Factors Explaining Variations in Soluble Solids Content of Apples during Ripening and Storage

Sweetness is one of the most important drivers of consumer preference in apples (Malus × domestica Borkh.). The increase in sugar during the ripening period of fruit is mainly brought about by the hydrolysis of starch accumulated before the ripening period. However, sugars are also continuously translocated into the fruit during the ripening period, as seen in watercored fruits. The objective of this study was to estimate the contribution of translocated sugars that accumulated in the apoplast to the increase in soluble solids content (SSC) of fruit during ripening. The amount of apoplastic solution (AS) tended to be high in the fruit of trees on vigorous rootstocks, such as ‘JM2’ and ‘Marubakaido’. On the other hand, fruit with more AS had lower SSC. Therefore, although AS increased during ripening, the contribution of AS to the increase in SSC was small. After fruit matured and during storage, dehydration increased the SSC of the fruit. On the other hand, the SSC decreased simultaneously due to a reduction in organic acids and any soluble solids, which was expressed as a decrease in titratable acidity (TA). Under standard refrigerated conditions, the increase in SSC due to dehydration and the decrease in SSC due to respiration were likely to be quantitatively comparable. The contribution of translocated sugars to the increase in SSC during ripening was small, suggesting that managing trees and fruit to increase starch accumulation before fruit ripening is crucial for the production of fruits with high sugar content.