This image is from the 1990’s when I was a Horticulturist for the BC Ministry of Agriculture based at Creston in the Kootenay area of BC. These are McIntosh apples, each row from a different orchard. Orchards in some areas of the valley (often those with sandy or gravelly soils) have more white indicating more advanced maturity.
This test is used with a series of other tests to determine when to harvest. The other tests include soluble solids (sugar), pressure (fruit firmness) and sometimes lab tests like acidity and ethylene production.
The starch Iodine Test is the most widely used test for apple maturity. As apples ripen, starch is converted to sugar. Iodine binds with starch granules in the fruit producing a dark purple to almost black color. Immature apples show little white as all their carbohydrate is all starch. The above visual rating scale was developed at Cornell University.
The charts below are variety specific, charts for other varieties can be found through Google searches. Charts are only available for varieties grown in commercial volumes, for new or heritage varieties you will need to extrapolate from existing charts.
Remember, these charts are primarily used to determine when to harvest for commercial storage and marketing. Starch Iodine (SI) readings of 3-4 Cornell Scale (CS) are suitable for long hold controlled atmosphere (CA) storage. These apples taste starchy as they go into storage, the ripening process is easily suspended at this stage of maturity. Apples measuring 4-6 CS can be put in CA storage for short periods. Fruit at or above 6 CS should be placed in regular cold storage or marketed immediately.
For home orchardists the fruit is best harvested at a reading of 6 0r 7 (CS), a reading of 8 indicates peak maturity for eating right off the tree but they will soften quickly. Apples often take 3 – 4 days to progress between each stage after stage 4, but it may be faster or slower depending on the weather. Rate of maturity development is variety dependent.