The ambient air is flushed out of the package and replaced by a different gas or a custom protective gas mixture. Most MAP gas mixtures include less oxygen, but much more nitrogen and/or carbon dioxide compared to ambient air. This is done to reduce bacterial reproduction, for example in cheese and other dairy products, or reduce the respiration rate of fruits and vegetables.
In contrast, certain other products such as beef can be packaged in a high O2-MAP procedure instead, which involves more oxygen compared to standard air. Oxygen is added to prevent discoloration.
There are 2 general types of modified atmosphere packaging: 1 – active and 2 – passive. We will introduce them in more detail and provide examples later. But first, we will take a closer look at the gases used in MAP packaging and their roles.
Oxygen
Oxygen is a highly reactive chemical. It is responsible for various chemical reactions and its presence accelerates microbe growth. In most cases, oxygen will facilitate food spoilage, so it should be excluded from MAP packaging. However, there are a few exceptions where adding oxygen into packages can have positive effects on the product’s shelf life.
Although individual cases can differ, one statement is generally true: Around perishable products, the reactivity of oxygen must be carefully managed.
Reducing oxygen in packages slows down lipid oxidation, browning and microbe growth, thus increases the shelf-life of most perishable products. On the other hand, increasing oxygen in packages helps red meat retain its red color and certain vegetables such as leafy greens retain their green color, texture and quality.
Carbon monoxide
Some MAP packages for red meat contain a low concentration of carbon monoxide (e.g. <0.4%). This gas reacts with myoglobin, a protein responsible for the red color of meat. Their reaction forms carboxymyoglobin, a bright red pigment which is more stable and even more visually appealing than myoglobin. The presence of CO in red meat packaging also inhibits lipid oxidation, which increases shelf-life and decreases flavor deterioration.
The use of carbon monoxide is controversial because the substance is toxic to humans. However, the concentration of CO in MAP packaging is very low, which also minimizes risk of exposure (this argument applies to consumers, not necessarily to factory workers in the red meat industry). Nonetheless, it is banned in food packaging in the EU, Japan and some other countries. In the United States, the use of CO is permitted within certain thresholds, but its presence must be clearly labelled on packaging.
Materials used in modified
atmosphere packaging
Polyethylene (PE), the most common plastic used in packaging applications, exists in three main variants:
All types of PE have low gas barrier but excellent water barrier properties. Because MAP packaging is an application with high gas barrier requirements, PE is typically used in combination with another material. Certain types of modified PE are sometimes added for enhanced heat seal layer properties. This includes composites with aluminum.
Polyamide (PA), also known as nylon, has high tensile strength, puncture and abrasion resistance, as well as gas barrier properties. It is used in vacuum bags for fresh meat, among other things. To reach the desired heat seal layer properties, it is usually laminated with PE.
Polyethylene terephthalate (PET) is the most common polyester used in food packaging. Flexible PET films are used as bags or top films to cover trays. They feature excellent gas and moisture barrier properties, high rigidity, good optical properties (high transparency) and temperature resistance. Due to its higher glass transition temperature, the opaque Crystallized PET (CPET) is used in microwavable packaging for ready-made meals.
Polypropylene (PP) has good moisture but bad gas barrier properties. It can be used in flexible, rigid and semi-rigid packaging. PP is microwavable if it contains low-fat foods. However, in the presence of fatty foods, PP packaging would get too hot if microwaved.
Expanded polystyrene (EPS) has poor gas barrier properties. However, if laminated with EVOH, it can be used in preformed bottom trays for MAP packaging, where it functions as a structural layer.
Polyvinyl chloride (PVC) is an oil and fat resistant MAP packaging material with excellent processing characteristics. It is commonly used in combination with PE to thermoform bottom trays with the required heat seal layer properties. However, certain organic solvents can soften PVC and inhibit its gas barrier properties, so the packages have to be stored and handled appropriately to avoid contamination.
Polyvinylidene chloride (PVDC) is a copolymer of vinyl chloride and vinylidene chloride. It has outstanding gas, moisture and aroma barrier properties and is thus considered one of the best barrier materials for food packaging. It is resistant to oil, fats, organic solvents and high temperature. Its characteristics remain very stable, even in the presence of moisture. However, there is no currently established recycling method for PVDC and the material intensifies the yellowish tint in food packaging.
Innovative MAP packaging materials from
Kuraray
EVAL™ EVOH
Ethylene vinyl alcohol (EVOH) is a copolymer of polyvinyl alcohol (PVOH) and ethylene. While PVOH forms an excellent gas barrier, it is vulnerable to moisture. This is why the more moisture and solvent-resistant EVOH is the preferred choice as a MAP packaging material. EVOH also features a high mechanical and thermal stability. EVAL™ EVOH is the brand name of the ethylene vinyl alcohol copolymers (EVOH) produced by Kuraray. This material is well-suited for various paper and plastic packaging applications.
Example of a conceptual structure for such packaging: Polypropylene / EVOH / polypropylene. The production process comprises cast film coextrusion at very low temperatures in combination with the water quenching or chill roll quenching method. PP is a better water barrier than polyethylene (PE). Moreover, the multilayer structure with PP has better cold impact resistance than MAP trays with a conventional polyethylene terephthalate (PET) monolayer. A one-millimeter-thin EVAL™ EVOH film, laminated on another water-barrier forming polymer such as PP, can create a functional barrier that is equivalent to a ten-meter-thick polyethylene wall. Such a multilayer solution can form super-clear to white recyclable trays and replace PVDC in MAP packaging applications. Likewise, inserting a barrier layer of innovative EVAL™ SC resin between two layers of PE produces a shrink bag for fresh red meat that is both super transparent and recyclable.
PLANTIC™
PLANTIC™ is a high-performance film manufactured by Kuraray using more than 80% renewable raw materials. It is produced from thermoplastic starch and is biodegradable and compostable (home and industrial composting). Due to its high gas barrier properties, this biopolymer from Kuraray can be used in packaging that preserves aromas and effectively keeps out oxygen. PLANTIC™ is therefore ideal for both MAP packaging for food with a short shelf life and packaging solutions for dry goods such as coffee, tea and animal feed. PLANTIC™ is usually covered in a layer of polyethylene terephthalate (PET) and polyethylene (PE) as a moisture barrier and heat-sealable layer.
Example of a conceptual structure for such packaging: Polyethylene terephthalate / PLANTIC™ / polyethylene terephthalate or polyethylene. Dry lamination and extrusion coating are used to produce the tray. In near- infrared (NIR), PLANTIC™ barrier films have a unique fingerprint, so the trays can be assigned to any sorting stream based on local requirements. PLANTIC™ is produced as monolayer film for lamination or extrusion coating. Once washed away in the recycling process, PLANTIC™ biodegrades in the waste water treatment plant and does not cause contamination with microplastics.
Are you interested in our innovative materials for MAP packaging? Do not hesitate, contact us and ask for EVAL™ EVOH or PLANTIC™:
