Unlike other chilled perishable foods that are modified atmosphere (MA) packed, fresh produce continues to respire after harvesting and any subsequent packaging must take into account this respiratory activity. The depletion of O2 and enrichment of carbon dioxide (CO2) are natural consequences of the progress of respiration when fresh produce is stored in hermetically sealed packs. Such modification of the atmosphere results in a respiratory rate decrease with a consequent extension of shelf-life (Kader et al., 1989). MAs can passively evolve within hermetically air-sealed packs as a consequence of produce respiration. If the respiratory characteristics of a produce item are properly matched to film permeability values, then a beneficial equilibrium MA (EMA) can be passively established. However, in the MAP of fresh produce, there is a limited ability to regulate passively established MAs within hermetically air-sealed packs. There are many circumstances when it is desirable to establish the atmosphere rapidly within produce packs. By replacing the pack atmosphere with a desired mixture of O2, CO2 and nitrogen (N2), a beneficial EMA may be established more rapidly than a passively generated EMA. For example, the use of flushing packs with N2 or a mixture of 5-10% O2, 5-10% CO2 and 80-90% N2 is commercial practice for inhibiting undesirable browning and pinking on prepared leafy green salad vegetables (Day, 1998).
The key to successful retail MAP of fresh prepared produce at the time of writing is to use packaging film of the correct permeability so as to establish optimal EMAs of typically 3-10% O2 and 3-10% CO2. The EMAs attained are influenced by produce respiration rate (which itself is affected by temperature, produce type, variety, size, maturity and severity of preparation); packaging film permeability; pack volume, surface area and fill weight, and degree of illumination. Consequently, establishment of an optimum EMA for individual produce items is very complex. Furthermore, in many commercial situations, produce is sealed in packaging film of insufficient permeability, resulting in development of undesirable anaerobic conditions (e.g. <2% O2 and >20% CO2). Microperforated films, which have very high gas transmission rates have been developed and are now commercially used for maintaining aerobic EMAs (e.g. 5-15% O2 and 5-15% CO2) for highly respiring prepared produce items such as broccoli and cauliflower florets, baton carrots, beansprouts, mushrooms and spinach. However, microperforated films are relatively expensive, permit moisture and odour losses, and may allow for the ingress of microorganisms into sealed packs during wet handling situations (Day, 1998).
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