Starch Metabolism in Apple Fruit and Its Relationship with Maturation and Ripening

The Starch Pattern Index is used to help assess fruit maturity.  Iodine forms a tri-iodine complex with starch molecule amylose and produces a black-blue staining.  However, since iodine only complexes with amylose the staining pattern can be misleading because its dependence on the ratio of amylose to amylopectin may make it less reliable for the assessment of the total starch changes over time. The cultivars ‘Empire’, ‘Gala’, ‘Honeycrisp’, and ‘McIntosh’ have been used to investigate relationships between the SPI, total starch and percentage of amylose. Starch concentrations are more variable at the lower SPI values where little hydrolysis has occurred. ‘Gala’, ‘Honeycrisp’, and ‘Empire’ show a wide range of starch concentrations at a SPI of 1, ranging from 100 to almost 400 mg of starch per gram (dry weight).  The starch concentration shows a curvilinear relationships with the SPI for all four cultivars, but linear when plotted over harvest date.  Later in the season the range of starch concentration at different SPIs varies among and within cultivars, and some of this variation can be explained by the changing percentage of amylose of total starch. The percentage of amylose decreases as the season progresses, in ‘Gala’ decreasing  from 62% to less than 1% (average 33%), whereas ‘Empire’ changed from about 58% to 15% during ripening (average 46%). ‘Honeyrisp’ and ‘McIntosh’ averaged 26.7% and 35.35% amylose, respectively. Those differences in total starch at different SPI levels could therefore, be related to the ability to stain the starch. At a very low percentage of amylose the staining is less clearly visible. Overall, the results suggest that the iodine test can be less reliable as an indicator of starch in the apple especially later in the season.