• IodineTest

Iodine Test

Iodine reacts with starch and turns a very dark (out of the bottle it is a brown colour). Since the mash is all about converting starch to sugar, this simple test gives a visual indication of how "converted" your mash is.

A few folk on rec.craft.brewing pointed out that this is "wrong".

1. Husks must be removed (obviously they weren't).
2. Samples should be from the same place of mash (it was).
3. Samples should be cooled (they were).
4. The reaction colour should be blue.
5. The reaction is easier to see when the iodine is diluted.

Fair enough but there is clearly some change in colour in the images below - the whole point is, if you are a homebrewer, learn how any measurement (temperature, pH, iodine, etc) relates to your brewing process.

Test for Starch in Food - more in depth information here.

Here are a series of pictures I took of iodine tests performed at 10 minute intervals through a mash. The recipe was a fairly conventional APA and the grain was freshly crushed (thanks to my homemade RollerMill) and the mash temperature was 67°C.

Please take into consideration that these photos were not all under identical conditions. For example, I took 3 samples and tested them before taking the first photo. Having said that, it didn't appear to have much bearing on the outcome, what you see after 10 minutes seems to be the same as what it was after 5 seconds.

Iodine test using dry crushed malt. This is the colour of soy sauce. Note that there was no mash water in this sample to dilute the iodine.

After 10 minutes. To me this looks very similar to the first one, some dilution notwithstanding. In both this and the first image, the stain on the plate is a dirty brown colour and the grain itself is very dark. The plate stain here is lighter but this may simply be due to the fact that this was taken from the mash, not dry crushed malt.

After 20 minutes. Here again the grain is very dark and there is some of that dirty brown stain on the plate. Also you will notice a purple colour stain, I have no idea what this means but it was present only in this sample.

After 30 minutes. The grain is still pretty dark but the free iodine smeared on the plate is not nearly so dark.

After 40 minutes. Even though the lighting here is a little different, there is lots of free, brown iodine so I conclude that conversion is well progressed. I had planned for a 60 minute mash which I did, but I didn't bother taking any more samples.