由于不同的水果或蔬菜都有其所特有的最佳包装空气浓度，即各有各的二氧化碳和氧气的最佳浓度及可容浓度，因此，挑选一种能在密封保鲜袋内适度调节空气，延缓包装食品变质的可透性塑料是十分重要的。
Because each different type of fruit or vegetable has its own optimum atmosphere--its own optimum concentrations of carbon dioxide and oxygen and its own tolerances-- it is important to select a film which modifies the atmosphere within the sealed package to the correct extent to retard changes associated with deterioration.