The Starch Conversion Test for Mash Conversion


Starch Conversion Test   Starch conversion is done by the enzymes in your malted grains. Your goal is to get 100% mash conversion.

How long should you mash? Most single infusion mashes are run for 60 minutes, but you’ll see some recipes calling for 45 minutes and some calling for 90 minutes. Which is right?

There seems to be a disagreement about what the optimum timing should be. Some say that the starch conversion takes place much quicker, and that 60 minutes is a waste of your valuable time.

Others suggest mashing up to 90 minutes to get full conversion of a high gravity mash. How about this idea, start running the starch conversion / iodine test after 45 minutes and quit mashing when you have full conversion?

Regardless of how long you plan to run your mash, you should do a standard starch conversion test to determine whether the mash has fully converted the starch to sugars.

This test is commonly called the iodine test. You may remember from high school science that iodine causes starch (usually a potato) to turn black.

During the mash, the enzymes should convert all the available starch to sugar. But you never really know when they have finished their job without running the test.

Starch Conversion Test-Methodology

Iodine bottle
Iodine bottle

Purchase all of your measuring and testing equipment at

To perform the test, place a few drops of wort (try not to get too many solids) in a small white bowl. (I like to buy the tiny white bowls from Walmart, these are with the Correlle dishes or you can get the small white ramekin bowls.)

Add a couple of drops of iodine to the wort.  If the starch conversion is incomplete, you will notice the iodine turn black. This means there is still starch in the mash that needs to be converted to sugars so you should continue mashing.

When there is complete starch conversion, the iodine will remain a reddish brown color indicating no starch is present. Check out the YouTube Video, ” How to do an Iodine Mash Conversion Test” by Nor Cal Brewing Solutions:

Sometimes you will get a false reading on the surface, ie. you put the iodine on the surface and it stays reddish brown. If you give the bowl a slight swirl, enough to get the iodine below the surface but now so much that it mixes with the entire contents, you will sometimes notice it turn black. If this is the case, continue mashing until it stays reddish brown after you swirl the cup.

Sometimes the simplest tests are the most informative. If you don’t like the complications testing involves and think they are a waste of time, start running your mash pH and starch conversion test. You might be surprised to find that your efficiency goes up and certain beers that have always come out astringent are now smoother and more of a pleasure to drink.

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