The food in modified atmosphere storage can maintain the quality and increase the service life thanks to the decrease in the speed of chemical and biochemical degradation reactions, and the elimination or reduction of the growth of micro-organisms responsible for the deterioration of the food. This is most evident in food products with high perishable nature, in which the modified atmosphere is highly effective (table 1).
The air in its gaseous state is composed of nitrogen (78,08%), oxygen (20,96%) and carbon dioxide (0.03%), along with varying concentrations of water vapour and trace amounts of inert gases.
The majority of foodstuffs degrade quickly with the air due to humidity or dryness, reactions with oxygen or the growth and activity of aerobic microorganisms. These factors can cause changes in texture, color, smell, taste, nutritional value and even toxicity in food, and ultimately the development of a product unacceptable and safe.
On the other hand, there is also the storage in controlled atmosphere (AAC), which is defined as the maintenance of the product in a gaseous atmosphere of fixed composition, through the control and the addition of gases. The main difference between EAM and AAC is that the gaseous composition of the food packaged in EAM is continuously changing due to chemical reactions and microbial activity, or to an exchange of gases between the interior of the container and abroad due to the permeability of the material of the container.
It is important to clarify that food in modified atmosphere packaging can extend life and improve the appearance of a commercial package, but you can do to improve a poor quality product. For this reason, it is important that the food be quality before packaging, to keep the MLE this quality.
The most commonly used in modified atmosphere gases are oxygen, carbon dioxide and nitrogen. You can use a gas or mixture of several in different proportions depending entirely on the food to increase the useful life of the product, at the same time maintaining its organoleptic properties.
CO2 is slightly corrosive in the presence of moisture. Dissolves readily in water to produce carbonic acid (H2CO3), which increases the acidity of the medium. It is also soluble in lipids and other organic compounds. This can cause the loss of the bright red in meat, due to the low pH near the surface of the food which induces changes in proteins (such as its denaturation, causing outliers in the absorption and reflection of light from the surface of the product). Furthermore, the solubility increases with temperature, so that the antimicrobial activity of CO2 is much larger at temperatures below 10 ° C than at temperatures above 15 ° c. This greatly influences in the MLE of food, already to the High solubility of CO2 may lead to rupture of the container by reduction in head space.
This is a highly reactive gas slightly soluble in water that causes several types of reactions of deterioration in the food (oxidation of fats, reactions of pardeamiento), oxidation of pigments, etc..
Most of the bacteria and harmful fungi for food need oxygen for growth, this increases the useful life of foods by lowering the concentration of residual oxygen in the atmosphere inside the container.
Should also be noted that a low concentration of oxygen can cause problems of quality and safety in some foods, such undesirable color changes in the pigment of red meat, essence in fruits and vegetables, growth of pathogenic bacteria, etc.
It is a gas little reagent and slightly soluble in water and other components of the food. Their presence helps to inhibit the growth of aerobic organisms by displacement of the air, but does not affect the anaerobes, it has no effect on the chemical and biochemical properties of food. It is also used to balance the gas pressure inside of the packaging, avoiding that they break when they contain food with plenty of moisture or FAT (meat, for example). The low solubility of nitrogen in food helps to counteract the effects of the solubility of CO2 in water and fat in the interior of the container atmosphere.
There are other developments today, in pursuit of improving the organoleptic characteristics and increase the service life by avoiding the health risks. In the table 2 currently displayed the most investigated gases:
Plastic materials that make up the containers have permeability that presented to the various gases that make up the modified atmosphere as one of its main features. Here is a problem in EAM packaging as an incorrect selection of materials can cause the loss of the atmosphere that we introduce initially.
The selective permeability of the main plastic materials used for the manufacture of food packaging is presented in table 3.
There currently are many types of plastic materials, each of them with different properties. On many occasions, it is difficult for a single material to present all the characteristics of protection, technical and commercial necessary for packaging in modified atmosphere of a particular food. For this reason, they are usually manufactured packaging with multilayer structures. The realization of multi-layer structures has increased applications of plastics for food packaging. These structures allow combining plastic materials with different properties in order to obtain a set for the final packaging. A structure based on a multilayer consists of a material located on the external party have structural properties, a material in the central barrier to gases layer and a third layer material internal to facilitate and allow the selleado with itself or with other additional material (table 4). These materials, normally, are not compatible between them by what they should go together by layers of adhesives (Figure 1). However, these structures can have a variable composition which may vary between two and eleven layers.
In the case of modified atmosphere packaging structures, the use of this type of structures, with the use of a material barrier to gases is necessary to keep the gas composition in the interior of the container during the lifetime of the food.
If the expiration of the food is very short, it is possible with material single sided with acceptable barrier properties such as, for example, the terephthalate (PET) or the polyamide (PA) can be enough. So check his behaviour on the basis of trials of permeability and shelf life of packaged food.
At the level of sort of container, it is convenient to indicate that they are currently available structures multi-layer in all known formats, film, trays, injected containers, bottles, etc. The main processes of fabrication of multilayer structures are the co-extrusión, the coating by extrusion, rolling, and the co-inyección.
For flexible packaging films and sheets multi-layer for the production of trays, include those materials that have been obtained by the combination of two or more layers of simple materials. These multilayer structures can be obtained by different procedures. When the structure is composed only of thermoplastic materials used the technique of co-extrusión or simultaneous extrusion of various polymeric materials usually interspersed with a thermoplastic adhesive.
In the extrusion coating is part of a material-base which joins another film with other features (for example, suitable for printing, sealing, etc) from the extruder machine. Both sheets are joined by action of the heat without the need for adhesives. Is trafficking in a rapid process because the structure multi-layer is obtained in a single step, although it has the limitation that should be applied to compatible materials.
On the other hand, there is the process of lamination occurring through application of adhesives and which enables to combine substrates of different nature, for example, non-compatible plastics or plastic combined with paper or aluminum foils. It is also the most appropriate procedure when required, prior to the Union, print materials, with the advantage that reports that the printing is carried out on the internal side of the material, this acting as protector of the printing.
The process of co-extrusión blowing of hollow body in a manner similar to the process of co-extrusión explained above can be used to obtain rigid containers, flasks or bottles.
On the other hand, there is the process of co-inyección, which is one of the variants of the multimaterial two-component injection molding process. The co-inyección process is characterized by its ability to encapsulate completely one of the materials injected into another. The mechanism of the process consists of sequential injections of two different materials through the entrance, is a three step process so that the second material is encapsulated by the first.
In short, the plastic container due to the variety of materials and processes allow to adapt current necessary packaging with modified atmosphere packaging processes including all kinds of formats, film, trays, jars, bottles, etc. However, a correct selection of plastic materials is necessary because due to the particularity of permeability plastics that have the use of any material is not valid.
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