Apple maturity guidelines
Editor’s note: This article first appeared in the September 5, 2006 issue of the Fruit CAT Alert.Check the label of any pesticide referenced to ensure your use is included.
Maturity and storage guidelines by variety have been developed over the years through the MSU Apple Maturity Program. These guidelines help target the harvest of apples at the optimum maturity and best storage life. Firmness and starch iodine index readings are easily performed in the field. Table 1 lists the suggested firmness and starch index levels for long-term and shorter-term storage by variety. The starch index "mature" level is a guide for long-term CA storage and the "overmature" level for short CA or cold storage.
The variety Empire has a firmness level guideline for CA empires that are to be held for export. Empire needs an additional two pounds firmness if they are to be exported. They need to be a minimum of 16 lbs. firmness out of storage at shipping time to arrive at their export destination in good condition.
Starch iodine index indicates the gradual change of starch to sugars in the fruit. In general, immature fruit are nearly 100 percent full of starch and will stain black (1 or 2) on the 8 point, New York Starch Iodine Index Chart. As the starch stain clears the core and starts to clear the flesh, the fruit then are mature and ideal for long-term CA storage. Fruits will often remain in the "mature" class for 7 to 10 days and then start to enter the "overmature" class. Overmature fruit are good for short CA or cold storage.
Brix readings are a guide for target sugar levels in fruit. The higher the better, but from year to year there is quite a lot of variation. Light crops will tend to have higher brix levels and heavy crops will be on the low end. Honeycrisp are expected to have higher brix readings.
Fruit firmness is often the major factor in deciding when to pick a block of apples. (view chart below) The ideal situation is a firm apple with a starch index in the mature zone. However, when fruit firmness drops close to the level for the intended storage (long to short CA, or cold storage), then it is time to harvest, regardless of the brix or the starch reading. The starch and brix readings at the time of harvest give a strong indication of the potential eating quality when the fruit comes out of storage.