Perhaps the most simple solution to limit or delay the adverse effect of oxygen in food products is the use of vacuum packaging. This technology is based on the reduction of the oxygen available in the headspace of the package by decreasing the headspace volume and/ or total pressure. Although some authors include vacuum packaging among the MAP technologies, the headspace composition in the package is identical to that of the external environment.
In vacuum packaging, the food is introduced in a flexible plastic pouch, which is sealed in a vacuum thermosealing machine. The pouch is loaded in the vacuum chamber where air is evacuated, then the open end of the pouch is sealed, and finally, the chamber is vented to atmosphere. The external pressure drives to a collapse of the package onto the product (see diagram in Fig. 1) (7).
The materials used in the design of these pouches should provide a high barrier to gases in order to maintain vacuum during shelf life. Besides this obvious requirement, the materials must be flexible enough to take the shape of the contained product, and present good puncture resistance. Among plastic materials, and suitable for vacuum technology meeting these requirements, is the polyamide family (PA), especially polyamide 6. Unfortunately, these polymers cannot produce a gas-tight seal by application of heat and pressure. Therefore, PA is commonly laminated with polyolefins (low density polyethylene [LDPE], linear low density polyethylene [LLDPE], etc.). In PA/PE structures, the PA provides a barrier to gases and mechanical strength and the PE provides sealing capacity and improved water barrier.
The use of vacuum packaging also requires good resistance of the food product to package collapse and to the external pressure. To avoid the adhesion of the slices, pieces of plastic film are used as slice separators. When the product cannot withstand this pressure, a rigid thermoformed tray can be used as container. Some of the structures used in the manufacture or these trays are polystyrene/ethylene-vinyl alcohol copolymers/PE (PS/ EVOH/PE), polypropylene PP/EVOH/PE, PA/EVOH/PE, or PET/EVOH/PE (8). These trays must be sealed with a film structure with high enough tensile strength to avoid packaging collapse. Bioriented films containing PET or PA are commonly used for this purpose.
Depending on the required shelf life and storage temperature, the level of barrier needed varies a great deal. Products with extended shelf life stored at room temperature require materials with superior oxygen barrier. Some designs include ethylene-vinyl alcohol copolymers, EVOH, such as PA/EVOH/PA/PE, films coated with polyvinyledene chloride such as PA/PVdC//PE, or coated with metal oxides such as PA/SiOx//PE, PET/AlOx//PE, or PET/SiOx//PE (8). Short shelf life and refrigerated temperatures permit a reduction of barrier characteristics and even single-layer HDPE may be an adequate solution.
Modified atmosphere packaging (MAP) is a technology in which headspace air is replaced by a gas mixture that improves and extends the preservation of the food product. There are two basic procedures: application of vacuum and venting with the gas mixture, or flushing the container with the modified atmosphere. The first procedure results in a very efficient replacement of air. The final headspace composition is practically identical to the gas mixture; the gas consumption is low but the packaging process time is longer. The flushing procedure always leaves a residual presence of air and increases the consumption of gas, although the packaging speed is much faster.
Modified atmosphere packaging is applied to many food products, red and white meats, fish, fresh produce, bakery products, and so on. The gas mixture composition varies with the product. For fermented and cured meat products, it is critical to reduce the presence of oxygen. Most solutions are based on the replacement of atmospheric air by carbon dioxide (20-30% carbon dioxide and 80-70% nitrogen). These atmospheres reduce fat oxidation and inhibit the growth of microorganisms (high levels of carbon dioxide have bacteriostatic and fungistatic effects) (9). Besides maintaining the quality of the product, MAP does not lead to package collapse; consequently, there is no adhesion between slices, and film separators are no longer needed.
The type of container used in MAP ranges from flexible pouches to semirigid or rigid trays. Flexible pouches are the simplest low-cost packaging technology and can be used in fast flow-pack machine with gas replacement by the flushing procedure. However, adequate information must be provided to the consumer. In many countries, consumers are used to buying fermented and cured meats in vacuum packaging. The presence of MAP pouches on shelves is interpreted by consumers as pouches that have lost the vacuum and thus are rejected. Also, the final presentation of the product differs among different pouches, becoming unappealing to the consumer. Therefore, the application of MAP for meat products is commonly carried out in semirigid and rigid trays and the gas replacement is obtained by the vacuum procedure. To better maintain the integrity of the package, total pressure inside the package is maintained slightly below 1 atm.
With respect to packaging material requirements, the properties of the package are similar to those described in the vacuum technology section. Therefore, packaging solutions are similar to those presented there.
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