There are relatively few different types of food packaging materials. However, there are many different variations within some material types, and many combinations of materials are utilized. The materials used to package meat products include fiber-based (paper, paperboard), glass, and metal. In addition, nearly all poultry packages have plastics as either coatings, linings, overwraps, or bags. The most common plastic materials, their use and properties are summarized in Table 6.1.
Paper, paperboard, and fiberboard
Paper, paperboard, and fiberboard differ in their relative thickness, paper being the thinnest, paperboard thicker, paper sheet more rigid, and fiberboard made by combining
layers of paper. The material used for secondary shipping cartons of poultry meat is most often corrugated paperboard, named because of the wavy inner layer of paperboard that adds strength. Although this material is commonly referred to as “cardboard,” “corrugated paperboard” is the term used in the packaging industry. These secondary paperboard boxes are sometimes produced from wood pulp and reprocessed paper, which is bleached and coated or impregnated with waxes, resins, lacquers, or plastics. The added layer improves the package’s resistance to high humidity and improves wet strength, grease resistance, appearance, and barrier properties. Acid treatment of paper pulp can result in glassine paper with high oil and water resistance. The acid modifies the cellulose, giving rise to long wood pulp fibers that also add strength to the paper.
Metals
Metals used for canned poultry meat include steel and aluminum. The steel can has greater strength and resistance to denting, while the aluminum can is lightweight and resistant to atmospheric corrosion. The steel can was at one time coated with tin to prevent corrosion at the food contact surface, however, this layer is now a steel alloy such as a chromium alloy, which is much cheaper than tin. The metal can is also coated with an additional organic layer on both the inside and outside can surfaces. This further protects the can from corrosion by the food constituents and also protects the food from contamination by the metal, particularly from metal-catalyzed degradative reactions. Phenolic compounds are used in this organic layer for meat spreads, while modified epons are used for other meat- containing products. Aluminum foil can be used in flexible pouches and is often combined with plastics and paper in layers. Foil offers a complete barrier to light, oxygen, and water vapor.
Plastics (polymers)
Plastics (polymers) comprise by far the most common packaging material for poultry meat products due to their versatility, cost, and convenience.
Polyethylene (PE)
MCH2MCH2MCH2MCH2MCH2M
Table 6.1 Plastics Used for Packaging Meat Products
Polymer type Use Features
Ionomer Heat-seal layer Resists seal contamination
Nylon (uncoated) Films, thermoformed trays Also used as bone guards Nylon (PVdC1coated) Films, thermoformed trays
PETE1
(uncoated) Films, trays Good clarity
PETE1(PVdC coated) Films
LDPE1
Bags, wraps Low cost, low gas barrier
LLDPE1 Heat-seal layer Good clarity
EVA1
-LDPE Seal layer, films, wraps Heat shrinkable
copolymer PP1
(non-oriented) Semi-rigid containers
PVC1 Fresh meat wrap Gas transmission rate depends
on plasticization
PVdC Barrier layer Barrier less affected by moisture
1
PVdC, polyvinyldienechloride; PETE, polyethylene terephthalate (polyester); LDPE, low density polyethylene; EVA, ethylene vinyl acetate; LLDPE, linear low density polyethylene; PP, polypropylene; PVC, polyvinyl chloride.
The molecular structure of PE is a (CH2)nwith short side ethylene chains located along the main ethylene chain, which prevents close stacking and results in a less dense structure. There are three major types of PE which differ in their structure, properties, and manufac- turing processes. These three types are high density polyethylene (HDPE), low density polyethylene (LDPE), and linear low density polyethylene (LLDPE). LDPE and LLDPE have different molecular structures but have similar densities (0.910 to 0.925 g/cm3). LDPE and HDPE differ in the length of side chains and thus, also differ in the overall density of the film. The HDPE is more dense, less clear, stronger, and stiffer than LDPE. The LLDPE is produced under higher pressure and results in a film with similar density to LDPE but with the strength and toughness of HDPE. HDPE also forms a good seal at relatively lower temperatures and has better grease and heat resistance than LDPE. LLDPE is stiffer and has a higher range of heat sealability than LDPE and is being used as a laminate layer as well as in bags and stretch wraps.
Polypropylene (PP)
CH3 CH3 CH3
| | |
MCHMCH2MCHMCH2MCHMCH2M
The structure of PP is a carbon chain with every other side group being a methyl (CH3) instead of a hydrogen as with PE. This structure results in a harder and more resilient poly- mer than HDPE with a permeability to water vapor and gases between those of LDPE and HDPE. The structure of PP can be varied several ways, including oriented or non-oriented, and can be extruded and coated to become heat sealable and change other film properties. The main application for meat packaging is in cook-in products, due to PP’s high heat tole- rance and impermeability to moisture during water bath or steam cooking.
Ionomers (Surlyn) CH3 | M(MCH2MCH2)xCH2MC | CM0M(Na or Zn) 0
Ionomers are polymers that have been copolymerized with an acid. Some part of the acid remains in the film structure in the form of an ammonium salt or metal, usually zinc or sodium. The incorporation of these ions increases the lipophilic nature of the polymer. The films are flexible, tough, and transparent, and are excellent heat-sealing agents. In meat packaging, ionomers are used as the food contact and heat sealing surface in laminated materials. They have a wide heat sealing range, possess good grease resistance, and will adhere well to most other packaging material, including aluminum foil.
Polyvinyl chloride (PVC)
M(MCH2MClCHMCH2M)nM
This polymer is similar in structure to PE except for having a chloride substituted for a hydrogen at alternating ethylenes. PVC is difficult to process since it begins to break down
at about 80°C. PVC is ideal for stretch and shrink retail packages where high oxygen and water vapor permeability (compared to PVdC) is desired and only limited shelf life is required. It is often used as in-store packaging for deli meat, fresh meats, and cured meat products.
Polyvinylidene chloride (PVdC)
M(MCH2MCCl2M)nM
Saran is a trade name for PVdC, and has an additional chloride atom included in the ethy- lene molecule compared to the PVC structure. This film is also clear and strong, with low permeability to gas and moisture. It is used as a layer in multilayer material for pouches, bags, and thermoformed packages for meat, where it functions as an oxygen and water vapor barrier. PVdC can be heat-sealed, is printable, and can withstand cooking or retor- ting. It is used to package frankfurters, luncheon meats, hams, or wherever modified atmosphere packaging is preferred.
Ethylene vinyl alcohol (EVOH)
( . . . CH2MCH2MCH2MCHMCH2MCH2MCH2MCHM)nM
| |
OH OH
This film is an excellent oxygen barrier, however, it is hydrophilic. Thus, its oxygen per- meability will fluctuate with high humidity. The hydroxyl groups in the polymer backbone make EVOH water soluble and disrupted by high humidity. To improve moisture resis- tance, EVOH is placed between layers of PP, PE, and/or PETE.
Polystyrene (PS)
M(MCHMCH2MCHM)nM
| |
| O | | O |
The PS structure has a phenyl (styrene) group substituted for a hydrogen in the PE structure. PS is clear, hard, brittle, and a low strength material. It is used as disposable con- tainers as well as packaging films. PS can be foamed to form expanded polystyrene (EPS) (Styrofoam), which is used as the tray in tray-packed poultry meat. Both the clear and foamed thermoformed trays have high oxygen permeability. High impact polystyrene (HIPS) has good tensile strength and stiffness. Styrene is one of the few materials with the thermal melt strength required to form trays.
Polyamides (nylons)
O O
H H
H(MNM(CH2)nMNMCM(CH2)nMC)nOH
Polyamides (nylons) include polymers formed by condensation of certain amino acids, and this is, therefore, the only food “plastic” containing nitrogen. The nylons are
designated with paired numbers, the first number indicating the number of carbon atoms in the amine portion and the second indicating the number of carbon atoms in the car- boxylic acid section. They have relatively high melting points and low gas permeability, but they will absorb moisture and lose strength when exposed to moisture. Nylons are used in cook-in-the-film meat applications, sometimes in combination with Surlyn (an ionomer). Polyesters O O M(MCH2MCH2MOMCM oMCMOM)nM
The most common polyester is polyethylene terephthalate (PETE), used in carbonated beverage containers. It has excellent strength, clarity, and heat stability and is used for vacuum packaging and cook-in applications for meat. PETE is strong, clear, and has very low moisture and gas permeability. PETE is also used in sterilizable pouches and boil-in- bag applications. Polycarbonates (PC) O CH3 O | C1MCMC1MOMOMCMOMOMCMOH | CH3
The PCs contain polyesters of carbonic acid. They are stiff, transparent, tough, and hard. PCs have high gas permeability and absorb moisture which causes them to lose mechani- cal properties. Despite relatively high cost, their inertness to food has promoted the use of PCs in plates for oven-treated dinners.
Cellophane
Cellophane is regenerated cellulose film made from trees and manufactured from sheets of wood pulp. The fibrous wood pulp is regenerated into a nonfibrous form, and with the addition of plasticizers obtains the needed degree of flexibility. Cellophane is a good gas and grease barrier but will break down in the presence of moisture, so it is often coated with a hydrophobic layer.
The physical properties of the polymer films are summarized in Table 6.2.