I would be interested to know if iodine tests are a reliable way to know that all the starch has been converted to sugar.

For instance, is it a good way to know if after 30mn of mash-in all the sugars have been converted? Is there a risk of false negatives? false positives?

Please note that this question is different from this one because I'm not asking if iodine tests are useful in practice (I do not use it for now and never felt that I had to) but I would rather like to know if they work effectively.

3 Answers 3


Yes, they are reliable if you perform them properly.

  • You need to be able to see this blue / green hue. It takes some skill to know what's starch and what's just a color of mixing iodine with wort. Worst in dark beers.

  • Test tells only about starch in sample. If you don't stir, you can have starch still in the grain or at the bottom.

  • Even if you stir, you can have starch trapped in grain. It wouldn't end in your brew, leaving it is safe, but it hampers your efficiency.


Yes, there is very much a risk of false readings. I know of people who kept mashing for 4-5 hours because the test showed no conversion. That's pretty much impossible. As was said from molot above, if you do it right, you can get useful info, but it's all too easy to mess it up. It also tells you nothing about the types of sugars you have.
I prefer to check conversion, if I do, using the braukaiser conversion efficiency chart.

  • Thanks a lot for the link to braukaiser, the entier page is super useful! Dec 15, 2016 at 14:29

Using Iodine works very well as long as you let the wort settle before taking a test sample. If you take a test sample that has ANY grain in it, the sample will continue to show high starch because of the grain. Make sure and let the wort settle before taking a sample so that you only pick up clear wort with no grain included. That will give you a good test. Usually within an hour or less.

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