For the first time in over 20 years, I cannot pass an iodine test

[prozac919]

I have been brewing since 1997. I have always done an iodine test after the initial mash. In the past, when the iodine test doesn't pass, I let it go for another 20 minutes or so and then all is well. Today, I am mashing a Northern Brewer German Alt Beer. For the life of me, I cannot get this test to pass. Any thoughts? Btw, I currently use a Brewers Edge Mash and Boil system for the last year which has been spot on all the time. The only change is this is the second time with this system I crushed the grains myself using an really old Phil Lysterman Phil Mill. I have used it in the past but have been lazy for the last decade and bought pre milled grains. I made sure to split the grain but leave the majority of the husks intact. I did not pulverize this.

 

[prozac919]

Think I found the problem. Mash is not as hot as the internal thermometer says. About 20 degrees F cooler. May have to go back to my old system of just using an industrial thermos if the thermometer has gone awry

 

[BrewMan13]

I've also seen the iodine test 'fail' when there was too much grain material in the sample, effectively 'skewing' the results.

 

[doug293cz]

I've also seen the iodine test 'fail' when there was too much grain material in the sample, effectively 'skewing' the results.

Actually, the iodine test should be used primarily on grain particles, if you are looking for 100% conversion. The starch that becomes soluble (after gelatinization) is very quickly converted to dextrins and shorter sugars. The unconverted starch in the mash is primarily in the un-gelatinized starch in the grain particles. If you test only wort (no grain) you can get a "done" indication while there is still a lot of starch in the grain.

The most reliable, and also quantitative, way to monitor conversion is to measure the SG of the wort in the mash using the method here.

Brew on

 

[BrewMan13]

Hmm, interesting. One of the instances I was thinking of was a decoction mash I did. By the time I did the iodine test, it had been well over an hour, and was just doing it out of habit, but it came up as still having starch.