What is starch?
Starch is a type of carbohydrate that is found in many plants, including fruits such as apples. In unripe apples, starch is the primary form of carbohydrate present, but as the fruit ripens, the starch is converted to sugar. The amount of starch present in an apple is one metric used to determine its level of ripeness, with more starch indicating that the fruit is less ripe.
The Starch Pattern Index
The starch test used in apple harvesting is based on the principle that iodine reacts with starch to produce a blue-black color. By applying an iodine solution to a cut surface of an apple, the amount of starch present can be determined by the staining pattern of the color produced. Different apple varieties may require different levels of staining to indicate the appropriate level of ripeness, so it is important to use the appropriate chart for each variety.
The starch-iodine test is one of the main preharvest tests apple growers use to determine the ideal harvest window. A Starch Pattern Index (SPI) is a visual tool used to estimate the starch levels in the fruit using an iodine solution. To determine the ideal window, testing for starch levels should begin at least four weeks before the expected harvest date, and additional tests should be done at least once each week until the optimum harvest window is determined.
Ingredients and Method
Always follow the label instructions and wear appropriate personal protective equipment when handling and mixing chemicals. Avoid contact with eyes and skin, and wear gloves when preparing the iodine solution.
Warning. Iodine is a poisonous chemical. This iodine solution should be correctly labeled and kept away from children and pets. Consider treated apples poisonous and do not feed to any animal or use in composting.
- Dissolve 8.8 grams of potassium iodide in approximately 30 ml of warm water. Gently stir the solution until the potassium iodide is properly dissolved.
- When it is properly dissolved, add 2.2 grams of iodine crystals. Shake the mixture until the crystals are thoroughly dissolved.
- Dilute this mixture with water to make 1.0 litres of test solution. Mix them well.
Use a fresh batch of solution and apples that have been recently harvested. If the temperature of the apples, or of the iodine solution, is less than 10°C, inaccurate readings may result.
- Select ten trees from different areas of the block and tag them for identification purposes.
- Take one or two apples from different sides of each tree. Choose the most average-looking sample from the outside of the tree rather than the inner shaded area.
- Cut the apple in half at the equatorial line (Test the apples within 24 hours of collection to obtain accurate results).
- Apply the iodine stain to the cut surface of the apple. This can be done using a brush, by dipping the apples into the stain, or with a spray bottle.
- Wait for at least one minute to allow the iodine to react with the starch in the apple tissue.
- Arrange the apples in order of stain percentage by visually comparing the amount of iodine staining on each apple.
- Use the chart for interpreting iodine staining for comparison to determine the starch level of each apple.
- Record the starch level. The less starch, the less iodine development there will be, so the darker the apple the less ripe it is.
- Compare the starch levels for the variety to the corresponding chart to determine the ideal harvest window for the apples.
SPI Scoring Systems:
While the testing process remains the same across the board, there are several scales used around the world to assess the results.
Cornell starch-iodine index
The Cornell starch-iodine test is scored out of 8 with one being the darkest stain, and therefore most starchy and least ripe, and eight representing the lightest, most ripe apples. This chart is generic across varieties. Some recent studies suggest that this method is less effective in determining true long term storage potential as it is based on tests done after only three months in storage.
Centre Technique Interprofessionnel des Fruits et Lėgumes (CTIFL)
The CTIFL scale was developed in France with particular emphasis on European cultivars specifically. This chart uses the classic 10-stage scale and is popular in Europe. The CTIFL starch conversion system recognizes two staining patterns, the circular staining pattern, and the radial pattern.
Laimburg scale
The Laimburg scale is a 1.0 - 5.0 ranking to one decimal place used most commonly in European regions around Germany, Austria and Italy. This score is often converted to a ten point scale after grading for use in the Streif Index Method.
The Honeycrisp Scale
The ever popular Honeycrisp apple is a sensitive cultivar with very specific storage requirements. The Washington State Treefruit Research Commision has created a specific starch index for these finicky apples, which stain in patterns inconsistent with the other varieties and starch scales.
The WA38 Scale
The WA38 Cosmic Crisp scale is a cultivar specific scale designed for the unique staining pattern that occurs in cosmic crisp apples. This scale is out of six with a 0.5 interval and has two staining patterns to account for variability.
The Granny Smith Scale
The Granny Smith scale was developed by the University of California Department of Pomology in cooperation with the California Granny Smith Apple Association. It is a 7 point scale starting at 0, with no decimal points.
The Streif Index:
Not strictly speaking an SPI, the Streif index is calculated from the results of three quality control tests; a starch test, pressure test, and a test of soluble solids concentration, or Brix. The Streif Index is meant to be a more thorough and accurate measure of maturity than using starch alone.
Calculating the Streif Coefficient
The equation F/(SSC x S) where F = Flesh firmness, SSC = Soluble Solids Concentration and S = Starch test results provides the Streif coefficient, which is then compared to the ideal values of individual varieties and regions.
Lower values mean the fruit is closer to maturity, and the Streif Index is particularly valuable not just as a tool to determine the start of harvest, but to determine the last possible date.
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