Ripening-related Physicochemical Changes of Polysaccharides of 'Golden Delicious' Apples and Their Relationship with Postharvest Fruit Softening

Several events have been related with the postharvest softening of apples, including the fruit dehydratation, modification of pectins and starch hydrolysis. However, the level of involvement of these factors on fruit softening is still unclear. In this work, Golden Delicious apples were harvested at 167 DAFB and stored under ambient conditions (air/23° C), refrigeration (air/0° C), and controlled atmosphere (2 kPa O₂-3 kPa CO₂) for up to 5 months. Samples of apples were monthly removed from storage, evaluated for moisture content and firmness, and used for the extraction of pectin and starch. Several physicochemical properties were evaluated in the extracted polysaccharides. The involvement of the moisture content of the fruits and the properties of extracted polysaccharides on fruit firmness was determined by correlation studies. The controlled atmosphere storage retarded the pectin degradation; however, the pectin content was similar for all treatments by the end of the experiment. The protein content in pectin dramatically increased after 1 month of storage and then decreased. This decrease was quicker in fruit kept at ambient conditions. The degree of methyl esterification of pectin increased during the first month of storage and then remained almost constant. The pectin from apples that had been stored in controlled atmosphere showed the lowest esterification degree. The extracted pectins contained two main fractions, which experienced the reduction of their peak molecular weight and abundance during storage. The intensity of these changes depended on the storage conditions, following the order of ambient > refrigeration > controlled atmosphere storage. The content of Gal, Ara and some mineral elements in pectins showed distinctive changes among experimental groups during storage. The starch content decreased as function of the storage time and conditions, with the starch hydrolysis being more intense in fruits hat had been stored at ambient conditions. The morphology of starch granules only could be determined during the first months of storage in refrigeration and controlled atmosphere due to the intense starch hydrolysis and the presence of unknown structures in the starch samples. The diameter of the starch granules was higher in fruit that had been stored in controlled atmosphere than in refrigeration. The starch granules were completely circular at the beginning of the experiment but this characteristic tended to diminish after 2 months of storage. The moisture content of fruits and some pectin properties seem to be more related fruit softening than starch content, hydrolysis, and morphology.