Test for Starch Conversion with Iodine Tincture.

(from Brew Chem 101 p.46) Lee W Jason, PH.D.

     During the starch conversion stage of mashing, the iodine test (or iodine conversion test) is performed, a chemical test that tells just how well the larger sugars have been broken down to smaller, fermentable sugars. But, how and why does the iodine test work?

      Iodine in solution is yellow. However, if iodine comes in contact with a long, linear starch molecule such as amylose, the iodine molecule fits into spaces within starch molecule. The solution turns dark blue (or black if there's enough starch). If the starch has been partially broken down or if has a lot of branches as with large starch molecules or amylopectin, the iodine and starch molecules won't fit together as well, and the solution will turn a reddish color. If no starch, amylose, or amyopectin are left to combine with the iodine, the solution will remain yellow. 

     Using this test, the 'All-Grain Brewer' can easily determine how much breakdown of the mash has taken place. For example, a mash solution that is barely broken down will produce a black iodine test, partial breakdown will result in a red iodine test, and full breakdown to fermentable sugars and a few medium-sized sugars will result in the solution remaining the same yellow color. Intermediate shades of these colors are also possible, indicating that the degree of starch and sugar breakdown is somewhere in between the three examples.

     But beware: grain husks can also react with iodine and produce a black solution, even when all the sugars have been converted. So, the iodine test has to be conducted very carefully ( be sure to get only clear mash liquid with no floating pieces of husk) and interpreted correctly.