Modified-atmosphere packing (MAP), also known as "maintain and preserve," is a technology employed to maintain and preserve the quality of fruit being packed and shipped. Modifying the atmosphere in which a fruit is stored can dramatically alter its rate of respiration and metabolism and increase its commercial storage life. A passive or active process can generate modified atmospheres. In the passive mode, respiring fruit, over time, generate their own atmosphere of elevated carbon dioxide and reduced oxygen. In an active system, desired endpoint concentrations are established more quickly by flushing the fruit with a desired mixture of gases.
Successful MAP depends on several factors. Each packed container has a bag or film material, usually a type of polyethylene, that surrounds the product and provides a barrier to gas transfer between the inside and outside. The oxygen transmission rate of the film will limit what products can be stored within each container. Other limiting factors include the respiration rate of the product, the amount of product, the temperature at which the fruit are being stored, and the amount of free space within the film.
Temperature is the major factor to manage in all postharvest handling. In MAP, temperature is very critical and must be maintained within specific limits. As temperature increases, so does the respiration rate. This is a dangerous situation for fresh fruit in a sealed bag and might result in the consumption of all available oxygen and the onset of anaerobic respiration, the production of carbon dioxide, eth-anol, and off flavor.
A MAP alternative for fruit with high respiration rates utilizes microperforation or combines the polyethylene film with a more porous membrane material attached as a patch. Microperforated film allows greater gas exchange and is more forgiving during episodes of moderate temperature abuse. Microperforations in films are produced by mechanical methods of laser, sharp-pointed instruments, or localized heat. The porous patch acts as a valve and allows greater exchange of gases than does the polyethylene film by itself.
A California corporation (http://www.landec.com/) has developed an "intelligent" polymer technology that is useful in MAP situations where temperature abuse is likely or insurance against improper temperature handling is warranted. The temperature switch technology uses a polymer that changes from a crystalline to a more fluid form in response to increasing temperature. As the polymer becomes more fluid, it allows greater exchange of gas and thus compensates for increased respiration rate by allowing more oxygen to diffuse into the package. The polymer changes its permeability over the range of a few degrees. Once recooled, the process reverses, the polymer acts as a barrier, and the fruit attains an equilibrium of oxygen and carbon dioxide relative to its respiration rate and temperature. The permeability of the polymer, or the "switch," can be adjusted to match critical temperatures and respiration rate requirements of individual products.
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