Relationships between starch pattern indices and starch concentrations in four apple cultivars

One of the most widely used measurements to assess apple [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] maturity is the starch pattern index (SPI), but relatively little is known about changes in its relationship with starch concentration in the fruit during maturation and ripening. Relationships between SPI and starch concentrations in 'Gala', 'Honeycrisp', 'McIntosh', and 'Empire' fruit harvested over several weeks have been investigated. While SPI values increased and starch concentration decreased over the sampling period, the relationship between the two factors was curvilinear for all cultivars. The binding capacity of iodine to starch is determined by the starch composition and consequently by the amylose (AM) concentration. Therefore, changes in AM concentration in total starch were assessed to evaluate whether changes in AM account for discrepancies between SPI values and changes in starch concentration. Low levels of AM at later stages of fruit development account in part for the discrepancy between SPI and starch concentration, due to the low staining potential of the remaining starch (amylopectin (AP)). Evaluation of SPI values was also carried out by calculation of percentage stained area by means of image analysis; changes in SPI and % staining were linear for all cultivars but to different degrees depending on cultivar. Computational analysis of SPI images has the potential to be more consistent, but interpretation of the index number assigned still relies heavily on a general understanding of fruit maturation. Understanding cultivar specific differences in the starch iodine staining patterns and the influential factors on starch during ripening can lead to an overall better understanding of SPI as a harvest index.