What is the iodine test for beer?

In this brief article, we will provide you with the answer to the question: “What is the iodine test for beer?”, discuss how to perform this test, and share some curiosities about beer. 

What is the iodine test for beer?

The iodine test confirms that all the available starch in the mash has been converted to sugars.

Turning grain to sweet wort is a laborious process. It is necessary to figure out how long to let the mash rest before you mash out and sparge. Mash duration is determined by a variety of parameters, including mash thickness, grist consistency, and the quantities of grain types you are mashing. 

So, a simple starch test using iodine can help you determine if it is ready to mash out. Iodine becomes purple-black in the presence of starch, so you may test a little sample of wort to see if the enzymes in your mash have completed their task or if they require further time. 

How to perform an iodine test?

It is quite simple to perform the iodine test.  

You only need a clean white surface and some iodine tincture. For the test, most individuals use a white plate. The color contrast is simple to discern between the mash dilution and the neutral backdrop. 

Iodine is available at drug stores, where you should get the smallest quantity possible because it will last indefinitely. Iodophor, an iodine-based sanitizer, can also be used, although it is not ideal. The tincture is reasonably priced and exhibits the color shift faster and more clearly than Iodophor.

Put a tiny amount of iodine on your plate or chalk; the color will range from amber to reddish-brown. After that, add a sample from your mash tun. The iodine should deepen immediately to purple or black. What you are searching for is a fast shift.

It might be highly educational to perform the exam at 15-minute intervals during the mash. Clean your plate each time, then take a couple of drops of wort from the mash tun using a spoon or an eyedropper and place them on the plate. Add a drop of iodine to the wort and wait for it to become purple or black. 

You may either drip the iodine straight into the sample or place a drop next to the wort and swirl them together; either way, the change should be abrupt. If this happens, give your mash another 15 minutes before trying it again. If the color remains pretty consistent, it means that the starches have been transformed. 

Tip: Keep in mind that the test simply indicates if there is any leftover starch to convert to sugar. Even if you do not see any color change after the first 15 minutes, give your mash at least 30 minutes to allow for a full fermentation process. 

The most common issue with this test is false positives, in which the iodine never stops becoming black. This frequently occurs when there are substantial particles of grain and husk in the wort samples. 

Even if the starches have been transformed in the available liquid, substantial bits of grain will cause the color to change instantly. You may avoid this by simply extracting liquid from your mash. 

Another confusion is that sometimes the test becomes darkening over time. This is usually due to very minute grain particles triggering the iodine. Because the desired impact is relatively rapid, these modest changes are essentially a negative signal for soluble starch.

Curiosities about beer

Beer is one of the world’s oldest beverages

Beer making was first documented around 6,000 years ago, most likely in Mesopotamia – an area that today corresponds to Kuwait and Iraq. There was evidence that the Sumerians, the indigenous people, used a fermented drink made from grains as early as 21000 before Christ.

The Egyptians built the pyramids under the influence of this beverage

According to University of Pennsylvania archaeologist Patrick McGovern, the builders of the pyramids were given around 4 liters of beer every day. 

In 1814, London was flooded with beer

In 1814, the main tank at The Horse Shoe Brewery in London ruptured and dumped nearly 1.5 million liters across the city. The accident caused the death of at least eight people.

US President George Washington brewed his own beer

According to his granddaughter Martha Washington, the president consumed a beer made on the Mount Vernon estate – a palace located in the state of Virginia. There are reports that George Washington also served the drink during parties.

The Czech Republic is the country that consumes the most beer in the world

According to the 2016 survey by the Bath Hass Group, the average annual consumption is 143 liters per resident of the country (based on the population over 18 years old).

For more curiosities about beer, click here


In this brief article, we provided you with the answer to the question: “What is the iodine test for beer?”, discussed how to perform this test, and shared some curiosities about beer. 


Craft Beer & Brewing. “On the Road to Conversion.” Accessed January 19, 2022. https://beerandbrewing.com/on-the-road-to-conversion/.

Forbes Brasil. “9 Curiosidades Sobre a Cerveja,” August 4, 2017. https://forbes.com.br/forbeslife/2017/08/9-curiosidades-sobre-a-cerveja/.

Hi, I am Charlotte, I love cooking and in my previous life, I was a chef. I bring some of my experience to the recipes on this hub and answer your food questions.