Lesson- 10 Other packaging materials
10.1. Edible films
Edible films and coatings formed from polysaccharides, proteins, lipids, resins, and/or waxes fall within the active packaging definition, since they can enhance the protective function, provide convenience, and minimize package environmental impact. Edible films placed or formed between components of a packaged food control transfer of moisture, oils, etc. over which the package has no control. Edible coatings or edible film pouches (as a primary package) work to complement the protective function of the nonedible (secondary) package. Such coatings and films can act as barriers to the external environment and maintain food integrity, thus reducing the amount of packaging required. Edible film pouches carrying premeasured amounts of ingredients can provide the convenience of placing pouch with ingredients into the food formulation. Edible coatings can also carry antimicrobials that can inhibit microbial growth at both the food-coating interface and the coating outer surface.
A number of food applications of edible films and coatings have been. Several polysaccharide-, sucrose-ester-,lipid- and resin-based edible coating formulations are available commercially to control moisture loss and respiration in fresh fruits and vegetables. Starch, hydroxypropyl methylcellulose (HPMC), zein, gelatin, and shellac coatings are available for confectionery and other food products. Edible collagen casings and wraps for meat and HPMC pouches for dry foods are available commercially. A large number of foods would benefit from development of suitable edible films or coatings.
Food materials can be protected from loss of volatiles or reaction with other food ingredients by being encapsulated in protective edible materials. This can be done by spray drying various flavoring materials emulsified with gelatin, gum Arabic, or other edible materials to form a thin protective coating around each food particle. The coatings of raisins with starches to prevent them from moistening a packaged breakfast cereal and the coating f nuts with monoglyceride derivatives to protect them from oxidative rancidity are additional examples of edible coatings.
Food materials such as amylase starch and the proteins zein and casein when solubilized can be cast to give sheets of edible films on drying. These films may then be used to fabricate small packets to hold other food ingredients. One application of such films has been to package baking ingredients which can then be added directly to the mixing bowl as an intact packet, on addition of water, the edible film dissolves and releases the packed ingredients.
Edible films are also used to coat fresh fruits and vegetables to reduce moisture loss and to provide increased resistance to growth of surface molds. The most common and oldest edible film is wax .A wide range of products such as apples are waxed for appearance and improved keeping quality. Newer edible films are being developed which can keep produce longer.
Packages made of polymer films are not absolute barriers against the transfer of water and O2 through the package, although they may be excellent barriers against microorganisms and dirt. Various flexible materials (papers, plastic films, thin metal foils) differ with respect to water vapor transmission, oxygen permeability, light transmission, burst strength, pinhole and crease hole sensitivity and so on. Multilayers or laminates of these materials that combine the best features of each can be used to produce packaging materials with combined properties such as the strength of paper, heat seal ability of plastics, and barrier properties of aluminum foils. Another new technique for combining different plastics is co extrusion simultaneously forces two or more molten plastics through adjacent flat dies in a manner that ensures laminar flow and produces a multilayer film on cooling. Such structured plastic films may be complete in themselves or be further bonded to papers or metal foils to produce more complex laminates.
10.3. Retortable pouches and Trays
Flexible pouches, semi-rigid/rigid plastic trays and cans, and paperboard-based cartons have been developed as alternatives to heat processing (retorting) in rigid metal cans or glass containers. The pouches, trays, and tubs are always multilayer laminate structures that contain different polymers which provide heat resistance, strength, and toughness (PET), pierce and pinhole resistance (nylon), oxygen barrier (EVOH, nylon or PVDC) and (for the pouches and trays) heat sealability (PP). An aluminum foil layer often serves as the moisture and oxygen barrier in pouches. The retortable paperboard cartons have external and internal PP layers that are impermeable to liquid and allow heat sealing, along with an internal aluminum layer that provides a gas and light barrier.
Retortable pouches can be either preformed or in-line formed using form/fill/seal equipment. Common pouch structures are PET/nylon/foil/PP and PET/nylon/EVOH or PVDC/PP. Retortable trays have a semi-rigid or rigid body and a sealable flexible lid. The trays are generally made from coextruded laminate such as PET/EVOH/PP by thermoforming. Retortable tubs are made from similar multi-layer laminates. An easy-open scored metal lid with pull ring is double seamed onto the tub body.
The advantage of retortable pouches and trays is that they have thinner profile than conventional metal or glass containers. The results are shortened process times, reduced energy consumption, and improved food quality due to more rapid and even heat transfer. In addition, retort pouches, trays, and tubs are convenient because of easy transport (due to shape and light weight) and easy opening. Plastic (with no foil layer) pouches, trays, and tubs are microwaveable. The main disadvantage of retortable pouches, trays, tubs, and cartons is more difficult recycling.
Flexible materials can be combined to withstand even the adverse conditions of retorting encountered with low-acid foods. Such “flexible cans” have becomes standard containers for some applications such as providing foods to soldiers in the fields. The advantages of pouches and trays over cans and jars of equivalent volume include shorter retort times, which can produce higher quality products and save on energy, lighter weight, increased compactness, easier opening and easier disposability. Retortable pouches are constructed of a three-ply laminate consisting of 1) an outer layer of polyester films for high – temperature resistance, strength and printability. 2) A middle layer of aluminum foil for barrier properties, and 3) an inner layer of polypropylene film that provides heat- seal integrity. Retortable trays are constructed from multilayers of polymers, one of which is ethylene- vinyl alcohol to provide an oxygen barrier. These trays are often sealed with a polymer-foil laminate film.
10.4. Cloth materials
Jute and cotton are woven materials which have been used for packaging foods. Sacks made of jute are used, to a limited extent, for fresh fruit and vegetables, grains and dried legumes. However, multiwall paper sacks and plastic sacks have largely replaced them for such products. Cotton bags have been used in the past for flour, sugar, salt and similar products. Again, paper and plastic bags are now mainly used for these foods. Cotton scrims are used to pack fresh meat. However, synthetic materials are increasingly used for this purpose.
10.5. Wooden Containers
Outer wooden containers are used when a high degree of mechanical protection is required during storage and transport. They take the form of crates and cases. Wooden drums and barrels are used for liquid products. The role of crates has largely been replaced by shipping containers. Open cases find limited use for fish, fruits and vegetables, although plastic cases are now widely used. Casks, kegs and barrels are used for storage of wines and spirits. Oak casks are used for high quality wines and spirits. Lower quality wines and spirits are stored in chestnut casks.
10.6. Composite Containers
So called composite containers usually consist of cylindrical bodies made of paperboard or fiberboard with metal or plastic ends. Where good barrier properties are required, coated or laminated board may be used for the body or aluminum foil may be incorporated into it. Small containers, less than 200 mm in diameter, are referred to as tubes or cans and are used for foods such as salt, pepper, spices, custard powders, chocolate beverages and frozen fruit juices. Larger containers, known as fiberboard drums, are used as alternatives to paper or plastic sacks or metal drums for products such as milk powder, emulsifying agents and cooking fats.
10.7. Regenerated Cellulose
Regenerated cellulose (cellophane) differs from the polymer films in that it is made from wood pulp. Good quality, bleached sulphite pulp is treated with sodium hydroxide and carbon disulphide to produce sodium cellulose xanthate. This is dispersed in sodium hydroxide to produce viscose. The viscose is passed through an acid-salt bath which salts out the viscose and neutralizes the alkali. It provides general protect against dust and dirt, some mechanical protection and is greaseproof. When dry it is a good barrier to gases, but becomes highly permeable when wet. Plain cellulose is little used in food packaging. Plain regenerated cellulose is mainly used coated with various materials which improve its functional properties. The most common coating material is referred to as ‘nitrocellulose’ but is actually a mixture of nitrocellulose, waxes, resins, plasticizers and some other agents.
10.8. Cellulose Acetate
Cellulose acetate is made from waste cotton fibers which are acetylated and partially hydrolyzed. The film is made by casting from a solvent or extrusion. It is clear, transparent and has a sparkling appearance. It is highly permeable to water vapor, gases and volatiles. It is not much used in food packaging except as window material in cartons. It can be thermoformed into semirigid containers or as blister packaging