Checking apple maturity: What to look for

Phil Schwallier, Michigan State University Extension - August 28, 2012

Follow these apple maturity guidelines to help target apple harvest at the optimum maturity and best storage life.

Maturity and storage guidelines by variety have been developed over the years through the Michigan State University 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 "over mature" level for short CA or cold storage.

Table 1. Suggested firmness and starch index levels for long-term and shorter-term storage by variety.

Variety	Firmness (pounds)*				Starch Index*
Variety	Short CA	Mid CA	Long CA	Export CA	Mature	Over mature
Braeburn	16	17	18		3	7
Cortland	14	15	16		4	6
Empire	14	15	16	18	3.5	6
Fuji	16	17	18		3	7
Gala	16	17	18		3	6
Golden Delicious	15	16	17		3	6.5
Honeycrisp	15	16	17		3.5	7
Idared	14	15	16		3.5	6
Jonagold	15	16	17		3.5	5.5
Jonathan	14	15	16		3.5	5.5
McIntosh	14	15	16		5	7
Mutsu/Crispin	16	17	18		3	6
Northern Spy	16	17	18		3	6
Red Delicious	16	17	18		2.5	5
Rome	15	16	18		3	5.5
Spartan	17	18	19		4	6
Winesap	17	18	19		2.5	6

*Firmness is measured with a mechanical 11-mm wide probe inserted into the pared flesh of a fruit to a distance of 8 mm. Starch index is measured on equatorial cross section of an apple stained with iodine solution and rated using the Cornell University starch-iodine index chart for apples on a 1-8 scale (Predicting Harvest Date Window for Apples by Blanpied and Silsbey, Cornell Extension Bulletin 221.)

Empire has a firmness level guideline for CA empires that are to be held for export. Empire needs an additional 2 pounds of firmness if they are to be exported. They need to be a minimum of 16 pounds firmness coming 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" for seven to 10 days and then start to enter the "over mature" class. Over mature 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 a brix.

Table 2. Brix readings on Honeycrisp and other varieties.

Brix Guide	Low	Fair	Good	Excellent
All Varieties	Less than 11	11	12	13
Honeycrisp	Less than 12	12	13	Greater than 14

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).