products, smoke treatment has a long tradition for taste and preservation reasons. In said tradition the smoking of the products in smoking rooms or chambers is widely used. However, in such a case the casings need to be permeable for the smoke, which means that the casing has a low water vapor barrier so that the meat product can lose weight during cooking and storage. To avoid weight loss during storage a secondary packaging may become necessary. However, smoke deposits on the walls and the ceiling of the smoking rooms can affect the appearance of meat products in the following cooking cycle. To avoid these drawbacks of a smoke treatment through a smoke-permeable casing, the inside of casings has been impregnated with liquid smoke, but also with colour solutions, e.g. solution on a caramel basis. For a transfer of substances like liquid smoke or caramel solutions onto foodstuff, several casing structures have been described. As inner layers of the casing, some of these structures use hydrophilic materials, which allow the absorbance of the substance into the material. For example inner layers comprising starch were used. Other structures use as an inner layer a layer made of cellulose which will come into contact with the foodstuff. In such a case, however, absorption of the liquid into the paper or cellulose is observed. In case of a flat film, alternatively the desired substance can be sprayed, printed or scraped onto the surface. In order to achieve a sufficient fixation, a drying step has to be carried out before the flat film is formed into a tube. The carrier film of these structures can be a fibrous layer, the major disadvantage of which is a low water vapor barrier effect. During a cooking process or storage of the product, the foodstuff loses part of its water content so as to reduce the yield. For this reason plastic coated fibrous casings and laminates comprising an outer plastic film and an inner paper or cellulose film have been developed. EP 0 992 194 A discloses a casing that consists of an impermeable film with a joined inner lining consisting of fibres made of cotton or cellulose, or woven, non-woven or knitted fabric, which is then impregnated and sealed to a tube or bag. Another example of a food casing which is capable of transferring food additives is described in US 2006/0003058 A, which teaches an at least two-layer tubular food casing comprising an inner layer formed of a thermoplastic organic polymer embedding a powdery organic filler such as starch. The casings described in these two documents combine the absorbance capacity of a material forming the inner layer with barrier properties provided by the outer (multi-layer) plastic films. However, such casings are disadvantageous because of a multi-step production process for preparing such coated or laminated casing structures, a possible mould growth on the inner starch, cellulose or paper layer and breaking problems during cooking. U.S. Pat. No. 7,615,270 B2 discloses casings comprising an inner layer made of hydrophilic materials, such as e.g. block-copolyether ester or block-copolyether amide. Such casings suffer from the disadvantage that their absorbance capacity for liquid substances is limited. In many applications, such an absorbance capacity is insufficient in order to provide the desired effect onto the foodstuff. Furthermore, if liquid substance remains on the casing surface, a non-uniform distribution and formation of droplets upon opening of the tubular casing is likely and the transfer onto the foodstuff shows a marble-like, non-uniform coloration of the foodstuff. DE 101 24 581 A teaches a food casing wherein liquid smoke is sprayed into the casing during shirring. As it takes a storage time of at least 5 days for the liquid smoke to migrate into the casing, the storage costs and the “time-to-customer” is high. In general, the outer side of a casing is easily accessible for coating and impregnation. But, in the case of barrier films the casing needs to be turned inside out so that the treated surface will come into contact with the foodstuff. EP 1 192 864 A teaches a step of coating or impregnating the outer side of a casing which is stuffed into the bore of the strand so that it can be turned inside out during stuffing. Thus prior to a stuffing of the strand the treated outer surface of the casing can be contaminated during handling processes. When the treatment (coating/impregnation) is carried out by using a printing process, the edges are printed twice, which leaves two longitudinal lines of higher intensity on the foodstuff after peeling off the casing. In order to increase the absorbance capacity of the thermoplastic inner layer, a formation of pores in that inner layer is desirable. Inner layers formed of a thermoplastic material as disclosed in EP 1 164 856 B1 show a network of interconnected interstices, which are in a range of 0.002 to 1 μm. Said inner layers are made from a food grade thermoplastic with the interstices being formed by a non-supercritical liquid pore-forming agent selected from soybean oil, peanut oil, corn oil, glycerine, sorbitol, polyethylene glycol, mineral oil or surfactants including polysorbate, polyoxyethylene (POE) 20, sorbitan monostearate, sorbitan monolaurate, sorbitan monooleate and glycerol monooleate. The thermoplastic material may further comprise an inorganic filler selected from silica (SiO2), talc (Mg2SiO4), aluminium oxide, hydrated alumina, titanium oxide, zirconium oxide, sodium silicate, silicate, sodium chloride, calcium, calcium carbonate, clay or calcined clay. EP 1 911 352 A1 discloses a multi-layered coextruded stretched thermoplastic food casing comprising at least one porous inner layer, wherein the porosity of the porous inner layer has been at least partially generated by stretching the coextruded casing, wherein at least an innermost porous inner layer has an interconnected porosity, such that said innermost porous inner layer is able to absorb, retain, desorb and to transfer at least one transferable functional additive from said at least one porous inner layer to food encased in said casing. The casing further comprises at least one layer having a barrier effect for water vapor, at least one layer having adhesion properties, said layer having adhesion properties can be the same or different from said porous inner layer and/or said layer having a barrier effect for water. The porous inner layer comprises at least one plastic material and at least one fine-grained filler and at least one pore-forming component of an oily pore-forming agent and an emulsifying agent. US 2009/214722 A1 discloses a multi-layered seamless tubular casing capable of absorbing, storing and releasing a food additive, comprising an outer layer of a thermoplastic polymer, at least one intermediate functional layer made of thermoplastic polymer with barrier properties against oxygen and/or water vapor and one porous inner layer made of a thermoplastic polymer with a network of spaces connected to one another, which are formed by using a non-supercritical liquid pore-forming agent bringing about a phase separation of the thermoplastic and the porosity modifier. The non-supercritical liquid pore-forming agent is selected from the group consisting of soybean oils, peanut oils, corn oils, glycerols, sorbitols, polyethylene glycols, mineral oils or surfactants, polysorbates, polyoxyethylene (POE) 20, sorbitan monostearates, sorbitan monolaurates, sorbitan monooleates, glycerol monooleates, Surf actol 365, or mixtures of the aforementioned substances. The inner layer may further comprise an inorganic filler selected from the group of silicon dioxide, talc (Mg2SiO4), aluminium oxide, aluminium hydroxide, hydrated alumina, calcined alumina, titanium oxide, zirconium oxide, sodium silicate and silicate. The inner layer of the casings described in EP 1 911 35 2A1 and US 2009/214722 A1 requires a complex recipe and compounding step. The compounds of the examples described in EP 1 911 352 A1 comprise a relatively high percentage of low molecular liquid pore-forming agents (19% oil and 8% glycerine), which make a compounding challenging and cost-intensive. In US 2009/214722 A1 the recipe of the inner layer is not disclosed, but the reported absorbance capacities are low. Furthermore, all such casings suffer from the fact that the liquid pore-forming agents used at least partially remain in the inner layer. Therefore, the object to be solved by the present invention is to provide a multilayered coextruded thermoplastic food casing having a high strength and improved peelability the inner layer(s) of which has/have a high porosity, high functional additive absorption capacity, excellent capability of transferring food additives to the enclosed foodstuff, wherein said inner layer(s) do(es) not comprise any liquid pore-forming substance, especially any non-supercritical liquid pore-forming agent after formation of the inner layer. Furthermore, it is an object of the present invention to provide a process for preparing such an improved multi-layered coextruded food casing. SUMMARY OF THE INVENTION According to the invention, there is provided a multilayered coextruded thermoplastic food casing comprising: at least one porous inner layer; at least one layer having a barrier effect for water vapor and/or oxygen; at least one adhesive layer, said adhesive layer optionally being the same as or different from said at least one porous inner layer and/or said at least one layer having a barrier effect for water vapor and/or oxygen; wherein the porosity of said at least one porous inner layer has been generated by (co)extruding a first polymer composition comprising a polymer and a supercritical pore-forming agent, wherein the porosity of the sum of said at least one porous inner layer is in a range of from 5 to 90% by volume, said at least one porous inner layer has a porosity comprising pores having a pore diameter of from 0.01 to 2000 μm, such that at least one of said at least one porous inner layer(s) is able to absorb, retain, desorb and to transfer at least one transferable functional additive from said at least one porous inner layer to food encased in said casing. In a preferred embodiment said multi-layered coextruded thermoplastic food casing is free of any liquid pore-forming agent, especially any non-supercritical liquid pore-forming agent. The present invention further provides a method for producing a multilayered coextruded thermoplastic food casing as defined above, comprising a step of subjecting a combination of a first polymer composition for forming at least one porous inner layer said first polymer composition comprising a polymer and a supercritical pore-forming agent, and at least one additional second polymer composition for forming a layer having a barrier effect for water vapor and/or oxygen to a co-extrusion process, wherein said polymer composition comprising a polymer and a supercritical pore-forming agent, after co-extrusion forms said at least one porous inner layer. The method for producing a multi-layered coextruded thermoplastic food casing optionally further comprises a step of subjecting the casing obtained by the co-extrusion process subsequently to a bi-axial orientation treatment. Furthermore, in a preferred embodiment of the present invention the first polymer composition which will form the at least one porous inner layer is a first polymer composition consisting of a polymer, a supercritical pore-forming agent and optionally a filler and/or optionally a nucleating agent. As according to the present invention the multi-layered thermoplastic food casing is produced by a co-extrusion process, any disadvantages due to lamination can be avoided and, thus, mould-risks due to organic substances such as paper, cellulose or starch will be prohibited. Furthermore, in view of the production process used according to the present invention for example tubular multi-layered food casings can be produced which do not comprise any weakening seam or seal so that a non-uniformly coloured surface of the foodstuff will not be produced. Furthermore, by subjecting said multi-layered coextruded thermoplastic food casing to a bi-axial orientation treatment, e.g. in a double- or triple-bubble-process, the strength of the casing structure can be further increased so that wrinkle-free and calibre consistent products can be produced. If the multi-layered coextruded thermoplastic food casing is not subjected to an orientation treatment, the strength of the multi-layered thermoplastic food casing can be increased by additionally co-extruding (a) non-porous inner layer(s) having an increased wall thickness in order to guarantee good peelability without splitting into multiple pieces. Such a non-oriented multi-layered food casing may be desirable in slicer applications. Furthermore, according to the present invention the functional additive absorption capacity can be easily adjusted by increasing or decreasing the thickness and/or the porosity of the at least one porous inner layer during co-extrusion. According to the present invention the term “functional additive absorption” should mean any absorption and/or adsorption of a functional additive into and/or onto the at least one porous inner layer. Contrary to the food casings known in the art, the porosity of the at least one inner layer of the food casing according to the present invention offers a complete absorption of the functional additive into the pores. Furthermore, the functional additive is practically completely transferred to the enclosed food article because the functional additive is not significantly retained in the material forming the inner layer(s). In spite of the high porosity of the at least one inner layer of the food casing of the invention, the producibility of the casings according to the invention is surprisingly good. At the same time, due to the use of a supercritical pore-forming agent such as supercritical nitrogen or supercritical carbon dioxide the production process used according to the present invention is highly efficient and excludes the presence of any residual pore-forming agent in the food casing of the present invention, and, thus, any migration of said pore-forming agent which may remain in the food casing according to the present invention, into the foodstuff. Thus, by using a food casing according to the present invention due to the complete absence of any non-supercritical liquid pore-forming agent in the final food casing, any possible organoleptic impact on the foodstuff enclosed in the food casing can be avoided. Preferably, the at least one porous inner layer of the food casing according to the present invention does not comprise (i.e., is free of) any non-supercritical liquid pore-forming agent, especially any pore-forming agents selected from the group consisting of the following non-supercritical liquid pore-forming agents: 1. mineral oils 2. biogenous oils such as peanut oil, soybean oil, sun-flower oil, rape-oil, olive-oil or any combination thereof, 3. esters of natural fatty acids, 4. synthetic fatty acid esters of glycerin, such as glycerin monolaurate, glycerin monooleate, glycerin dioleate, glycerin trioleate or any combination thereof, 5. mono- or diglycerides of fatty acids, which optionally have been reacted to esters with any organic acid e.g. like acetic acid, lactic acid, citric acid or biacetyl tartaric acid or any combination thereof or any combination of substances within the different chemical groups, 6. polyglycerol esters of fatty acids, 7. alkyl polyglucosides, 8. esters of saccharose, 9. sugar glycerides, 10. sorbitan fatty acid esters such as sorbitan monostearate, sorbitan tristearate, sorbitan monolaurate, sorbitan monooleate, sorbitan monopalmitate or any combination thereof, 11. polyoxyethylene-sorbitan fatty acid esters such as Polysorbat® 20 (polyoxyethylene-(20)-sorbitan monolaurate), such as Polysorbat® 40 (polyoxyethylene-(20)-sorbitan monopalmitate), such as Polysorbat® 60 (polyoxyethylene-(20)-sorbitan monostearate), such as Polysorbat® 65 (polyoxyethylene-(20)-sorbitan tristearate) or like Polysorbat® 80 (polyoxyethylene-(20)-sorbitan monooleate), 12. C12- to C18-fatty alcohol ethoxylates, 13. pore-forming liquids based on glycerin or lecithin, such as viscous oil-like, but more polar liquids for example compounds on the basis of glycerin, lecithin or any combination thereof, and 14. glycolic compounds such as polyethylene glycol, propylene glycol ester especially of natural fatty acids like oleic acid, lauric acid, stearic acid, palmitic acid or any combination thereof. The at least one layer having a barrier effect for water vapor and/or oxygen in the food casing according to the present invention assures a sufficient barrier effect so that the to-be-transferred functional additive does not readily diffuse to the outside of the casing so that the environment and machinery during handling and processing will not be contaminated. Furthermore, by using a food casing according to the present invention weight loss during cooking and storage can be greatly decreased in comparison to conventional casings made of fibrous, cellulose or collagen. Furthermore, in case of using as the at least one layer having a barrier effect for water vapor and/or oxygen at least one layer having a barrier effect for water vapor, the water vapor transmission rate of said at least one layer having a barrier effect for water vapor of the food casing according to the present invention can be adjusted in such a way that drying of the surface still can take place during cooking. Thus, the water vapor transmission rate of said at least one layer having a barrier effect for water vapor can be decreased for example by producing at least one polyolefin-based barrier layer. On the other hand the water vapor transmission rate of the at least one layer having a barrier effect for water vapor can be increased for example by reducing the wall thickness of the at least one layer having a barrier effect for water vapor or by incorporating hydrophilic components into the at least one layer having a barrier effect for water vapor and/or by sandwiching a microcellular layer between two layers having a barrier effect for water vapor. Thus, a surface wetness resulting from a diffusion of the food additive to the outer surface of the casing can be prohibited according to the present invention. Also the oxygen barrier effect of the at least one layer having a barrier effect for water vapor and/or oxygen can be adjusted via adjusting the composition of the at least one layer having a barrier effect for water vapor and/or oxygen. For example, blends of polyamide and polyethylene vinyl alcohol or polyamide resins having a barrier effect for oxygen such as polyamide resins produced through polycondensation of meta-xylylene diamine (MXDA) with adipic acid (e.g. Nylon-MXD6® from Mitsubishi Gas Chemical Company) can be used. In a further embodiment of the present invention a multi-layered coextruded thermoplastic food casing according to the present invention as defined above may comprise at least 3 layers, the additional outer layer of which being a functional layer providing optical properties (e.g. colour or natural appearance) and/or haptical properties (e.g. roughness or soft touch) to the food casing. For example, an outer functional layer may provide a natural matt appearance of the food casing. Alternatively, use of two different colours in such an outer porous functional layer and a further more inner layer of the food casing according to the present invention can provide appealing optical effects. Depending on the polymer used and the porosity level produced, soft touch or rough surfaces on the most inner and/or outer microcellular layers of the food casing according to the present invention can be obtained. Contrary to a roughness created by the addition of inorganic fillers in the food casings known in the art, the surface roughness created by the process according to the invention, does not give rise to any abrasion on the rubber brakes of the stuffing machines. In a further embodiment of the present invention, outer functional layers having an improved printability, optical and/or haptical properties (e.g. colour, roughness e.g. by fillers, additives or low vapor pressure liquids) can be produced by using recipes known to a person skilled in the art. In a further embodiment the present invention provides a multilayered coextruded thermoplastic food casing comprising: at least one inner layer having a barrier effect for water vapor and/or oxygen; at least one porous outer layer; at least one adhesive layer, said adhesive layer optionally being the same as or different from said at least one porous outer layer and/or said at least one inner layer having a barrier effect for water vapor and/or oxygen; wherein the porosity of said at least one porous outer layer has been generated by (co)extruding a first polymer composition comprising a polymer and a supercritical pore-forming agent, wherein the porosity of the sum of said at least one porous outer layer is in a range of from 5 to 90% by volume, said at least one porous outer layer has a porosity comprising pores having a pore diameter of from 0.01 to 2000 μm. Furthermore, in a preferred embodiment of the present invention the first polymer composition which will form the at least one porous outer layer is a first polymer composition consisting of a polymer, a supercritical pore-forming agent and optionally a filler. In a further embodiment the present invention provides a method for producing a multilayered coextruded thermoplastic food casing comprising: at least one inner layer having a barrier effect for water vapor and/or oxygen; at least one porous outer layer; at least one adhesive layer, said adhesive layer optionally being the same as or different from said at least one porous outer layer and/or said at least one inner layer having a barrier effect for water vapor and/or oxygen; said method comprising a step of subjecting a combination of a first polymer composition comprising a polymer and a supercritical pore-forming agent, and at least one additional second polymer composition to a co-extrusion process, wherein said first polymer composition comprising a polymer and a supercritical pore-forming agent, after co-extrusion forms said at least one porous outer layer. DETAILED DESCRIPTION OF THE INVENTION A food casing according to the invention may be used as a packaging for any type of food including but not limited to meat products, sausage products, milk products, cheese products and dishes, whole or processed—especially to meat containing products like fermented sausages, cooked-meat sausage (Kochwurst), scalded emulsion sausage (Brühwurst) like frankfurters, cooked ham (Kochschinken), ham, pickled meat, smoked turkey breast and salt meat (Pökelware), vegetables, dairy products like cheese, carbohydrates, soybean products as well as different mixtures or in any desired application. The food casings according to the present invention may be used in any desired form such as in the form of “endless” tubes, sections, rings such as sausage rings, customized products such as shirred sticks, and the food casing according to the present invention can be empty, stuffed and further processed if desired. Food is often processed, i.e. cooked in a plastic film package, for example by at least partially immersing the package in hot water or placing the package in a steam cabinet. The processed food package may then be refrigerated until the processed food is prepared for a meal or is to be consumed, or it is peeled and further packaged after portioning into pieces or slices. During the cooking process e.g. of meat, for example, smoke or other modifiers for color, flavor or fragrance diffuse into the food material. Transferable Functional Additives The at least one transferable functional additive may be, for example one or more of colorants such as caramel or paprika extract, flavorants such as glutamates, fragrances such as terpenoides and/or any other desired food additives. Many transferable functional additives can be effective as different (such as two or three) types of transferable functional additives selected from colorants, flavorants and fragrances. For example liquid smoke functions as a colorant, a flavorant as well as a fragrance. A colorant, flavorant, fragrance and/or any other additive or any combination thereof may be applied—in one of the possible embodiments—to the casing in any desired way e.g. in a tube containing the liquid, dispersed, dissolved or in any combination. The transferable functional additive can be in a liquid mass which may preferably be distributed with the aid of a liquid bubble such that the additive may be moved or distributed or both. In this way, the content of the transferable functional additive may be directly applied to a coextruded and optionally stretched casing. The transferable functional additive may be applied in its commercially available condition—preferably in a liquid form, especially in a dissolved form, dispersed form or in a dissolved and dispersed form, e.g. in water—or in a modified condition. More preferred, it is dissolved in water. It is typically applied onto an inner surface of the casing, which is the outer surface of the innermost of the at least one porous inner layer (P) or onto the whole film(s), or onto the whole casing e.g. by dipping, flooding, spraying or even by squeezing of the tube containing the liquid mass. This can be effected e.g. with the aid of squeezing rollers and distributing the liquid at least partially on the surface of the inner layer of the casing e.g. with the aid of such squeezing rollers. Preferably at least one of these compounds or a mixture containing at least one of these compounds—preferably in a liquid form—may be absorbed, charged, distributed, incorporated, injected, applied as a film or coating or in any combination thereof onto/in/into a film or an inner layer of the casing. It is especially desirable that the additive(s) are provided onto/in/into the at least one porous inner layer or onto/in/into parts of the at least one porous inner layer e.g. by the effect of capillary forces, and are optionally maintained at least partially in the layer. In one embodiment, it is preferred that only the at least one porous inner layer is contacted with such compound(s) or their mixture(s). The surface of the film, casing or the at least one porous inner layer (P) may be dry after the application, especially, if only surfaces of porous layers are contacted with such fluid or liquid. The transfer of these compounds/mixtures from the at least one porous inner layer (P) of the food casing to the food may occur to a great extent or nearly completely via their interface. This is particularly the case if the at least one porous inner layer is formed of non-polar polymers such as polyolefins. If the colorant, the flavorant, the fragrance and/or any other additive is polar in nature, the transmission of such compound(s) may sometimes be nearly complete or perhaps even complete. The fluid or liquid of the at least one transferable functional additive is preferably taken up from the pores contained in the at least one porous inner layer. A casing according to the present invention preferably has at least one porous inner layer, but it is preferred in several embodiments that most or all the other layers of the casing have low, nearly no or no jaclynmyers@example.com. It is preferred that such other layers show a high strength. If a casing has at least one non-porous layer having a barrier effect for water vapor and/or oxygen and at least one porous inner layer, casings which are sufficiently porous can be formed. Such casings have a sufficient strength, a sufficient flexibility and/or a barrier effect for water vapor or gases like oxygen gas or both. The at Least One Porous Layer In one embodiment the food casing according to the present invention typically comprises at least one porous layer serving as the porous inner layer. Alternatively two, three or four or more porous inner layers may form a group of porous inner layers, of which one is the innermost porous layer whose surface is adjacent to the encased food. Typically, at least one porous inner layer is designed to absorb/adsorb and therefore to immobilize the at least one transferable functional additive. The additive can be in liquid form and after it is immobilized, it can be desorbed, mobilized and transferred to food it is in contact with. In another embodiment the food casing according to the present invention comprises at least one porous layer serving as the porous outer layer. The at least one porous inner or outer layer preferably contains at least one thermoplastic polymer component. Thermoplastic polymer components within the meaning of this invention include organic polymers having an essential content of thermoplastic organic polymers. Typically such thermoplastic organic polymers have a transition region of flowing above the temperature of their use and below their melting temperature—especially for at least partially crystalline organic polymers. In many embodiments, the at least one porous inner or outer layer contains more than one typically thermoplastic organic polymer component. Often there are two or three, and sometimes even more than three different thermoplastic polymers or even at least two different thermoplastic polymers which differ in at least one chemical group contained therein. Of the typically thermoplastic organic polymers usable according to the present invention, the following are especially preferred: (Co-)Polyamides, such as homo-, co- or ter-polyamides, preferably aliphatic (co-)polyamides, partially aromatic (co-)polyamides; polyolefins, preferably polyethylenes, polypropylenes or copolymers based on e.g. ethylene, propylene, or other -olefins, poly(iso)butene or any mixture thereof; vinyl copolymers like ethylene vinyl acetate copolymers, polyvinyl alcohols, ethylene vinyl alcohol copolymers (EVOH) or any combination thereof, which optionally is partially or totally saponified, such as ethylene vinyl alcohol copolymers; polyvinylpyrrolidone, polystyrene, polyvinylchloride, polyvinylfluoride or any combination thereof; vinylidenechloride (co-)polymers (PVDC), e.g. copolymers of vinylidene chloride with comonomer(s) such as vinyl chloride or (meth)acrylate; (co-)polyesters of aliphatic, (partially) aromatic or aliphatic and aromatic character e.g. polylactide, polycaprolactone, polycarbonate or (co-)polymers of aliphatic diols with aliphatic or aromatic dicarboxylic acid(s) such as terephthalate(s), such as poly(butylenes glycol terephthalate). In a preferred embodiment as the thermoplastic organic polymers there are used copolymers of ethylene or propylene, more preferably linear alpha-olefins with 3 to 8 C-atoms with alpha-beta-unsaturated carboxylic acids, more preferably acrylic acid, methacrylic acid and/or their metal salts and/or their alkylester or corresponding graft-copolymers of the above-mentioned monomers onto polyolefins or partially saponified ethylene/vinylacetate-copolymers, which are optionally graft-polymerised with an alpha-beta-unsaturated carboxylic acid and have a low degree of saponification, or their blends. Furthermore, modified polyolefins such as modified homo- or copolymers of ethylene and/or propylene and optionally further alpha-olefins with 3 to 8 C-atoms, which contain grafted monomers such as alpha-beta-unsaturated dicarboxylic acids, preferably maleic acid, fumaric acid, itaconic acid or their acidic anhydrides, acid esters, acid amides or acid imides can be used according to the present invention. Most preferred are polyolefins which contain grafted maleic anhydride, because grafted maleic anhydride groups provide an adhesive function so that delamination of e.g. polyamide-based and polyolefin-based layers can be avoided. In another preferred embodiment of the present invention polyolefins can be used as thermoplastic organic polymers, such as homopolymers of ethylene or propylene and/or copolymers of linear alpha-olefins with 2 to 8 C-atoms, preferably linear low density polyethylene, low density polyethylene, high density polyethylene, homo-polypropylene, block-polypropylene and random copolymers of propylene. In an even more preferred embodiment low density and linear low density polyethylene can be used. In another preferred embodiment of the present invention polyamides can be used as thermoplastic organic polymers, such as homo-, co- or ter-polyamides which can be produced from the corresponding monomers such as caprolactam, laurinlactam, omegaaminoundecan acid, adipinic acid, azelainic acid, sebacinic acid, decandicarbonic acid, dodecandicarbonic acid, terephthalic acid, isophthalic acid, tetramethylendiamine, pentamethylendiamine, hexamethylendiamine, octamethylendiamine and xylylendiamine. Preferred polyamides to be used according to the present invention are homo- and copolyamides such as polyamide 6, polyamide 12, polyamide 66, polyamide 610, polyamide 612, polyamide MXD6, polyamide 6/66, polyamide 66/6, polyamide 6/12 and polyamide 6I/6T. In an even more preferred embodiment polyamide 6 and polyamide 6/66 can be used. Furthermore, the thermoplastic organic polymer to be used according to the present invention may comprise additional hydrophilic components such as a polyetherester copolymer, polyvinylalcohol, polyesterblockamide, copoly etheresteramide or block copolymer polyether amide so as to facilitate the water vapor transmission through the matrix of the porous inner layer(s). Such a hydrophilic component may be used if a higher water transmission rate of the food casing is desirable, e.g. if a drying during cooking and/or storage of the foodstuff is desired. The at least one porous inner or outer layer usually has a wall thickness of 1 to 300 μm, preferably 5 to 200 μm, more preferably 10 to 160 μm. The layer thickness depends on the desired absorption capacity and the number of layers available to achieve the desired absorption capacity. According to the present invention the porous structure of the at least one porous inner or outer layer is created by a supercritical pore-forming agent introduced into the molten polymer or polymer mixture in the extruder designated to form the porous inner or outer layer(s), in such a way that a solution is being generated. In contrast, in the food casings known in the art the hollow spaces or interstices of the porous inner or outer layer(s) are formed by using non-supercritical liquid pore-forming agents such as hydrophobic oils or oily substances. One benefit of using a supercritical pore-forming agent is that according to the present invention no pore-forming agent will remain in the at least one porous inner layer after extrusion of the polymer mixture. Furthermore, the pore structure of the at least one porous inner or outer layer such as pore size, porosity level and so on, can be controlled much more exactly according to the present invention. Any of a wide variety of supercritical pore-forming agents such as supercritical nitrogen, supercritical oxygen, supercritical helium or supercritical carbon dioxide, more preferably supercritical carbon dioxide or supercritical nitrogen, even more preferably supercritical nitrogen may be used as the supercritical pore-forming agent to be used according to the present invention. Said supercritical pore-forming agent is introduced into the extruder and mixed with at least one first polymer (for forming the at least one porous inner or outer layer) for example a thermoplastic organic polymer so as to form rapidly a single-phase solution with the polymeric material by injecting said pore-forming agent as a supercritical fluid. Usually, the supercritical pore-forming agent is introduced into said polymeric material which is designated to form the porous inner layer(s), via a port of an extruder in an amount of 0.001 to 10 weight-%, based on the weight of the polymer mixture forming said inner or outer porous layer(s). As outlined above, the supercritical pore-forming agent to be used according to the present invention preferably is selected from the group consisting of supercritical nitrogen and supercritical carbon dioxide. In principle, other supercritical fluids such as supercritical helium or supercritical oxygen can be used as well. However, even if the supercritical point of such agents allows their usage, flammability, costs and/or health and environmental concerns may create disadvantages compared to supercritical nitrogen or carbon dioxide. The supercritical point of nitrogen is at 34 bar and −147° C., while the supercritical point of carbon dioxide is at 72 bar and 31° C. The concentration of the supercritical pore-forming agent in the polymer mixture forming the inner or outer porous layer(s) is usually in a range of from 0.001 to 10 weight-%, preferably in the range from 0.005 to 1 weight-%, most preferably in the range from 0.01 to 0.5 weight-% based on the weight of the polymer mixture forming said at least one inner or outer porous layer. In addition, optionally the at least one porous inner or outer layer may comprise a filler. Preferably, said filler is an inorganic filler. In a preferred embodiment said filler—if used—is selected from silica(s) such as quartz, fused quartz, cristobalite, diatomaceous earth, silica sol, silica gel, precipitated or pyrogenic silica or any combination of these silica types, talc, calcium carbonate(s), barium sulfate, alumina(s), aluminum hydroxide(s), magnesium hydroxide, titania(s), zirconia, silicate(s) preferably as precipitated Ca-, Al-, CaAl-, NaAl-silicates, as mica(s), kaolin, wollastonite or any combination of these silicates, NaCl or any combination between substances of these different substance groups. Such fillers are preferably added in a fine particulate form, although other forms like gel etc. can be used if desired. In a more preferred embodiment the filler is selected from silicates, carbonates, or hydroxides known to a person skilled in the art. Most widely used is talc or calcium carbonate. Preferably, the filler(s) used has/have an average grain size in the range from 0.02 to 12 μm, more preferred in the range from 0.05 to 8 μm, often in the range from 0.2 to 5 μm. Such filler(s) may also improve absorption/adsorption of the at least one transferable functional additive in at least one porous inner layer. Preferably, at least one porous inner layer is able to carry the at least one transferable functional additive. Typically the filler is added in an amount of from 0.1 to 20 weight-%, preferably from 0.1 to 15 weight-%, most preferably from 0.2 to 12 weight-%, based on the weight of the polymeric material used to form the porous inner layer(s). Furthermore, optionally the at least one porous inner or outer layer may comprise a nucleating agent. According to the present invention the nucleating agent may be selected from the group consisting of carbonates (e.g. sodium bicarbonate), hydrazine derivatives (e.g. 4,4′-oxibis(benzenesulfonylhydrazide)), azo compounds (e.g. azodicarbonamide), semicarbazides, tetrazoles, nitroso compounds and/or citric acid and derivatives thereof. In a preferred embodiment of the present invention in the multi-layered coextruded thermoplastic food casing according to the present invention (i.e., after its production) the at least one porous inner or outer layer consists of polymer and optionally a filler and/or optionally a nucleating agent. Especially, in the multi-layered coextruded thermoplastic food casing according to the present invention (i.e., after its production) the porous inner layer of the food casing according to the present invention does not comprise (i.e., is free of) any liquid pore-forming agent, especially any non-supercritical liquid pore-forming agents selected from the group consisting of the following pore-forming agents: 1. mineral oils 2. biogenous oils such as peanut oil, soybean oil, sun-flower oil, rape-oil, olive-oil or any combination thereof, 3. esters of natural fatty acids, 4. synthetic fatty acid esters of glycerin, such as glycerin monolaurate, glycerin monooleate, glycerin dioleate, glycerin trioleate or any combination thereof, 5. mono- or diglycerides of fatty acids, which optionally have been reacted to esters with any organic acid e.g. like acetic acid, lactic acid, citric acid or biacetyl tartaric acid or any combination thereof or any combination of substances within the different chemical groups, 6. polyglycerol esters of fatty acids, 7. alkyl polyglucosides, 8. esters of saccharose, 9. sugar glycerides, 10. sorbitan fatty acid esters such as sorbitan monostearate, sorbitan tristearate, sorbitan monolaurate, sorbitan monooleate, sorbitan monopalmitate or any combination thereof, 11. polyoxyethylene-sorbitan fatty acid esters such as Polysorbat® 20 (polyoxyethylene-(20)-sorbitan monolaurate), such as Polysorbat® 40 (polyoxyethylene-(20)-sorbitan monopalmitate), such as Polysorbat® 60 (polyoxyethylene-(20)-sorbitan monostearate), such as Polysorbat® 65 (polyoxyethylene-(20)-sorbitan tristearate) or like Polysorbat® 80 (polyoxyethylene-(20)-sorbitan monooleate), 12. C12- to C18-fatty alcohol ethoxylates, 13. Pore-forming liquids based on glycerin or lecithin such as viscous oil-like, but more polar liquids for example compounds on the basis of glycerin, lecithin or any combination thereof, and 14. glycolic compounds such as polyethylene glycol, propylene glycol ester especially of natural fatty acids like oleic acid, lauric acid, stearic acid, palmitic acid or any combination thereof. Usually, the at least one porous inner or outer layer contains a total porosity in the range of from 5 to 90% by volume, preferably in the range of from 40 to 90% by volume, more preferably of from more than 70 to 90% by volume, most preferably of from 72 to 90% by volume. If more than one porous inner layer is included in the casing, the porosity may preferably be varied from layer to layer, especially in such way that the innermost or the second innermost porous layer of all said inner porous layers has the highest porosity. Nevertheless, the group of all porous inner or outer layers of a casing usually shows a total porosity in the range from 5 to 90% by volume, preferably in the range of from 40 to 90% by volume, more preferably of from more than 70 to 90% by volume, most preferably of from 72 to 90% by volume. The pore diameters of the group of pores and the porosity channels in the porous inner or outer layers making up the largest distribution may preferably be in the range from 0.01 to 2000 μm, often in the range from 0.05 to 1200 μm. Preferably, the at least one porous inner or outer layer of the food casing has pore channels predominantly having diameters in the range from 0.02 to 1000 μm, more preferred in the range from 0.1 to 800 μm, as calculated via the numbers of well visible pores and their smallest diameter in the plane of the surface under a scanning electron microscope (SEM) or on SEM photographs. Often, the pore size distribution of the pores of the at least one porous inner or outer layer or of the whole food casing or of both shows a pore size distribution that has two, three or four peaks. The average diameter d50 of the pore diameter distribution in the porous inner or outer layers may preferably be in the range from 0.1 to 500 μm, more preferred in the range from 0.5 to 400 μm or from 1 to 300 μm, most preferred in the range from 1.5 to 200 μm or from 2 to 150 μm. In many embodiments, the d95 data of the pore diameter distribution in the porous inner or outer layers may preferably be in the range from 10 to 3500 μm, more preferred in the range from 20 to 3000 μm or from 30 to 2400 μm, most preferred in the range from 40 to 1800 μm or from 50 to 1400 μm or from 60 to 1000 μm. The porosity of the different layers of the food casing according to the present invention may be measured on cross-sections of casings or films prepared with a cryo-microtome. The measurements of the (total) porosity may be performed under a scanning electron microscope (SEM) or on SEM photographs, e.g. with the aid of measurement lines counting the number of points or measurement lengths for the porosity in relation to the total number of points respectively length of the total measurement length. According to the present invention the porosity is the sum of the interconnected porosity (pore channels etc.) and of the closed porosity (closed pores). The ratio of the interconnected porosity to the closed porosity may vary in broad ranges. Depending on the pore size the interconnected porosity may be measured according to the mercury pressure penetration method with the aid of adequate high pressure equipment. However, in cases, which involve larger pore sizes, the absorption capacity of the porosity formed cannot be adequately determined by the evaluation of cross-sections or the mercury pressure penetration method. In these cases the porosity can be easier determined by measuring the total wall thickness of the at least one inner or outer porous layer using a thickness measurement gauge and determining the free volume via subtraction of the non-porous volume of said at least one layer which was extruded. For the non-porous volume the layer thicknesses of barrier layers in the final casings are evaluated by light microscopy. Knowing the extruded volume for every individual layer via the rotations per minute of its melt pump, the layer thickness of the inner or outer layer when non-porous is calculated by dividing the thickness of all barrier layers by their extruded volume per minute and multiplying by the extruded volume of the inner or outer layer. The porosity is calculated using the ratio of the thickness of the at least one porous inner or outer layer when non-porous to the layer thickness of the porous inner or outer layer, resulting from the difference between mean wall thickness and the sum of all thicknesses of the barrier layers. The layer thickness of the different layers may also be measured under a light microscope (LM) or on LM photographs, especially in a cross-section cut vertically or nearly vertically through the casing. The layer thickness of a single porous inner or outer layer may preferably be in the range from 1 to 300 μm, more preferred in the range from 5 to 200 μm or from 10 to 160 μm, most preferred in the range from 30 to 140 μm or from 56 to 130 μm. A single porous inner layer or a group of porous inner layers is preferably suitable or able to take up the at least one transferable functional additive such as liquid smoke within a contact time e.g. of up to 15 to 25 s at 20° C. and at atmospheric pressure when assuming a total layer thickness of the at least one porous inner layer as being in the range from 30 to 150 μm, in an amount of preferably in the range from 1 to 100 g/m2, more preferably in the range from 5 to 90 g/m2 or from 10 to 80 g/m2, most preferred in the range from 15 to 70 g/m2 especially of liquid smoke of the product “Hickory Teepak Alkalized Smoke” of Kerry Ingredients & Flavours. With this liquid smoke, absorption capacities of more than 0.3 g/m2 per μm porous inner layer thickness, more preferably more than 0.4 g/m2 per μm porous inner layer thickness, most preferably more than 0.5 g/m2 per μm porous inner layer thickness. In many embodiments, the surfaces of the hollow spaces and channels of the interconnected porosity of at least one porous inner layer are able to absorb/adsorb a transferable functional additive, to store it and, subsequently, to release and to transfer it. In many cases the functional additive is stored in the at least one porous inner layer then comes in direct contact to the food when food is encased in the casing and then the at least one functional additive is transferred to the food. The compositions and other properties of more than one layer of the porous inner layers within one casing may differ between an innermost porous layer and one or more other porous layer(s), although they may be partially or totally identical if desired. If there is present at least one additional porous inner layer adjacent to the innermost porous layer contacting the foodstuff, the at least one additional porous inner layer can increase the amount of functional food additive, which can be absorbed/adsorbed by the casing, and/or it can improve the adhesion of the adjacent layers, which may show less adhesion if the additional porous inner layer were not present. A typical example is a porous inner layer based on modified polyolefin as an adhesive layer between a layer based on polyamide and a layer based on polyolefin. The Layer Having a Barrier Effect for Water Vapor Usually the at least one layer having a barrier effect for water vapor is predominantly made of a thermoplastic polymer. A layer having a good barrier effect for water vapor shows a water vapor transmission rate of 0.01 to 500 g/m2 d, preferably 0.1 to 100 g/m2 d, more preferably 1 to 20 g/m2 d, even more preferably 1 to 4 g/m2 d or less at 23° C. and 85% relative humidity according to ASTM E398-03. If the food casing according to the invention shows a water vapor transmission of 20 g/m2 d or less, then the encased foodstuff does not easily loose its moisture and the encased meat products stay fresh for extended time periods. Said thermoplastic polymers to be used for the at least one layer having a barrier effect for water vapor are typically the same thermoplastic polymers as those mentioned above with respect to the at least one porous inner or outer layer. In a preferred embodiment said thermoplastic polymers to be used for the at least one layer having a barrier effect for water vapor are selected from the group consisting of polyolefins, copolymers comprising ethylene and/or propylene and/or linear alpha-olefins with 3 to 8 C-atoms, copolymers to be used for an optional adhesive layer as described below, polyamides (homo-, co- or ter-polyamides), thermoplasts having an oxygen barrier effect and (co)polyesters. Mixtures of these thermoplastic polymers can be used as well. Where necessary, compatibilizers known to a person skilled in the art can be added to the thermoplastic polymer. Compatibilizers can for example be selected from the group consisting of copolymers of ethylene and vinyl acetate (EVA), ethylene and acrylic acid (EAA), ethylene and methacrylic acid (EMAA), ethylene and methyl acrylate (EMA), ionomers and/or anhydride modified polyolefins. The water vapor barrier properties may be provided by using at least one polyolefin layer. These layers typically predominantly comprise organic polymers based on polyethylene, polypropylene, polybutylene, copolymers containing units of ethylene, of propylene, of α-olefin preferably with 4 to 8 carbon atoms, of dienes and/or any combination of these units or any combination of
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