Introduction
The history of food packaging can be traced back to the advent of human civilisation. In ancient times, human beings might have used the utensils made from plant or vegetable leaves. As time passed, packages and containers became an essential part of human life for distribution, handling, storage and transportation of food and presently various types of packaging materials are being used for fresh and processed food. Packaging provides convenience of handling and extension of shelf-life by keeping the products fresh-like as well as free from contamination.

Need for Packaging
Most food products after harvest receive artificial treatments for preservation. The main purpose of the treatments is essentially to lengthen the safe handling time from harvest to consumption. The longer the time, the farther the food can be distributed from the site of the harvest to the consumer. Modern science has developed drying, cooking, heating, refrigerating, freezing, pasteurising,sterilising, irradiating and packaging.Packaging is extremely important and can maximise the time of handling from harvest to consumption and therefore warrants the longest shelf-life of any food product.
 

Modern Packaging of banana.

Food packaging has become an increasingly important area in modern society. The purpose of food packaging is to safely preserve the food and distribute them in near to fresh, nice, sweet, tasty, and delicious form to the consumer, preventing deterioration under various environmental conditions.

Different packaging materials and their application technologies have been developed for food packaging which include:The history of food packaging can be traced back to the advent of human civilisation. In ancient times, human beings might have used the utensils made from plant or vegetable leaves. As time passed, packages and containers became an essential part of human life for distribution, handling, storage and transportation of food and presently various types of packaging materials are being used for fresh and processed food. Packaging provides convenience of handling and extension of shelf-life by keeping the products fresh-like as well as free from contamination.
• Retortable packaging
• Aseptic packaging
• Free-oxygen scavenging packaging
• Vacuum packaging
• Controlled atmosphere packaging
• Modified atmosphere packaging
• Active packaging
• Anti-microbial packaging
• Nano-technological packaging
• Smart or intelligent packaging
• Micro-perforated packaging

These packaging techniques have been further divided into different sub-techniques depending upon the requirement.
 

Principles of Packaging
Packaging is defined as techniques and states for applying suitable materials or containers to goods in order to protect and maintain product quality during shipping and storage. Since packaging includes all operations necessary before the product reaches the consumer, it should be planned to meet distribution processes and the external environment.

The principles of packaging include:

• Containment for handling, transportation and use.
• Preservation and protection of the contents for required shelf-life.
• Protection from external environmental and mechanical hazards.
• Identification of contents, quantity, quality and manufacturer.
• Food safety and quality retention.
• Facilitation of dispensing and use.
• Ease of opening, re-closure, portioning application, unit of use, multipacks, second use or re-use.
• Working features such as those found in the cook-in pouches.

Packages must be designed to meet internal or external difficulties that may be encountered at respective stages of distribution, to protect goods safely from such difficulties, and to send goods to the consumers satisfactorily. The packaging engineer and researcher must know that the food packer’s main aim is not to sell a package, but its contents.

Primary, Secondary and Tertiary Packaging
There are three kinds of packaging. First is primary or item packaging, which is defined as packaging of individual goods and states for applying suitable materials or containers in order to attract the consumer or to protect product quality or in other words, primary packaging is the main package that holds the fresh or processed food. The primary packaging is mostly carried out at the producer’s level and depicts the manufacturing date, status, date of expiry etc. on each package; e.g. each milk pouch contains the information about its date, place of manufacture, selling price and date of expiry, batch number etc. Secondary packaging combines the primary packages. It is defined as the inner packaging of packaged freight to protect goods; e.g. the individual milk pouches are either bagged or taken in plastic containers for transport. Tertiary packaging combines all of the secondary packages into one pallet and is defined as outer pack of packaged freight for the purpose of shipping.

Overview on Modified Atmosphere Packaging
Modified atmosphere (MA) results in a gaseous equilibrium inside the film packages containing fruits or vegetables which is different from the normal atmospheric composition of oxygen and carbon dioxide. Here, target is to keep oxygen level low (2-5%) and carbon dioxide level high (up to 12%). Therefore, creation of an appropriate modified atmosphere, which may ultimately reduce respiration rate, ethylene production, decay and the physiological changes in the packaged produce, is of utmost importance. The dynamics of MAP essentially consists of a continuous interaction between respiration of packaged produce and gas permeability characteristics of the polymeric film package under a specified environmental condition of temperature and relative humidity. The environment or storage room is maintained at the desired temperature and relative humidity.Usually, low temperature is maintained inside the storage room, which besides controlling the produce metabolism reduces the activity of certain pathogenic bacteria. Now-a-days, nanotechnology based stores are also available, which can work in combination with modified atmosphere technology to store the produce and maintain the low temperature even in case of a power failure.Further, the transport containers working on nanotechnology are also available which does not need any power supply as required in case of refrigerated vans. These containers can maintain low temperature during storage of the fruits and vegetables packaged in modified atmosphere. Using them, fruits and vegetables can be transported for long distances.

In this technology, packages of appropriate dimensions and containing a known amount of produce are placed inside the storage room. The film package is prepared from a polymeric film having selective amount of permeability values for O₂, CO₂, N₂ and water vapour. The empty space inside the package above the packaged produce is known as package headspace. The produce starts consuming O₂ from the package headspace and at the same time CO₂ evolution takes place. Oxygen from the storage room starts diffusing into the package and the CO₂ diffuses out from the film package due to the gaseous partial pressure differential between the storage room and the film package. The convective currents of cold air in the storage room are the major carriers of various gases, water vapour and heat of respiration across the polymer film package and into the storage environment. The influx or efflux of gases and water vapour takes place through the entire surface area of the film package. A stage of equilibrium comes at which the partial pressures of O₂ and CO₂ inside the film package do not change appreciably with the passage of time. At this stage, the rate of O₂ infux into the film package equals the rate of O₂ consumption by the packaged produce.Similarly, the rate of CO₂ evolution from the produce equals the rate of CO₂ efflux from the package. Each of the modified atmosphere generated vis-a-vis the keeping quality of the fruit or vegetable may be suitable or not. Therefore, among the different atmosphere so generated, the most suitable atmosphere for different fruits and vegetables has to be found in terms of levels of O₂ and CO₂.

Mostly, single layer films like Polypropylene (PP), Oriented polypropylene (OPP), Oriented Polysterene (OPS) and Low density/high density polyethylene (LDPE/HDPE) are used for vegetable packaging and recently, highly permeable films and micro-perforated films are used for packaging of vegetables. However, the problem with the LDPE and HDPE is that water accumulates in these packages. For this purpose, either micro-perforated films can be used or macro-holes can be made in the package which needs to be tandardised for the individual vegetable. However, microperforated film packages are expensive and are not easily available; therefore macroperforations using small pin-hole technique are better alternatives for this purpose. The size of the package should be selected in such a way that the gaseous equilibrium remains nonfermentative during the entire period of storage under MAP. In case of non-perforated, about 30% space above the filled vegetable should be empty. On the other hand, in case of perforated packages, the headspace should be around 15% of total package volume.

Advantages of Modified Atmosphere
• Potential increase in shelf-life.
• Reduced economic loss.
• Products can be distributed over longer distances and without loss.
• Maintains quality of high value perishable produce like fruits and vegetables.
• Retention of pigments, antioxidants, particularly, phenols, flavonoids etc.
• Increases or maintains chlorophyll in leafy vegetables and their shelf-life as well as appearance is maintained.

Creation of Modified Atmosphere
Modified atmosphere packaging (MAP), can be created passively by using proper permeable packaging materials, or by actively using a specified gas mixture, together with permeable packaging materials. The purpose of this procedure is to create an optimal gas balance inside the package, where the respiration activity of the product is as low as possible; on the other hand, the oxygen concentration is not detrimental to the product.

One limitation in the design of modified atmosphere packaging is in finding good permeable material that will match the respiration rate of the produce; only a few choices are available in the market. Most films do not result in optimal O₂ and CO₂ atmosphere in products with high respiration rates. This problem can be tackled by making micro and macro holes of defined size and quantity in the material to prevent anaerobiosis. Another alternative is to combine high permeability materials with oriented polypropylene and low-density polyethylene at a specified thickness. These materials have significantly higher permeability than the polyethylene or oriented polypropylene. The modified atmosphere that best maintain the quality and storage life of fresh fruits and vegetables as well as ready-to-cook ones have been found to have an oxygen range of 2 to 8 percent and carbon dioxide concentration of 5 to 15 percent.

Gas Transmission through Plastic Films
For modified atmosphere packaging of respiring products, gas transmission through the polymeric film at certain rate is a must. The transmission of molecules through the polymeric films is called as gas permeability or gas transmission. Gas transmission rate for a particular thickness of polymeric film is defined as the capacity of a film to transfer a particular amount of gas and is expressed in cc/m².day.atm or ml/m².h.kPa. The most important characteristic of packaging materials is the varied permeability of the packaging material to different gases. Permeability of packaging materials to the three kinds of gas exchange is highest for CO₂ followed in order by O₂ and N₂ gas in the ratio of 3-5:1:1/3-1/5. Gas permeability of packaging materials changes with temperature and it
varies exponentially with temperature (Table 1). Moreover, it is also influenced by humidity of the surroundings. For some of the films, like polyester and vinylidene chloride, gas permeability is hardly affected by the humidity while it is highly influenced by humidity in case of films made from polyamide, ethylene-vinyl alcohol co-polymer or cellophane. Mostly, single layer films like OPP, OPS (Oriented Polystyrene) and LDPE are used for fruit and vegetable packaging and recently, highly permeable films and micro-perforated films are being used for packaging of highly perishable crops such as mushroom, broccoli and leafy vegetables. Since micro-perforated films are expensive, therefore gas transmission rates can be increased through macroperforations in the normal films like polypropylene or low density polyethylene (LDPE). This way one can reduce cost and get a longer shelf-life. Our results have suggested that spinach, okra and betel leaves can be stored in macro-perforated films for as long as 10 days without much loss of quality (Fig. 1). Also, the mushroom, broccoli, mustard leaves can also be stored for much longer time using this technique. Further, the technique can be used to store and extend the shelf-life of minimally processed, ready-to-cook vegetables such as shredded cabbage, cutbeans, diced carrots, spinach etc. which are highly perishable otherwise (Fig. 2).



At CIPHET, Ludhiana, the parameters like type of packaging material, size of bag, microperforations required for exchange of gases as well as specific pre-treatment required prior to packaging have been standardised. The shelflife of various vegetables has been found to increase sufficiently using this technique.

Some of the examples are as follows: 

• Okra or lady finger which has the problem of blackening and flaccidity during its storage can be stored for 9 days using this technology in macro-perforated PP film packages.
• Broccoli which has a normal shelf-life of 2-3 days can be stored using modified atmosphere packaging technology for 7-8 days in macro-perforated PP film packages in the winter season keeping all its bioactive constituents intact.
• The shelf-life of betel leaf which is a commercial crop of immense national importance can be extended to 13 days in non-perforated PP packages at the retail level without development of harmful pathogens which is much more than 3-4 days storage at retailer’s level through regular sprinkling of water.
• The leafy vegetables such as spinach and mustard leaves; along with an additional shelf-life by 4-5 days have been found to retain their green colour (chlorophyll), turgidity and other physico-chemical attributes through packaging in nonperforated PP film packages.
• Modified atmosphere packaging of ready to cook vegetables.

In a nutshell, modified atmosphere packaging is a technology suitable to maintain the fresh like quality as well as prevention of physical and nutrition loss in fruits and vegetables. Using this technology, the farmer can gain by reducing his losses and can store the fruits and vegetables for a longer period of time. Further, the unsold fruits or vegetables or those highly perishable in nature such as leafy vegetables can be easily saved by this technology.