The main way in which packaging helps to preserve food is by preventing bacteria from contaminating its contents, but modern packaging is very sophisticated. Some foods are vacuum packed, so that plastic wrappings exclude any air from the product. Other kinds of packaging are designed to trap gases such as oxygen, nitrogen and carbon dioxide. Mixtures of these help to preserve different foods and to give them a pleasant appearance. Meats, for example, can be kept pink and fresh-looking. Sometimes you will see that meat looks browner where it touches the packaging. This is because the gases cannot reach it at this point. As these are gases that we breathe in every day, they are perfectly safe.
Because packaging helps to control the immediate environment of a food product, it is useful in creating conditions that extend the storage life of a food. Packaging materials commonly used for foods may be classified as flexible (paper, thin laminates, and plastic film), semi-rigid (aluminum foil, laminates, paperboard, and thermoformed plastic), and rigid (metal, glass, and thick plastic). Plastic materials are widely used in food packaging because they are relatively cheap, lightweight, and easy to form into desired shapes.
The selective permeability of polymer-based materials to gases, such as carbon dioxide and oxygen, as well as light and moisture, has led to the development of modified-atmosphere packaging. If the barrier properties are carefully selected, a packaging material can maintain a modified atmosphere inside the package and thus extend the shelf life of the food product.
Dehydrated foods must be protected from moisture during storage. Packaging materials such as polyvinyl chloride, polyvinylidene chloride, and polypropylene offer low moisture permeability. Similarly, packaging materials with low gas permeability are used for fatty foods in order to minimize oxidation reactions. Because fresh fruits and vegetables respire, they require packaging materials, such as polyethylene, that have high permeability to gases.
Smart packages offer properties that meet the special needs of certain foods. For example, packages made with oxygen-absorbing materials remove oxygen from the inside of the package, thus protecting oxygen-sensitive products from oxidation. Temperature-sensitive films exhibit an abrupt change in gas permeability when they are subjected to a temperature above or below a set constant. These films change from a crystalline structure to an amorphous structure at a set temperature, causing the gas permeability to change substantially.
Food storage is an important component of food preservation. Many reactions that may deteriorate the quality of a food product occur during storage. The nutrient content of foods may be adversely affected by improper storage. For example, a significant amount of vitamin C and thiamine may be lost from foods during storage. Other undesirable quality changes that may occur during storage include changes in color, development of off-flavours, and loss of texture. A properly designed food storage system allows fresh or processed foods to be stored for extended duration while maintaining quality.
The most important storage parameter is temperature. Most foods benefit from storage at a constant, low temperature where the rates of most reactions decrease and quality losses are minimized. In addition, foods containing high concentrations of water must be stored in high-humidity environments in order to prevent the excessive loss of moisture.
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