Article originally written by Cindy Tong, UMN Postharvest Specialist, in 2018 and updated by Madeline Wimmer, UMN Extension Fruit Production Educator in 2026.
As fall approaches and apples continue to ripen, what is the best way to determine when to harvest? It all depends on whether they will be sold more immediately for fresh market, or stored for a longer period.
There are several ways to determine apple ripeness. This involves changes in…
1) …peel background color from green to yellow.
2) …peel “overcolor” from green to yellow or red.
3) … fruit firmness from rock hard to being able to bite into easily (it shouldn’t break your teeth!). Fruit firmness can be measured using a tool called a penetrometer.
4) … preferred flavor, which varies from person-to-person.
5) … flesh carbohydrate conversion that changes the taste from starchiness to sweetness.
6) … soluble solids levels (measured in degrees brix). This method is especially helpful for cider apples where total soluble solids can help estimate final alcohol content for hard cider.
Not all of these methods work for all varieties. Peel background color can be hard to see, overcolor can change when background color is still green, and some apple varieties stay firm or crisp (like Honeycrisp). The take-home for the first four listed above is that it is still important to taste test apples to evaluate their ripeness.
The Starch Iodine Test
As fruit ripen, they lose starch. Change from starchy to sweet is generally a good indicator of ripeness, and is used by many apple growers, along with the other methods mentioned above, to determine when to harvest their fruit. This is because apple ripening involves converting starch into simpler sugars, which starts in the core, and then continues throughout the apple flesh.
Starch-iodine testing involves spraying an iodine solution on the apple flesh. The solution is commonly composed of a 4% potassium iodide along with 1% iodine solution that only reacts with the starch, leaving a blue stain without staining sugars like sucrose, fructose, and glucose. Apples with larger blue staining areas are generally less ripe, while more ripe apples will have less and less staining over time. ‘
Images: Three apples that were assessed for ripeness by the UMN Horticultural Research Center apple team, first by taste testing, followed by a starch test, which stains starches blue.
This article from the University of Vermont does a good job of explaining how to make the iodine solution that you will use to stain the apples.
This generic chart showing change in starch pattern was developed by Blanpied and Silsby: 1992 Cornell Cooperative Extension Bulletin 221, and is often referred to as the “Cornell chart”. It was originally meant as an aid for predicting harvest windows for fruit that would be placed in long-term cold storage. The bulletin also lists the steps to sample and stain the apples.
The example in the Cornell chart is for McIntosh, and ranges from a scale of 1 to 8. But not all apple varieties lose starch in the same pattern as McIntosh. McIntosh apples first lose starch from the core area around the seeds, then from the fleshy part that we eat.
Honeycrisp apples do not always lose starch from the core before the flesh does. Ines Hanrahan, who works for the Washington State Tree Fruit Research Commission, developed a chart specifically for Honeycrisp apples that uses a 1-6 scale. So we can use the Cornell chart for many, but not all apple varieties.
Growers who anticipate storing apples for longer periods of time may want to consider harvesting apples with slightly more starch since apples continue to ripen slightly post-harvest*.
*How much ripening occurs during storage can relate to factors like how quickly fruit is cooled, oxygen levels in storage, and the use of ethylene action inhibitors like 1-MCP.
Penetrometer and Brix Testing
Images: A penetrometer is used by first removing the apple skin and applying the penetrometer to the apple flesh (left). Refractometers can be used to measure sugar level in cider apple production, primarily (right).
How easily the flesh of an apple is penetrated using a penetrometer tool can help measure how firm a fruit is and indicate its ripeness. Higher readings typically indicate firmer apples, whereas low readings can be a sign that an apple is becoming overly ripe and mealy, although extremely crisp varieties may easily be penetrated without being mealy.
Brix testing, using a refractometer, is frequently practiced in wine and cider production to measure total sugars present in the fruit, which steers the final alcohol concentration after fermentation. Testing brix levels on fresh eating apples can be helpful and complementary to starch testing, but may not be the most accurate way to measure maturity, as it does not always correlate with overall ripeness.
Thank you to our farm and ag professional partners for contributions to the UMN Fruit Update series. Non-credited photos in this article were either taken by Madeline Wimmer or within the UMN Extension system.
