On the Road to Conversion

Performing a starch test with iodine can help you know when your grain is ready to mash out and sparge.

Jester Goldman Mar 18, 2016 - 6 min read

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Some brewers begin with malt extract before making the leap into all-grain brewing, while others dive right in. Either way, converting grain to sugary wort takes some work. You can buy the extra equipment, and you can choose how much of the chemistry you want to understand, but you’ll still be left with the question of how long to let the mash sit there before it’s ready for you to mash out and sparge. A lot of books and recipes offer the general rule of thumb that 1 hour is plenty of time, although some outliers swear by anything from 20 minutes to 2 hours. The truth is that mash time is dependent on a number of factors, including thickness of the mash, the consistency of your grist, and the proportions of grain types you’re mashing (for you serious nerds, I’m talking about diastatic power). It’s complicated, and a single size doesn’t fit all.

Enter iodine. Performing a simple starch test with iodine can help you figure out whether it’s time to mash out. Iodine turns a purplish black in the presence of starch, so you can test a small sample of wort to indicate whether the enzymes in your mash have done their job or whether they need some more time. All you need is a clean white surface and some iodine tincture. Most people use a white plate for the test, but some prefer to use a piece of white chalk. Either one works well, even with dark beers such as stouts; between the mash dilution and the neutral background, it’s easy to recognize the color distinction. You can easily find iodine at the drug store, where you should buy the smallest container you can because it will last forever. It’s also possible to use the iodine-based sanitizer Iodophor, but I don’t recommend that. The tincture is cheap enough, and in my experience, it shows the color shift more quickly and clearly than Iodophor.

To get a sense of how the test works, try it just after you mash in. Put a small bit of iodine on your plate or chalk; it will have an amber to reddish brown color. Then add a sample from your mash tun. The iodine should immediately darken to purple or black. That quick shift is what you’re be looking for.

Performing the test at 15-minute intervals during the mash can be very educational. Each time, clean your plate, collect a couple of drops of wort from your mash tun using a spoon or an eye dropper, and put them on the plate. Add a drop of iodine to the wort and watch for a quick change to purple or black. You can drip the iodine directly into the sample or put a drop next to the wort and swirl them together; either way the change should be sharp and sudden. If you see that, then give your mash another 15 minutes before testing again.


If the color stays relatively stable, you have an indication that the starches have been converted. Keep in mind, though, that the test is only showing you whether there’s any remaining starch to convert to sugar. Even if you see no color change after the first 15 minutes, it’s a good idea to give your mash at least 30 minutes to allow for full saccharification. Otherwise, you might hit your target starting gravity but without as many of the short-chain fermentable sugars you want.

The biggest problem that people have with this test is in getting false positives, where the iodine never stops turning black. Often, this happens when there are solid bits of grain and husk in the wort samples. Even if the starches have been converted in the available liquid, solid chunks of grain will immediately trigger the color change. You can avoid this by ensuring that you pull only liquid from your mash. The simplest technique is to make a depression in the top of your mash and let it settle to clarity before you collect your wort sample.

Brewers can also be confused by seeing the test darken gradually over time. This generally happens because very small particles of grain trigger the iodine. The effect you’re looking for is very swift, so these slow changes are effectively a negative indicator for soluble starch.

Once you gain experience, you may decide to abandon this test, but it’s a good idea to revisit it whenever you make significant changes to your equipment or your mash process.

Want to get the most from your grain? Sign up for CB&B’s _Advanced All-Grain Method _online class and take your all-grain brew day to the next level.