Iodine Test – Avoiding a False Positive

[dbüscher]

Here is an interesting thing I learned about the iodine test for starch conversion in your mash...

So you don't have to read the rest of my story, I learned that it is best for the test wort sample to be clear, ie, post-vorlof with little or no bits of grain in it in order for the test to work properly.

If the wort you are testing has grain material in it, particularly if you are brewing with a wheat malt, then the prevalence of those grains in your sample could lead to a false positive reading.

Of course this is common knowledge to some (or most) since I read it in a 1999 article of BYO and then heard in on the BN.

I learned my lesson after mashing my Weizenbock. Each time I drew a sample after the first hour and it came back negative, I kept telling myself "Just a half-hour more." This continued on until I mashed for approximately 2.5 hours. During this time I was furiously searching the interwebs looking for answers, thinking that somehow I effed up my mash and didn't get proper conversion. In the end, I said fuck it and made a brief offering to the brewing gods (they live in a shrub out back where I sometimes pee after drinking during a brew session) and just went on with my sparge, lauter and boil.

The beer turned out fine, but since then I have started taking my sample using clear wort after vorlof.

You can read all about my brewing shenanigans on my new blog: buscherbrauhaus@blogspot.com

[Rob Martin]

Well, I've been using iodine tests for years but never knew this false positive issue. Explains why sometimes we get a quick mash and other times it just goes on and on and on and on....

[Jensen]

I wish Brewing Techniques magazine was still in publication...

[Pentaquark]

Query: Can this test be done with iodiDE? Is there a major chemical difference? And do I have to dilute the iodine to enable better results?

[JMcG]

Just use "Tincture Of Iodine" straight off the shelf from the disinfectant section. One drop onto a small puddle of (clear, no grain husks or cloudiness) wort on a white plate and you should see no color change. If it turns black or purple it means starch is present and the mash hasn't finished converting to fermentable sugars. Cloudiness in the wort implies starch is present. The sugars dissolve in the wort so its "clear", but the starches have limited solubility and are sort of suspended in the wort making it cloudy.
I know a lot of brewers don't bother to check the test because with well modified modern malts conversion happens pretty quick (sometimes 15-20 min!). Most just go ahead and mash for 60 min and if they are seeing clear wort after "vorlaufing" the wort is pretty much ready.
jim