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Jun 08, 2022 View:

Iodine Test

Hello all,

Looking for feedback on experience doing iodine test on grain-in mashes. Have read the husk can cause false positive. Having used recommended dosage, temp ranges, and hold time and still not getting negative starch test results (using commercial enzymes). I use a hammer mill with 1/16th screen. Have considered taking a sample and essentially sparging or rinsing it through a fine screen (coffee filter) and doing starch test on the run off. Photo is of cooled sample after 2 hours rest at 135F with glucoamylase. Still very dark purple.


Run the mash through a coffee filter to remove the solids and you should be good to go.


You need a clean sample of liquid, any solids (which I see there are quite a bit) will give a false negative.  You'll need to let it sit for a few minutes and then just barely skim the surface to collect a couple drops of mash. I prefer to use an eyedropper to grab mash samples when I do a test (which is rare, I know my process and enzymes are working).

Is your S/G or Brix where you expect it? 



We use a paddle to stir the mash and then let the liquid drip from the end of the paddle into a dish. We can usually get a pretty grain free sample that way. 


Thanks for the tips everyone. I'm sure the solids were the issue. I just did another test at 2 hour rest filtered through coffee filter and got a dark maroon or brown color. Not purple or blue. I've read that red/maroon/brown means starch is saccharified, but I was thinking it'd be lighter like the color of thinned out iodine, or pale. Perhaps not?

Starting Brix: 20

Ending Brix: we shall see - doing a longer gluco hold than normal to see if i can get it to go lower. in the past i've been getting about 12 point drop. Trying to troubleshoot lower than desired ABV.


If your refractometer is reading 20 brix before fermentation and 8 brix after fermentation, than that is about right for full attenuation.

Reply:1 hour ago, Blockader said:

That's correct. Strain out enough liquid to do a gravity reading with a hydrometer once fermentation stops. Than compare that reading, hopefully 1.00 or a little below, with what your refractometer reads, likely around 8 with a starting Brix of 20. As long as you keep the same starting gravity you will have confirmed your ending brix and can use the refractometer for future final gravity readings.