Iodine Test Failed with adjuncts

I just brewed Biermuncher's Cream of Three Crops last weekend and after a 90 min mash I did an iodine test and failed. I figured my thermometer was out of whack so I checked, but it was fine. Decided to just give it more time. At two hours I redid the test and failed again. I tasted the wort and it was very sweet though, so I vorluafed and sparged anyway. My preboil gravity was .008 points above my estimated at 1.038 so I did get good conversion.
This is my first time brewing with flaked maize, minute rice, and rice hulls. I guess I'm just wondering if any or all of these make an iodine test unreliable? Thanks.

maybe all the rice and maize gave a lot of starch in the mash, and it has not been all converted in sugar

didinho said:

maybe all the rice and maize gave a lot of starch in the mash, and it has not been all converted in sugar

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Once the starch is gelantinized, conversion occurs rather quickly. More likely that the starch from the adjuncts did not completely gelatinize, so were not available to the amylase for saccharification. Starch from different sources gelatinizes at different temperatures, which is why some grains need cereal mashes. Flaked grains are pre-gelatinized.

Brew on

Thanks for the link to that article Doug. Not being much of a cook I didn't know there was a long and short grain rice. I checked the box I used and it is long, so maybe my mash temp was a little low (152*) for that even though it is "instant". If that was the case I probably got some when I added my mash out water and during the 15 min I let the sparge water sit, which would explain why my efficiency was still okay. Thanks again for the info.

HomebrewHandicapper said:

My preboil gravity was .008 points above my estimated at 1.038 so I did get good conversion.

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I don't think that necessarily follows. Both starch and sugar will impact gravity. A gravity reading alone cannot tell you whether or not you've got conversion. A wort loaded with starch could have the same gravity as a wort loaded with sugar. The gravity reading alone simply tells you you've got "stuff" dissolved in the water - not whether it's starches or sugar (or both).

kombat said:

I don't think that necessarily follows. Both starch and sugar will impact gravity. A gravity reading alone cannot tell you whether or not you've got conversion. A wort loaded with starch could have the same gravity as a wort loaded with sugar. The gravity reading alone simply tells you you've got "stuff" dissolved in the water - not whether it's starches or sugar (or both).

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Uh-oh 😬. I'm a couple of days away from taking a final gravity. I guess that will tell all. But you are right, I hadn't even thought of that.

I never liked the iodine test myself. Its very subjective to your available light, repeating on the same dish, and like you have discovered, perhaps the adjuncts can throw it. I think nailing down all your other processes including consistently hitting mash temps and holding for allotted time with the same crush and vessel will ultimately yield more confidence than the iodine test Just MHO on a Tuesday morning with Starbucks coffee...

Well I kegged this today and it had dropped to a 1.010 final gravity. Not record setting but more or less what I was expecting. So, I give up. Mysteries of beer I guess. If I ever come up with an answer I'll let you all know. Thanks for your help.

Did you use a paper plate? They are held together by starch.

Nope, ceramic. Good idea though and it got me thinking. I did mill my grain and mash in my kitchen. If the plate got milled dust on it though or something. Same thing I always do but maybe this time.....??? I'm thinking it was something like that though, just an odd happening. I am guessing I am gonna brew this again at some point (sample was tasty even warm and flat) so I'll try the test again if I do and report back. Thanks again for the help everybody.