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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.

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