Ask Mr. Wizard

Iodine Starch Testing and Defining Mash Conversion


Andrew Robb — Sydney, Australia asks,

I’ve been brewing for a few years now and have very recently joined up with BYO. In the last few days I’ve reviewed the Double IPA recipe listings, which look really good and I’m gearing up to start my first clone attempt. In the instructions is the phrase “until the enzymatic conversion is complete.” Problem is I’ve never heard that phrase before so could you help by explaining what it means and advising how I can tell when it’s complete.


Welcome to BYO where we are committed to providing current, helpful, and technically sound brewing advice to our readers! It’s always nice seeing great homebrewing questions from all parts of the world and we thank you for the query from down under. Now, onto the question at hand: Mash conversion.

Brewing is an ancient practice and the terms used by brewers are a hodgepodge of words, phrases, and units used by brewers who came before us. Some of these terms, like conversion, are commonly used by brewers but not always very helpful when it comes to really trying to make meaningful changes to beer flavor. Conversion simply refers to how starch, specifically amylose, reacts with iodine. Take a sample of wort early in the mash, place it on a white plate, hit it with a drop of iodine solution, and a deep purple/inky black color immediately forms. As the mash rest is extended and starches are transformed by malt enzymes, especially alpha amylase, the color-forming reaction between wort and iodine ceases and the mash is said to be converted. Although this may sound like some religious experience for the mash, conversion is a bit nebulous. Let’s dig a bit deeper into malt starch, malt enzymes, and what happens during mashing.

There are two types of starch found in cereal grains ranging from Amaranth to Zizania: Amylose and amylopectin. Starch is a glucose-based polymer, where each glucose sugar molecule is a monomer. And these glucose monomers in starch are either linked by a bond referred to as alpha 1-4 or alpha 1-6. Amylopectin is a branched molecule including alpha 1-4 and alpha 1-6 bonds, but amylopectin doesn’t react with iodine and is not part of this discussion. However, all of the glucose monomers in amylose are linked by alpha 1-4 bonds and amylose polymers react with iodine to form colors ranging from colorless to deep purple/inky black. Although the color is related to the size of amylose polymer, the iodine test used by brewers is declared in absolute terms of positive and negative. Iodine-positive wort appears purple/black in the presence of iodine and iodine-negative wort has no real color change. Pretty subjective for sure. So, what does this test reveal?

Although beta-amylase is the enzyme in malt that produces fermentable sugars, mainly maltose, from amylose and amylopectin, it does not have a drastic effect on the reaction between amylose and iodine. Alpha amylase, on the other hand, produces little fermentable sugar from amylose and amylopectin, but it handily chops large starch molecules into smaller chunks that beta amylase can digest. Brewers need both enzymes to produce wort that has a high percentage of fermentable sugars. The iodine test is really telling us something about the action of alpha amylase on amylose. Iodine-positive wort is low in fermentable sugars and the iodine test is a handy way of signaling brewers that the enzymes need more time to do their thing. That’s an anti-climatic conclusion, but that’s really all the iodine test tells us. Wort could be very low in fermentability and be iodine negative, but in most cases, conversion signals that things have generally gone as planned. In that regard, an iodine-negative test result is a pretty good indicator of a successful mash.

Time for a Mr. Wizard digression about this test. There was a time when I followed the rules of brewing and always checked for mash conversion before mashing out because good brewers follow rules and enzyme denaturation is irreversible. After umpteen negative iodine tests after the 70 °C rest (158 °F for brewers who are not yet hooked on metric) I said to myself “Ashton, this is silly. Modern malts are super consistent compared to malts from the past when this test was made popular. It’s time to stop pulling samples that don’t tell me anything new.” And that was when I stopped performing the iodine test. That was about 25 years ago. I thought this was maybe my lazy little secret and kept it to myself.

While amylopectin, pictured here, doesn’t react with iodine, it’s sister starch amylose does. Alpha amylase and beta amylase will chop both molecules down but amylopectin cannot be fully broken down into simple sugars unless exogenous enzymes are added to either the mash or the fermenter.

A few weeks ago, I was having a phone meeting with a great brewer at a brewery that will remain nameless and two of my work colleagues who are both brewing scientists. We were discussing the use of exogenous enzymes, amylo-glucosidase and pullulanase, to produce highly fermentable wort (you can learn more on this topic in the October 2020 issue’s “Advanced Brewing” column). One of my colleagues asks the brewmaster we were meeting with “was the wort iodine negative?” And the brewmaster said “I am embarrassed to admit that we did not perform an iodine test because they are always negative and we eliminated that step in our process years ago.” Oh boy, I was smiling a toothy smile after that confession because I knew that I was not alone! Indeed, many modern brewers declare starch conversion complete when the mash cycle, defined by ratio of malt and water, mash pH, mash temperature, and mash time has ended. Some always use the iodine test just to be certain and others get on with the day. One counter argument to this though is that your thermometer is not reading correctly and you have missed your target mash temperature. You can see one such example in the May-June 2020 issue of Mr. Wizard, available at

And that, Andrew, is what brewing recipes really mean with the phrase “when conversion is complete.” Now, it’s time to vorlauf; ciao!

Response by Ashton Lewis.