There are various ‘rules of thumb’ for assessing the maturity of cider apples in order to commence harvest. These range from the colour of the seeds to the proportion of fruit that has dropped off the tree.
Sampling the apples for juice and then determining the specific gravity of the juice can be useful but it doesn’t necessarily prove that all the starch within the fruit has been converted to sugars.
It is possible to assess the starch content of fruit visually using the starch iodide test: https://www.canr.msu.edu/uploads/files/Research_Center/SWMREC/Apple_Maturity_Protocol.pdf
This utilises a solution of potassium iodide and iodine in water which is applied to the cut surface of the apple. The degree of blackening will show whether starch remains unconverted and how much. The pattern of the blackening varies with the apple variety. An absence of black indicates that all the starch has been converted.
There will be variation with apples picked from the tree. How critical the assessment is will determine how many fruit should be sampled from the tree, high or low, shaded or sunny, north or south etc. Images with four halves have, on the left, an apple that had fallen to the ground. On the right is an apple picked from the tree. Apples that have fallen could be ripe and drop from the tree. Alternatively the fallen apple could have been squeezed off as other apples in its bunch expand.
Examples of the current season’s testing are linked below.
Sturmer Pippin is not thought of as a cider apple in its country of origin (Britain) but was a mainstay of the cider industry in Tasmania for many years and is still highly thought of for cider there and in other places within Australia.