New starch charts for evaluating the maturity of apple varieties in Michigan
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EmailThis collection of user-friendly Starch Index Scales features high quality images of iodine-stained apples across the range of starch values and detailed instructions for iodine testing.
Starch testing is a foundational step in assessing apple maturity. As apples ripen, the starch they contain is converted to sugars to produce the sweet fruit we expect from ripe apples. To test maturity, fruit may be cut at the equator and the flesh stained with iodine. The starch in the apple reacts with iodine to produce a vivid, dark stain on freshly cut fruit. We can use that stain to assess how much starch remains in the apple. While this is a useful metric, it can be difficult to objectively interpret the iodine stains because the pattern of the iodine stains differs between varieties and the target index we interpret as optimum for storage (e.g., short, medium or long-term storage) differs as well.
New Starch Index Scales for 13 commercial Michigan apple varieties were produced in 2022 by Michigan State University Extension in collaboration with Storage Control Systems of Sparta, Michigan. Over the course of two seasons, fruit of 13 varieties was collected, stained and photographed by Randy Beaudry, Anna Wallis and Nicole Norris. Photographs were screened and organized into representative scales for each variety from completely unripe (0) to completely ripe (8). For some varieties, the starch never completely converted to sugars.
This collection of user-friendly Starch Index Scales features high quality images of iodine-stained apples across the range of starch values and detailed instructions for iodine testing. In addition to the starch index scale, this guide includes suggested controlled atmospheric conditions based on the firmness and starch index for each apple variety. The commercial varieties covered in this guide include Ambrosia, Braeburn, Empire, Evercrisp, Fuji, Granny Smith, Honeycrisp, Idared, Jonagold, Jonathan, Pink Lady, Red Delicious and Rome.
Download the PDF version of the Starch Index Scale
The printed version of the Starch Index Scale will be available via regional tree fruit Extension educators.
This is the first version of this Starch Index Scale. If you would like to provide feedback for future versions, please find the survey below.
Thank you to Storage Control Systems for their generous support in the production and publication of these starch guides.