Iodine test never turned negative

[GlowingApple]

I brewed an amber ale today and decided to try using chalk for an iodine test (did a vial test for my previous couple of all-grain brews). Even after mashing for over 90 minutes the test still wasn't negative on chalk. In the vial it looked negative, but the color is really hard to tell in solution.

After 30 and 60 minutes it looked like B in the image from Braukaiser: http://braukaiser.com/wiki/images/4/42/Iodine_test.jpg

After 90 minutes it actually got darker (darker than B, not quite as dark as A though). Even after 105 minutes it still was closer to B than C.

I carefully decant the wort so there shouldn't be any grain hulls.

I mashed at 152 ˚F. My mash tun is losing about 4 ˚F/hr, so it was somewhat low toward the end of mashing, but the specific gravity (by refractometer) pretty much leveled off at my calculated pre-boil SG around 90 minutes (30 min = 1.040, 60 min = 1.046, 90 min = 1.048, 105 min = 1.049, bringing to a boil = 1.048), so I can't imagine there were unconverted starches after 105 minutes.

Any ideas what went wrong?

 

[tezcatlipoca]

Not a clue, but I'd like to be here for when someone does let you know what happened.

 

[broadbill]

With the iodine test, I find that even a converted mash will eventually go to the "B" sample if given enough time. If the iodine doesn't go purple in the first 2-3 seconds after adding, I call that a negative test. For comparision, try doing a test at the beginning of the mash to see how fast you get a positive result (almost instantaneously after adding iodine).

I also tried the chalk thing, I didn't see the benefit. I still do the white plate as it allows me to spread the sample around and test a few spots.

 

[Oginme]

I use a small plate with a couple of ml of water on it. I place a couple of drops of the wort on the plate to mix with the water, then rotate so that only the liquid moves and place a drop of iodine on that. Small suspended particles of unconverted starch will give you a false blue reading, when the dissolved materials are fully converted.

 

[doug293cz]

I use a small plate with a couple of ml of water on it. I place a couple of drops of the wort on the plate to mix with the water, then rotate so that only the liquid moves and place a drop of iodine on that. Small suspended particles of unconverted starch will give you a false blue reading, when the dissolved materials are fully converted.

How can your mash be fully converted if you have "suspended particles of unconverted starch"? My understanding of the mash kinetics is that the conversion of solubilized starch occurs faster than the solubilization (at least for coarser crushes.) Thus to know if your mash is done, you need to test the particulate mater to check for starch.

Brew on

 

[Oginme]

Ever hear of mash efficiency? My mash efficiency runs right around 90%, which means that I am extracting and converting 90% of the possible starches. 10% still remains. I probably should have used suspended grain particles in my description, as they are probably mostly protein with some unconverted, some unextracted starch molecules in those particles.

 

[Gavin C]

Ever hear of mash efficiency? My mash efficiency runs right around 90%, which means that I am extracting and converting 90% of the possible starches. 10% still remains. I probably should have used suspended grain particles in my description, as they are probably mostly protein with some unconverted, some unextracted starch molecules in those particles.

Not so.

What you are describing is conversion efficiency. (How much of the starches converted). Typically this should be north of 95% with all brewing models having it knocking on the door of ~100% (Not obtainable but you should be close)

Mash efficiency is lower because not all of this sugar makes it to the kettle. (Spent grain is wet with sweet-wort)

An iodine test for starch has a very poor specificity with the starches in solution rapidly converting.

Mash efficiency=conversion efficiency x lautering efficiency

Positive test for starch right after dough-in

Negative test

 

[GlowingApple]

Just wanted to follow-up. Next time I'm going to try a white plate and see if I get different results. I think the chalk test was accurate though... after cold-crashing with gelatin and sitting in the keg for over a week the beer is still cloudy. Warming it up the cloudiness doesn't change, so I think it might be starch haze.

 

[GlowingApple]

Brewed this amber ale again yesterday, and had the same issues with the iodine test. I checked the SG every time with a refractometer and used that as a judgement for when the mash was complete.

I tried the test on chalk and in a bowl and it seemed that while the chalk test was always positive, the bowl test seemed like it was negative after the dough-in test? Unless I'm reading that test wrong (see attached picture). I discovered that if I took a sample from the wort before stirring the iodine test on chalk came back less positive, maybe negative, but if I took a sample after stirring it was always positive. During mash-out I took a sample before and after removing the grain bag, tested both on chalk (both were positive), and then let the samples settle while boiling. I tested them later on and they were both negative.

So I'm thinking that the fine flour in the mash is causing false-positives? Next time I might try diluting my iodine solution further to see if that helps at all. Anyone have any other ideas?

 

[urg8rb8]

What's in the bowl shows that there is a lot of starch in the wort.

 

[Bear979]

The way malt is currently made, most conversion is done in 15-40 minutes. I recommend stirring everything up really well with the mash paddle, then using a clean spoon or whatever to take a same. If you use the mash paddle any little bits of grain or dust will create a false negative. If youve milled so fine its basically flour, that would also create the false negative.
Hope this helps for the next one!