Iodine Test
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This article explains why an iodine test should be performed, how it works and how it is best done. ContentsPurposeThe color reaction between iodine and glucose chains (dextrins and starch) is used to detect their presence in wort. Aside from producing a wort of desired fermentability it is the goal of mashing to reduce the maximum length of dextrins in the sweet wort to less than 9 glucose molecules for unbranched and less than 60 for branches chains. At this point they don't show a reaction with iodine anymore and the wort or mash is said to be iodine negative [1]. If that is not done they final beer may develop a so called "starch haze". Despite its name in most cases this haze is not caused by starch but by long dextrines which become less soluble and precipitate in the presence of alcohol. Those dextrines give a red to purple color reaction with iodine [1] How does it work?When glucose chains are sufficiently long they coil up like springs. This coil is supported by weak links between the glucose molecules. These links break down at high temperatures and yhe glucose chains uncoil. When the chains are longer than about 9 glucose molecules a triiodide ion (I3-) fits inside the coil (Figure 1). The resulting iodine dextrin molecule absorbs light and the cause of the typical color reaction between iodine and starch. The longer the glucose chains are the more iodine molecules fit into the coils and the more intense the color reaction will be. Iodine solutionsIodine by itself is very poorly soluble in water. One way to dissolve iodine in water is to add potassium or sodium iodine. Those salts dissolve into potassium or sodium ions and iodine ions. The iodine ion (I-) reacts with the free iodine (I2) to form a triiodide ion (I3-) which is still soluble in water and can react with glucose chains. A solution of iodine and potassium iodine is also called Lugol's iodine and was one of the first uses of iodine as a disinfectant. further improvements have lead to the use of other solubilizing agents. That lead to the iodine products that we use today for sanitization purposes. All those products enable the presence of free iodine in solution and are therefore suitable for an iodine test. For starch test purposes in brewing the concentrated iodine solutions are best diluted with ethanol. This is mainly for keeping the color of the test solution to a light yellow which allows for a better observation of the color reaction. Ethanol works better for diluting iodine solution than water due to the better solubility of the iodine in alcohol. A simple starch test solution can be made from 1 part Iodophor and 9 parts rubbing alcohol. Those are both ingredients that brewers tend to have at hand. If you don't use Iodophor in your brewing and have none at hand, you may also buy Lugol's iodine. And make a starch test solution from 1 part Lugol's iodine and 9 parts rubbing alcohol. Lugol's iodine can be found online. While iodine's use in Methamphetamine production [2] has caused iodine sales to be restricted, small amounts can still be bought without any problems [3]. When looking for iodine products at the grocery store or pharmacy make sure not to get a substitute which doesn't actually contain iodine. A starch test solution made from Lugol's iodine tends to give a clearer reaction with starch than one prepared from Iodophor. This might be because the iodine in Iodophor is release slower compared to the iodine in Lugol's iodine, but both solutions are still equally well suited for starch testing in brewing. The iodine test solution is best kept in a small eye-dropper bottle clearly labeled "starch test", obviously. Perfoming an iodine testSo much about the science. In brewing an iodine test is best done on a piece of white chalk or drywall. In my own brewing I use a piece of white sidewalk chalk that I took from the kid's bucket of chalks.
Alternate starch test methodsMost brewers have been taught to do a starch test on a white dish. The problem with this method is that the mash sample taken may contain husk and grit pieces which can react with iodine and lead to a false reading.
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