The Iodine Test for Checking Mash Conversion

This week I show you how to do an iodine test to see if your mash has completed and the starches in the mash have been fully converted to sugars.

The Iodine Test for Mash Completion

How does an all grain brewer know when the mash is complete? It turns out there is a very simple chemical test you can run to check to see if you have a complete conversion.

The purpose of the mash in all grain brewing is to convert complex starches from the malt into simple sugars that yeast can easily consume. With modern highly modified malts, which are high in enzymes, it is possible to complete your mash in a very short amount of time – sometimes as little as 20 minutes. So you may not need to mash for an hour or more, but how do you know?

A simple method is the iodine test:

  • Purchase some common liquid iodine, which is available at virtually any drug store. It typically is located with the first aid supplies, as it is sold as an antibiotic for use on cuts and scrapes.
  • To run your test, draw a small sample of wort from the top of your mash tun. You only need a teaspoon size sample, and put it on a white plate so you can observe the color.
  • Add a few drops of iodine to the wort. If there are unconverted starches in the wort it will quickly turn dark blue or black. This indicates that the mash is not complete.
  • If the mash is complete it will be either clear or a slight shade of brown that is very close in color to the wort itself. This indicates that the mash starch conversion is complete and you can proceed with the sparge.

Using the iodine test can be a real time saver on brew day. With modern malts it is rare that you need a full 90 minute or even hour long mash. In many cases you can complete a mash in under 30 minutes.

I hope you enjoyed this week’s article from the BeerSmith Home Brewing Blog. Please subscribe for regular weekly delivery, and don’t hesitate to leave a comment or send this article to a friend.

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