How to do an Iodine Test for Starch

This is a way to tell if your apples are ripe or not. It relies on the fact that unripe apples contain starch granules which have not yet been converted to sugar by natural enzymes in the fruit. Once the fruit is ripe, all that starch will have been changed to sugar and will be undetectable.  One of the properties of iodine solution is that it reacts with starch to give a blue-black colour, but it doesn't react with sugars. Many mid and  late season apples are not physiologically ripe when they are picked or fall from the tree, and they need a period of storage before all the starch is converted into sugar and are at peak flavour. This is a way to monitor that process.

You need a bottle of 'tincture of iodine' which you can get at a High Street pharmacist or from an online supplier, and ideally a small dropping pipette, as in the picture.

Starch Test

Slice the apple and drip a drop of iodine onto the cut surface. In a few minutes it will turn dark blue or black if there is significant starch present (unripe fruit). If not, the drop will just leave a slight yellow stain of the apple flesh as in the bottom row of the picture (ripe fruit).  The sample at top left is a 'positive control' i.e. a potato or an apple known to be unripe, which shows the characteristic blue-black reaction. That shows that your iodine is OK and is doing its stuff. (BTW the apples are Foxwhelp and Frederick which have notably red-tinged flesh below the skin).

A note about iodine supply

Tincture of iodine isn't so readily available on the High Street as it once was. Newer types of topical antiseptic have taken its place. But you can still buy it from mail order pharmacists online. It is also available from some home brew suppliers since it's used to test for starch conversion during grain malting and mashing.  

However, avoid formulations of Povidone-Iodine aka Betadine. These don't work in the starch test. Although they do contain plenty of iodine and so work as antiseptics, the iodine is already complexed with PVP (povidone) and so it's no longer free to form a complex with apple starch. There is a slight reaction, but it's very very weak and doesn't have the sensitivity that we need.

If you are semi-profesional and want to do lots of iodine tests, you'd be better off to make up your own solutions. There are many recipes used by commercial growers, but here's a link to just one (the Cornell test) which also gives a rating scale for starch conversion depending on colour intensity. It's used by commercial dessert apple growers to tell when to pick their apples for controlled-atmosphere store.

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Last revised 3rd September 2016