With Jonagold there seems to be a wide variation in readings. A starch index of seven or eight could be easily found with readings of three. We suggest harvesting Jonagolds for CA storage with the index is between three and 5.5 for Michigan. This is when the flesh in Jonagold are starting to change to sugar. Our studies with Jonagold indicate that the change in starch is quite linear though out the maturity season.
Red color is of course a prime concern.
If the color has not developed enough it will not be shipped as a fresh apple. Good quality fruit and not over mature fruit will than be marketed. The green background color change can be helpful in determining maturity. When the green changes to a cream color, which can be seen at the calyx end of the fruit and backside of fruits, the apple is ready for CA storage. Jonagolds are a sweet-tart variety and develop excellent sugar content at 15 brix plus.