The function of packaging is to surround or wrap meat products with suitable protective material (Fig. 335). Packaging materials were in the old days simple natural materials, e.g. leaves, but nowadays exclusively manufactured materials such as paper or synthetic films.
Purpose of packaging
The basic purpose of packaging is to protect meat and meat products from undesirable impacts on quality including microbiological and physio-chemical alterations. Packaging protects foodstuffs during processing, storage and distribution from:
Adequate packaging can prevent the above listed secondary contamination of meat and meat products. But the further growth of microorganisms, which are already present in meat and meat products, cannot be interrupted through packaging only. To halt or reduce microbial growth, packaging has to be combined with other treatments, such as refrigeration, which will slow down or stop the further growth of microorganisms, or with heating/sterilization, which will reduce or completely eliminate contaminating microorganisms.
Fig. 335: Meat products in different packages of synthetic materials
The packaging procedure results in an inner package, where the packaging material is in direct contact with the product. In some cases it is combined with an outer package often a cardboard boxe, or other materials.
There are various synthetic packaging films available for the inner packaging, e.g. transparent or opaque, flexible or semi-rigid, gas proof or permeable to certain gases. These materials are selected to serve specific purposes, such as protection from unwanted impacts or attractive presentation.
Requirements for packaging materials
A range of synthetic materials suitable for meat packaging are available mainly in the form of plastic films1 or foils.
Packaging films must be/have:
Barrier against gases
Good barrier properties against oxygen and evaporation are the most important features in order to ensure:
a) Exclusion of oxygen
Air contains about 20 percent oxygen. Oxygen negatively affects unpackaged meat and meat products during prolonged storage periods. It changes the red meat colour to grey or green and causes oxidation and rancidity of fats resulting in an undesirable off-flavour.
1) Oil is the raw material for plastics, 4% of the annual production of crude oil is used for making plastics.
The oxygen permeability of films used for the packaging of meat products varies. The lower the oxygen permeability the more efficient the protection of product quality. The best protection will be achieved using oxygen-proof packaging films together with vacuum packaging of the product (see page 271). This ensures that practically no oxygen remains in the package and no oxygen will penetrate from the air into the packaged product.
While oxygen is generally undesirable in packages of meat and meat products, there is one exception where oxygen-permeable foils are desirable, namely for fresh ready-to-sell meat portions in self-service outlets. In this specific case the utilization of oxygen-permeable foils produces a desirable bright red meat colour (see page 269 “Fresh Meat Packaging” and page 276 “Modified Atmosphere Packaging”)
b) Prevention of evaporation of product moisture
Fresh meat or fresh sausages, cooked ham, etc. have a relatively high moisture content and will suffer considerable weight and quality losses by evaporation and drying during storage if such products remain unpacked. The packaging material must therefore be sufficiently water-vapour-proof.
Barrier against light
The prolonged exposure of meat and meat products to daylight or artificial light accelerates unattractive colour changes, oxidation and rancidity because light provides the energy for these processes. Transparent packaging films normally used for meat products allow attractive product presentation as the packaged product is visible. However, such films provide no protection against light impact. Normally products in transparent packaging films are sufficiently protected when stored in the dark or moderate llight conditions. For light sensitive products or products exposed to strong light, coloured or opaque films (Fig. 336) should be used. Films laminated with aluminium foil (Fig 337) are very effective.
Fig. 336: Opaque and printed films as light barrier
Fig: 337: Aluminium foil bag
Some packaging materials are required to have good thermoplastic properties. They are heat sealable, which means that two of these films, put closely in contact to each other under slight pressure and with simultaneous high temperature application, will melt or seal together along the heated area, resulting in hermetically closed plastic pouches or bags.
Types of packaging films
Practically all films used for meat packaging derive from synthetic “plastic” materials.
Cellulose, which is not a synthetic but a natural material derived from wood, was formerly widely used in the form of transparent films. It is now no longer of great importance in meat packaging although still used for some specific purposes. However, cellulose is still important for the manufacture of certain kinds of artificial sausage casings (see page 261).
The most common synthetic materials used for meat packaging are:
|Polyethylene (PE)||(oxygen + , water vapour -)|
|Polypropylene (PP)||(oxygen + , water vapour -)|
|Polyvinylchloride (PVC) (soft)||(oxygen + , water vapour -)|
|Polyester (PET)||(oxygen ± , water vapour -)|
|Polyamide (PA)||(oxygen - , water vapour +)|
|+ = relatively permeable|
|- = relatively impermeable|
Ethylenvinyl alcohol (EVOH)
|used as barrier plastics|
(see Fig. 342B)
Foils made from the above synthetic materials are selected based on their different properties related to oxygen and water vapour.
For the various purposes in the meat industry packaging films can be divided into
Fig. 338: Single-layer film
One common use of single-layer films (Fig. 338, 339) is the wrapping of meat pieces, processed meat products, bone-in or boneless meat cuts or even entire carcasses. These films are usually self-adhesive, i.e. they cling together -“cling film”- in the overlapping areas. Hence they provide good protection from external contamination and to some extend from evaporation, but no protection from oxygen, as they are not hermetically closed or sealed packages. Foils with good self-adhesive properties are PE, PA, PVC and PP.
Fig. 339: Wrapping with singlelayer film
Another important utilization for single-layer films is in freezer storage. For meat blocks, meat cuts or smaller portions of meat or meat products, single-layer films are stretched tightly around the meat surface before freezing. The tight film prevents evaporation losses, which occur during freezer storage of unpacked products. The film is in tight contact with the products surface, in order to avoid evaporation, ice formation and freezer burn at non contact spots (Fig. 340). Suitable cold resistant films for freezer storage are PA or PE.
Fig. 340: Freezer burn developed during freezer storage because of insufficient protection against evaporation. Meat surface with greyish colour and dry (left)
One specific utilization for single-layer films is the wrapping of chilled meat portions for self-service outlets (supermarkets, etc.) (Fig. 341). Those meat pieces (beef, pork or chicken) are placed in a hygienic cellulose or plastic tray and tightly wrapped with single-layer plastic film. The ends of the foil are overlapped at the bottom side of the tray, where they firmly cling together. Films to be used should have low water vapour permeability to avoid the drying out of the meat during storage. But for making attractive to customers, such meat needs to retain an attractive bright red meat surface colour (oximyoglobin) (see page 7) especially in the case of fresh portioned beef. For this reason the plastic foils to be used shall have a high oxygen permeability so that the oxygen of the air can react with the myoglobin of the meat and form the bright red oximyoglobin. Oximyoglobin is not a chemical compound but a loose aggregation of oxygen to the red meat pigment myoglobin, which keeps meat bright red for a number of hours. Suitable single-layer films for fresh meat packaging are PE or soft PVC. Formerly cellulose films were also used, which have the same permeability pattern but are less self-adhesive and the overlapping ends do not cling together very well.
Fig. 341: Fresh meat being packed in self-service tray, covering foil is cut to right size for overlapping at the bottom
Practically all the other films used for meat packaging are designed as strong oxygen and water-vapour barriers. In order to fully achieve these requirements, films with good barrier properties for oxygen and water vapour respectively are combined.
Fig. 342: Typical multi-layer films (a = two layers, b = three layers)
Layer A: Outside layer (mechanically strong, gas barrier to oxygen)
Layer B: Middle layer (barrier to oxygen)
Layer C: Inside layer = sealant layer (capable of being melted and welded under pressure to the sealant layer of the opposite sheet of the bag/pouch, serves also as barrier to water vapour)
A very efficient combination is PA/PE. PA is used as the outside layer for example for films for vacuum bags. PA is relatively oxygen proof but permeable to some extend to water vapour. PE has the opposite properties, it is water vapour proof but permeable to oxygen. The combination of both renders such a multi-layer film very tight against oxygen and water vapour evaporation. Moreover, the PE used as the inside layer has good thermoplastic properties and is therefore well suited for heat sealing. The PA/PE combination is the most simple structure for a multi-layer film (Fig. 342a). The packaging industry has refined the systems by introducing additional layers which serve as strong oxygen barriers (Fig. 342b).
Sealant layers consist mostly of Polyethylene (PE) or Ionomer (I) (Fig. 345, 346, 347). Outside layers may be Polyamide (PA), Polyester (PET) or Polypropylene (PP). Barrier layers for oxygen are made of Polyvinylidenchloride (PVDC) or materials with similar properties (see page 268).
Fig. 343: Sausage slices placed in vacuum bag prior to sealing
Vacuum bags, used for vacuum packaging machines (Fig. 343, 344, 350) are composed of two or more sheets of multi-layer films. By drawing the vacuum and sealing of such bags, the air is excluded from the package and the damaging effects of oxygen such as rancidity or discoloration of the packed products will be significantly slowed down or not develop at all. However, exposure to strong light may cause discoloration even under vacuum.
Fig. 344: Start of evacuation in vacuum packaging machine
|Fig. 345: Evacuated and sealed product||Fig. 346: Good sealing seam|
|Fig. 347: Faulty sealing seam: risk of air penetrating inside the package because of film overlapping||Fig. 348: Simple vacuum|
Processed meat products in slices or as entire pieces are packed in small to medium-size vacuum bags. For larger sized products, bags made of shrinkable films can be used where, after vacuum-packaging, the product in its package of synthetic film is sprayed with or dipped into hot water (80°C). The contact with the hot water causes the shrinkage of the thermoplastic film and results in tight impermeable wrapping of the goods. Shrink films may for example be composed as follows: PET/PA/EVOH/PO
Vacuum packaging for fresh chilled boneless beef cuts is mainly done to promote meat ripening. The beef meat cuts can remain for a number of weeks (maximum 3 months) in the vacuum bag, provided oxygen remains excluded and the storage temperature is kept at -1°C (which is just above the freezing point of meat). During these storage conditions lactic acid bacteria (which do not cause spoilage!) will inhibit the growth of most other bacteria, which results in the prolonged microbial stability. Beef becomes very tender during such extended ripening periods without losing much of its typical flavour (Fig. 349).
Fig. 349: Vacuum packed beef cuts / rump, tenderloin, roast beef, (three layer co-extruded film with EVOH as oxygen barrier, shelf-life of chilled beef up to 3 months at -1°/0° can be achieved)
Fig. 350: Manual vacuum packaging machine – Phases of operation
|a Meat product
b Vacuum bag
c Atmospheric air
d Vacuum packaging machine-body
e Vacuum packaging machine - lid|
f Plastic boards to adjust volume of chamber to size of product
g1,2 Sealing bars
h Air outloe/inlet
Heat treatment or cooking for some meat products can be carried out in the package after vacuum-packing. Temperatures of 60°-80°C or even higher up to sterilization temperatures (above +100°C) can be employed for hams, sausages, etc. In these cases a pasteurization or sterilization effect of the uncooked packaged products is achieved and re-contamination avoided as long as the package is not opened.
For specific products such as entire sausages, semi-automatic vacuum-packaging can be employed. A bottom film is moulded according to the shape of the sausages by using heat and force (by compressed air or mechanical) (Fig. 351). These machines are called thermo-formers. The sausages are loaded (Fig. 351) and a rigid top film is sealed on after evacuating the moulded spaces. Individual product portions are cut apart along their sealing layers (Fig. 352).
|Fig. 351: Semi-automatic vacuum packaging. Loading sausages in moulded bottom film. Top film to be sealed on bottom film after drawing vacuum. Individual portions to be cut apart (see Fig. 352)||Fig. 352: Product vacuum packed in moulded bottom film and rigid top film. (For presentation to customers the bottom film becomes the top of the package and verse visa)|
A packaging method commonly used in larger meat industries is skin packaging. For this method the products are placed in the packaging machine, usually on a rigid film, which serves as the bottom layer of the final package. Another flexible film (top layer, which is heated for increased flexibility) drapes itself from above around the product, resembling a tight “skin” on the product surface avoiding wrinkles and purges. The skin-like coverage of the product takes place in a sealing station in the packaging machine, where the top and bottom film are sealed around the edges. Individual packages are separated by cutting around the bottom seal perimeter (Fig. 353, 354).
Fig. 353: Skin packaging
The latest development in this sector is the “form-shrink” packaging technology. Products e.g. meat cuts, chicken carcasses, entire sausages, smaller portions of meat products, are placed between two shrinkable films, which are moulded without wrinkles around the goods. Sealing seams can be kept extremely small. This technology is very cost-effective in terms of usage of packaging films but requires high-tech equipment and is only of relevance for large-scale industries.
Fig. 354: Skin packaging
A useful technology is Modified Atmosphere Packaging (MAP) of meat and meat products. The packaging materials used are gas-proof multi-layer films composed for example of PE, PA and barrier layers. Rigid films may be used to mould cup or box shaped containers which are filled. A flexible lid foil is then sealed on (Fig. 355). MAP packaging can also be done for ordinary plastic bags/pouches. MAP packages are firstly subjected to a vacuum. A mixtures of gases is the introduced into the air-free space before sealing. The gas mixture usually contains nitrogen (N2) and carbon dioxide (CO2). N2, which is also the major constituent of atmospheric air, is inert, i.e. it does not react with meat product components such as fat or myoglobin. Its function is to replace the atmospheric oxygen (O2) and thus prevents O2 induced negative impacts (see page 266). The other component of the gas mix, CO2, has a protective function, as it inhibits to some extend the growth of bacteria and moulds.
Fig. 355: Modified atmosphere packaging MAP). Lid foil of one package segment opened (right)
The gas mixture commonly used is 20%-30% CO2 and 70%-80% N2. This is applicable for all processed meat products. If fresh meat pieces are to be packed in gas-proof packages instead of wrapping them with oxygen-permeable foil (see Fig. 341), the bright-red fresh meat colour can be achieved by adding oxygen to the gas mix to be injected into the package and replacing the N2 content accordingly. As sufficient oxygen is needed to maintain the bright-red colour, gas mixes for fresh meat are usually composed of 70%-80% O2 and 20-%30% CO2.
Skin- and MAP-packaging are often too sophisticated for the small producers, but may be of increasing interest to medium-size meat plants. There are now small manual and semi-automatic packaging machines available (Fig. 356, 357, 358), which are designed for smaller throughputs. However, the utilization of those machines implies that the necessary types of synthetic films, and in the case of MAP-packaging, also the relevant gases, are available.
Fig. 356: Simple manual machine for sealing plastic trays with flexible lid foil (tray-sealing machine)
Fig. 358: Small equipment for skin packaging and MAP-packaging
Fig. 357: Close view of manual tray sealing machine