Packaging of foods is a major outlet for plastics in packaging with an estimated share of 55% in case of flexible packaging materials such as Coextruded Multilayer Films and Laminates. With an ever growing requirement of ready-to-use convenience foods, packaging has to play an important role to meet the performance requirements of a products with respect to compatibility, barrier properties, sterilization methods, shelf-life and last but perhaps the most important, the aesthetic (sales) appeal. Some trends are wrapped-up in the following paras :


RETORT PACKAGING :

Retortable flexible packaging materials are defined by the ASTM as 'those capable of withstanding specified thermal processing in a closed retort at temperature over 100 �C'. Some products are retorted upro 130�C for 40 minutes.
Retortable package must maintain their material integrity as well as their required barrier properties for their designated end-product during products - to package handling, thermal processing and subsequent handling and transport abuse. For shelf-stable foods, the materials used must be retortable and still maintain extended barrier characteristics against such effects as light, moisture, oxygen, microbial penetration, etc. The polymers most commonly used by coextrusion are PET, PC and PP. Low acid, homogeneous and small particle foods are tending towards aseptic packaging, whereas retort packaging is preferred for low acid foods with moderate to large-size particles because of ease of removing oxygen from the head space by gas flushing and also because a crisp texture obtained through freezing is possible only with the retort packaging system.
Convenience and ready-to-serve foods are generally warmed-up in microwave ovens. Few consumers would use conventional ovens, since the concept of fast foods is totally lost if one has to wait for 30-40 min to serve, which is the case if conventional ovens are used. The current demand is for the dual-ovenable retort concept.
Microwavable retort packages, on the hand, offer consumers convenience and time saving. With the availability of microwave ovens indigenously, the demand for such packaging is expected to grow very rapidly. The important properties of a retort container are shelf-life, dimensional stability through the retort and abuse resistance in shipping. Coextrusion offers the greatest potential for retort packages but material selection is critical, especially in areas of structural and physical properties, gas and moisture barrier properties, and adhesive compatibility.

Material Selection :

The selection of a polymer and combination is based on the requirement of barrier properties. They are in the form of flexible pouches of coextruded and/or laminated films of PET, PC, PP with aluminium foil, thermoformed when it comes to trays and containers by injection blow-moulding process. Where a glass look is desired, PC offers excellent optics and low haze for improved products identification and market share. A coextruded structure on PC has four time the relative modulus at 250 �C of coextrusion based on filled PP. In addition, PC improves the tensile strength of a container. Ease and flexibility of processing also come into play here. A material must be able to be blow moulded, or extruded and thermoformed. Both PP and PC can be extruded and blow-moulded. However, PC has an advantage when it comes to thermoforming because it is amorphous and thus has a wider thermoforming window. In most extrusions, the structural layer is printed. Unlike PP, PC needs no special treatment before printing. The barrier properties of the materials within a coextruded package determine the shelf life. For expended shelf-life, moisture and oxygen barrier is needed. EVOH and PVDE offer the lowest oxygen permeation rates for retortable applications. Considering all the factors, PC and EVOH appear to offer the most promise in restorable high oxygen-barrier applications. To sum up, the best structure available for a microwavable retort container is a five-layer coextrusion with PP the food contact.

RETORT POUCH :

The term is used to describe a flexible package into which a good product is placed, sealed and then sterilized at temperatures in excess of 100 �C. Generally the process temperature is between 110 - 140 �C. Most foods with a pH above 4.5 requires this type of process. The finished product is commercially sterile, shelf-stable, and requiring no refrigeration. The term 'retort' itself refers to the pressurized container in which packaged foods are cooked by the food processor. Manufacture Retort pouches are made from laminates such as polyester/aluminium foil/modified high-density polyethylene or polyester / aluminium foil/propylene-ethylene copolymer. Some two-ply laminates give the longer shelf-life, the aluminium foil supplying a complete barrier to oxygen, moisture and light (provided that heat seal integrity is maintained). The latter, of course, depends on the material use for the inner ply of laminate. The outer ply must be tough, withstand abrasion and give good print quality. The adhesive used to laminate the various plies is important because of the dangers of delamination during storage and transit.
Seal integrity is easier to achieve using pre-made pouches where three seams can virtually be guaranteed. However, reel stock is also used and pouches are formed, filled and sealed on the appropriate equipment. When pre-made pouches are used, the filling equipment consists of rigid, open mouth plastic containers mounted on a re-circulating conveyor system. At the appropriate point in the system, the plastic container empties its contents into the pouch then returns for washing and refilling. The foregoing description applies to solid products, such as pieces of fish, hamburgers, etc. Liquids, such as sauces for fish, are filled from an integral hopper. After filling, the machine removes head space air and the pouch is sealed, using a double heat seal to minimize the risk of faulty seals. The seal is cooled and the pouch passes on to a rotating reservoir for loading on to the retort tray. A residual air level of below 2% pouch capacity is normally achieved. Retort or autoclave systems are based either on water or steam and air. The latter system is more expensive but employs less energy in the long term. Pouches, unlike cans, should not be agitated during the processing period because the burst strength of the pouch decreases as the pouch temperature is increased.
After removal from the retort, the pouches are dried and overwrapped with a polyester/polyethylene laminate or placed into pre-erected cartons. Although the retort pouch is tough and resists a great deal of abuse, it is normally the practice to overwrap or place it in an outer carton as a moisture-barrier layer. A trim layer next tot he PP layer improves the economics. An adhesive layer bonds the scrap tot he oxygen-barrier EVOH layer. Lastly, second ties layer bonds the PC outside structural layer to the EVOH layer.