Blocking Microorganisms: Many harmful, infectious microorganisms are contagious; once a piece of fruit inside a package begins to rot or become infected, the infection can quickly spread to adjacent fresh fruit. Preservation paper acts as an effective barrier against pathogenic microorganisms, preventing such transmission. This is particularly evident in the packaging used for fruits like grapes and peaches.
Retaining Moisture: Significant moisture loss during packaging and transport inhibits the fruit's essential respiration processes, leading to a decline in nutritional value. By coating the surface of the preservation paper or incorporating water-resistant substances into its structure, the paper gains moisture-blocking properties; this reduces moisture loss and helps maintain freshness. This method has been successfully applied to the packaging of fresh pears.
Chemical Action: Fruits can sometimes "fall ill" during storage and transport due to changes in their internal chemical composition. To address this, therapeutic agents required by the fruit can be incorporated into the preservation paper. For instance, the green skin on the shaded side of certain apples is prone to discoloration-a condition known as "scald" (or "tiger skin disease")-which can cause the flesh to deteriorate. Scald is caused by the accumulation of harmful substances (oxidation products) within the apple skin. Spraying the preservation paper with an anti-scald agent can chemically eliminate the discoloration, thereby preserving the fruit's quality.
Preventing Premature Ripening: Ethylene gas significantly accelerates the ripening of fresh fruit. By incorporating ethylene-adsorbing active materials-such as tuff, quartz, silica, or clay-into the inner preservation paper, ethylene can be absorbed and infrared radiation reduced, thereby delaying the fruit's ripening and senescence. This technique is used, for example, in the packaging of "Red Delicious" apples (which ripen in autumn and are prone to premature ripening) and bananas during long-distance transport.
Sterilization: Harmful microorganisms can be inhibited or destroyed by coating the preservation paper with volatile bactericidal or antioxidant substances, or by soaking the paper in a solution containing these agents and then drying it. The bactericidal substances used can be either chemical or natural agents. The sterilization efficacy also depends on external temperature, the modified atmosphere conditions, and bacterial concentration. In some instances, applying safe, hygienic coating materials directly to the fruit's surface can also provide an antimicrobial effect.
Gas control: Fruit requires oxygen and moisture for aerobic respiration, yet it also needs inhibitory gases to delay senescence and the depletion of reserves; consequently, controlling the levels of various gases-often to specific ratios-is crucial. This can be achieved by selecting specific types of preservation films or using coating methods to reduce the film's gas permeability. As the fruit continues to respire aerobically, the oxygen concentration within the package decreases while the carbon dioxide concentration rises, thereby reducing the rate of respiration.
