This is an old article that has been replaced by Iodine Test
An starch test uses the reaction between starch and iodine to check for the completion of the starch conversion in the mash. It is not required to perform such a test, but it is seen as good practice as it would alert the brewer of problems during the mashing process. Reasons for a mash to fail to convert could be improper temperature (faulty thermometer), completely inadequate mash pH or insufficient diastatic power of the malts (large amounts of adjuncts, decoction mashing).
When mixing starch or large chained dextrines and iodine a color reaction will appear. The darker and more intense the color is, the longer the chains and/or higher the concentration of starch is. [Noonan, 1996]:
"Blue-black indicates the presence of native starch (amylose); deep mahagony/red-brown evidences gelatinized starch (amylose fragments and large a-limit dextrins), faint red simple a-limit dextrins. A faint mahagony to violet-reddish reaction denotes a mix of small dextrines. Total mash saccrification (a solution of some small a-limit dextrins with maltotriose, maltose and simple sugars) causes no change in the yellow color of iodine"
More on the reaction between iodine and starch/dextrins can be found in Carbohydrates - Reaction with Iodine
How to perform an iodine test
An iodine test requires a white saucer, a small bottle of iodine with a dropper, and a small sample of liquid from the mash tu.
Let the liquid drip onto the saucer, being careful not to include any solid material such as grain husks, which could give a false positive reading. Then add a drop or two of iodine. If there is still unconverted starch in the mash, the wort will quickly turn dark blue or black.
Iodine is a poison: regardless of the color, do not return the sample to the mash tun.
Iodine test on chalk
Another elegant way of performing an iodine test is to test for iodine on chalk. Take a piece of side walk chalk or dry wall and drip one drop of mash liquid onto it. This can easily be done with the probe of the thermometer or the mash paddle. Now add a drop of iodine and observe the reaction. To test again use a knife to cut off of the piece of chalk that was used for the test. Now you have a clean surface again. The following pictures of an iodine test on chalk were taken during a mash of Briess Pale Malt at 153 *F (67 *C):
The iodine reaction shortly after dough-in. Note the strong dark blue to purple reaction.
The iodine reaction 10 min later. As you can see the reaction is significantly less, but there are still many long chained dextrines left.
At this point the reaction is only faint and the mash is almost converted. Because of the high diastatic power of the pale malt and a relatively high mash temperature the conversion was done in less than 30 min. However, the mash was done for 60 min since the mash temperature and time were chosen to achieve a targeted fermentability.
No significant reaction left after 30 min.
The same after 50 min.
- [Noonan, 1996] Gregory J. Noonan, New Brewing Lager Beer. Brewers Publications, USA, 1996