The major factors responsible for extending the shelf life of fruits and vegetables include: careful harvesting so as not to injure the product, harvesting at optimal horticultural maturity for intended use, and good sanitation When these are practiced, the implementation of optimum storage conditions through modified atmospheres can be quite effective at maximizing the shelf life and quality of the product.

A modified atmosphere can be defined as one that is created by altering the normal composition of air (78% nitrogen, 21% oxygen, 0.03% carbon dioxide and traces of noble gases) to provide an optimum atmosphere for increasing the storage length and quality of food/produce . This can be achieved by using controlled atmosphere storage (CAS) and/or active or passive modified atmosphere packaging (MAP). Under controlled atmospheric conditions, the atmosphere is modified from that of the ambient atmosphere, and these conditions are maintained throughout storage. Examples of this type of storage and the commercial systems available are MAP uses the same principles as CAS; however, it is used on smaller quantities of produce and the atmosphere is only initially modified. Active modification occurs by the displacement of gases in the package, which are then replaced by a desired mixture of gases, while passive modification occurs when the product is packaged using a selected film type, and a desired atmosphere develops naturally as a consequence of the products' respiration and the diffusion of gases through the film . The numerous film types used in MAP and some commercially available MAP systems are listed in Table .

Oxygen, CO2 , and N2 , are most often used in MAP/CAS (Parry 1993; Phillips 1996). Other gases such as nitrous and nitric oxides, sulphur dioxide, ethylene, chlorine (Phillips 1996), as well as ozone and propylene oxide (Parry 1993) have been suggested and investigated experimentally. However, due to safety, regulatory and cost considerations, they have not been applied commercially. These gases are combined in three ways for use in modified atmospheres: inert blanketing using Nitrogen Gas , semi-reactive blanketing using CO2 / N2 or O2 /CO2 /N2 or fully reactive blanketing using CO2 or CO2 /O2 (Parry 1993; Moleyar and Narasimham 1994).

Normally, the concentration of O2 in a pack is kept very low (1-5%) by use of Nitrogen Gas Generators to reduce the respiration rate of fruits and vegetables (Lee and others 1995). Reducing the rate of respiration by limiting O2 prolongs the shelf life of fruits and vegetables by delaying the oxidative breakdown of the complex substrates which make up the product. Also, O2 concentrations below 8% reduce the production of ethylene, a key component of the ripening and maturation process. However, at extremely low O2 levels (that is, <1%), anaerobic respiration can occur, resulting in tissue destruction and the production of substances that contribute to off-flavors and off-odors (Lee and others 1995; Zagory 1995), as well as the potential for growth of food borne pathogens such as Clostridium botulinum (Austin and others 1998). Therefore, the recommended percentage of O2 in a modified atmosphere for fruits and vegetables for both safety and quality falls between 1 and 5% (Table VI-4). However, it is recognized that the oxygen level will realistically reach levels below 1% in MAP produce. It is generally believed that with the use of permeable films, spoilage will occur before toxin production is an issue; MAP of produce, however, should always incorporate packaging materials that will not lead to an anoxic package environment when the product is stored at the intended temperature. This recommendation should be qualified, however, by saying that all films are permeable to oxygen to some degree; the difference pertains to the rate of gas transfer through the film, with some films allowing greater transfer rates than others. Moreover, the elimination or significant inhibition of spoilage organisms should not be practiced, as their interaction with pathogens may play an integral role in product safety. A number of packers of fresh prepared green vegetables in the United Kingdom have been experimenting with O2 mixtures between 70 and 100% (Day 1996). The treatment, referred to as "oxygen shock" or "gas shock," has been found to be very effective in inhibiting enzymatic discoloration, preventing anaerobic fermentation reactions, and inhibiting aerobic and anaerobic microbial growth. High levels of O2 can inhibit the growth of both anaerobic and aerobic microorganisms since the optimal O2 level for growth (21% for aerobes, 0-2% for anaerobes) is surpassed. However, there have also been reports of high O2 (that is, 80-90%) stimulating the growth of foodborne pathogens such as Escherichia coli and Listeria monocytogenes (Amanatidou and others 1999). Recent studies by Kader and Ben-Yehoshua (2000) and Wszelaki and Mitcham (2000) examining the use of superatmospheric O 2 levels to control microorganisms on produce, have found that only O 2 atmospheres close to 100 kPa or lower pressures (40 kPa O 2 ) in combination with CO 2 (15 kPa), are truly effective. These requirements may be difficult to achieve in industry since working with such high O 2 levels can be hazardous due to flammability issues. As with most MAP gases, superatmospheric O2 has varied effects depending on the commodity, and further research is required in this area to elucidate the utility of this technique in the fresh-cut produce industry. A high O2 MAP group has been formed in the United Kingdom and includes a number of industry groups, notably Marks and Spencer plc, one of the first retail chains to distribute MAP foods. More recently, the "high O2 MAP club" has provided a base for the new "Novel Gases MAP Club" in the United Kingdom, a group that will investigate the use of novel high O2 , argon and nitrous oxide MAP for extending shelf life and quality of fresh-cut produce. Their main focus is research into the commercial application of this process.