EFFECT OF APPLICATION OF VARIOUS CHITOSANS WITH DIFFERENT MOLECULAR WEIGHTS ON THE STORABILITY OF ¿JONAGOLD¿ APPLE

Chitosan derived from crab- or shrimp-shell after deacetylation of chitin is known to be effective as a coating in prolonging fruit shelf life.
Difficulty of application, however, is still a problem due to its relatively high viscosity arising from its high molecular weight.
This study used ‘Jonagold’ apple to evaluate chitosans having different molecular weights (MW) as shown in Table 1, with aim of improving the method of chitosan application.
As chitosan is insoluble in water, solutions were prepared by dissolving 1% chitosan in 0.4% acetic acid then adjusting the pH to 5.2, with 0.4% acetic acid used as control.
Fruits were individually dipped in the treatment solution and then air-dried.
The coated fruit were stored at 5°C, RH 60–70% in a coolstore.
Physiological characteristics such as firmness, respiration rate and internal CO₂ and O₂ concentrations were measured at 3-week intervals.

There were significant differences in firmness of fruits among the treatments (Figure 1). Chitosan treatment delayed softening and ripening in parallel with the difference in molecular weight, i.e. the most effective was the high MW chitosan followed by medium MW, and then low MW. High and medium MW chitosan treatments depressed the respiration rate significantly, while low NW chitosan did not.
It was found that there were two typical patterns in internal gas composition: CO₂ accumulation and O₂ depletion in the case of high and medium MW chitosan treatments, with little change from air composition in the low MW chitosan treatment and control.
No off-flavors were found in chitosan-coated apples.
The beneficial use of chitosan application arises out of its particular functional properties, being semipermeable to some gases including ethylene and carbon dioxide and antifungal to several phytopathogens.
Marked differences were seen in SEM studies of the peel of fruits treated by chitosan compared to controls. (Du et al., 1998). The surface seemed to be less cracked and there were changes in the intercellular space.
This different structure arising from the coating may help in limiting the transpiration rate.

It seemed that high and medium NW chitosans had potential to be used as effective coating materials for apple fruit, similar to sucrose polyester.
With respect to handling properties of the chitosan, the medium MW chitosan solution is not so viscous.
Thus, the possibility of using medium NW chitosan commercially could be considered.

International Postharvest Science Conference Postharvest 96

H. Gemma, J. Du

https://doi.org/10.17660/ActaHortic.1998.464.124