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Testing Mash Starch Conversion

As I was doing my second mash ever, I started to worry that my starches weren’t converting to sugars.  For the sake of “better safe than sorry”, I ran over to the drug store and picked up a bottle of iodine.

As it turned out, I had nothing to worry about, but it’s nice to have the bottle around in case I ever decide to worry about this again!  I took some pictures of both passing and failing tests to show the difference.

The Iodine Test

An Iodine Test uses the reaction between starch and iodine to check for the completion of the starch converted in the mash.  All-grain brewers often use the iodine test to confirm the success of their mash.  A successful mash means that all of the starch in the grains will have been converted to sugar.

To perform an Iodine Test, you’ll only need a few items.  A white plate or bowl, a small bottle of iodine, and liquid from the mash tun.

Pour a bit of your mash liquid in to the plate or bowl.  Make sure not to include any solid material (grain husks), as they’ll taint your results.  Now add a drop or two of the iodine.

If there is still unconverted starch, the iodine will quickly turn the wort to a black / dark blue color.  If the starch conversion is complete, the iodine will remain a reddish brown color.

NOTE: Iodine is a poison.  Do not return the sample to the mash tun.

Additional Resources

BYO – brewing tools
Home Brew Wiki – Iodine Test
Winning Homebrew

{ 1 comment }
  • WVhomebrewer September 6, 2015, 11:30 pm

    Have you heard of iodine sanitizing solution used for the iodine test?

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