If produce is sealed in a film which is not sufficiently permeable, undesirable anaerobic conditions (<1% oxygen and >20% carbon dioxide) will develop. This will result in deterioration of the quality. Conversely, if produce is sealed in a film with excessive permeability, the modified atmosphere will not be retained and moisture loss will also lead to accelerated deterioration in quality. Examples of materials that are suitable for Modified Atmosphere Packaging (MAP) of fresh fruit and vegetables are microporous film or LDPE/OPP.
The key to successful MAP of fresh produce lies in packaging film of correct intermediary permeability. This establishes an optimum equilibrium modified atmosphere (EMA) as the rate of oxygen (O2) and carbon dioxide (CO2) transmission through the film equals the produce respiration rate.
Typically, optimal EMAs of 3–10% oxygen and 3–10% carbon dioxide, can dramatically increase the shelf life of fruit and vegetables. The EMA is influenced by numerous factors such as respiration rate, temperature, packaging film, pack volume, fill weight and light.
The respiration rate is affected by the variety, size, maturity and preparation of the produce. Consequently, working out the optimal EMA for a particular item is a complex equation that can only be solved through experimental testing.
To establish a beneficial modified atmosphere, fresh produce can be sealed in air or flushed with gas (3 –10% O2 / 3 –10% CO2 / 80 –90% N2).
As previously explained, modified atmospheres evolve within an air sealed pack due to produce respiration. However, in certain cases it may be desirable to gas-flush to accelerate establishment of a beneficial EMA. For example, enzymic browning of salad vegetables can be delayed by gas flushing instead of air packing.
Experimental tests should be undertaken to establish the optimum solution. Different conditions may apply, for example, to peeled potatoes and apples (which should not be packed with oxygen to avoid enzymic reactions that result in brown discolouration). Pre-peeled potatoes, for example, can be packed in 20% CO2 + 80% N2. As a result, this extends the shelf life from 0.5 hours to 7–8 days at +4 to +5°C (+39.2 to + 41 ºF).