Are your late season storage apples ripe? Find out with the Iodine Starch Test

One simple, effective method to check apple ripeness is the iodine starch test. This test reveals how much starch remains in the apple, which directly relates to its maturity and sweetness. Let’s explore how to perform this test, why it works, and how it helps growers and enthusiasts get the best apples.

Fig. 1 A selection of apples tested Sept 29th. Only Wickson Crab (top R) shows very sugar levels

Understanding Apple Ripeness Through Starch Levels

Apples store energy as starch during growth. As they ripen, enzymes break down starch into sugars, making the fruit sweeter and softer. The iodine starch test directly measures this starch content by reacting with starch molecules, producing a blue-black color. As apples ripen, starch converts into sugars, so less starch, or a lighter colour, means the apple is sweeter and ready to eat. For example:

• High starch (dark blue areas) means the apple is still immature.

• Moderate starch suggests the apple is nearing ripeness.

• Low or no starch indicates the apple is ripe and ready to eat.

  
  

Many storage and late season apples are not ripe at the time of harvest, they require several weeks or months to finishing the maturation process and turn from a starchy bland apple into something sweet and delicious.

Fig 2. This photo shows WA38, now known as Cosmic Crisp, at two different stages of ripeness, showing different radial patterns. (Source: WSU Tree Fruit, 2017)

But when are storage apples ready to eat?

By testing your apples regularly you will see a difference in colour/starches over time, and gain insight into when your apple variety is at it's peak for eating or using in the kitchen. Different apple varieties will also have varying starch conversion rates, so it helps to learn the typical starch pattern for your specific types. Yes, you could just taste test an apple a week from your stash to see if they are ripe, but this starch test is simple, fun and adds some data to your ripeness assessment.

Fig 3. Court Pendu Plat (top) Liberty (bottom) starch tested Nov 13 after 3 weeks of storage at room temp. These were all sugar, and over ripe for my taste.

How to Perform the Iodine Starch Test on Apples

The iodine starch test is straightforward and requires only a few materials.

Materials needed: iodine solution (available at pharmacies or garden centers), a knife, and a clean surface.

• Step 1: Cut the apple in half across the middle, exposing the core.

• Step 2: Spritz the iodine solution on the open surface, or dip the surface in a small bowl of solution.

• Step 3: Observe the color change. Areas with starch will turn a dark blue or black color, while ripe areas with less starch will remain lighter or unchanged. You will see some changes right away but wait 30 min for final colour.

Additional Resources

If you find harvest and ripeness indicators interesting, check out this page by the Washington State Extention service: https://treefruit.wsu.edu/web-article/harvest-apples/

It covers harvest maturity indicators such as firmness, seed colour, ethylene gas release and more. It will help you gain a more nuanced understanding of when to harvest your fruit next year!