[joe6pack]

The iodine test only tells you if there's free starch in the mash tun, not if you've converted all the available starches in your grist.  On my last brewing session, I checked the gravity of the wort in the mash tun to see how the sugars increase over time.  This is what I read with my refractometer:

     45 min  13.4brix
60 min  13.7
75 min  14.0
90 min 14.3
105 min 14.3

This was a single infusion at 152*, 2.0 qts/lb thickness.  So this tells me my mash was complete at 90 minutes.  I wouldn't know that with the iodine test.  I would have left unconverted starches in the tun at shorter times.  I'll be using the refractometer from now on.  What do you guys think?

[tygo]

I think you're on to something   

I never bother with the iodine test.  I never bother with the refractometer to test conversion though either.  Might be an interesting experiment to try one of these brew days.

[davidgzach]

I have not performed an iodine test in 20 years.  Agreed that this would be a good experiment.

Dave

[liquidbrewing]

I think you already answered your question.    Why bother with either of them, if you got full conversion at 90 mins? 

 I have been mashing for 90 mins for the last 2 years, after starting with really poor efficiency on my system.  Mashing 90 minutes was my solution.  I've never tested anything in the mash.

[tomsawyer]

I did this same thing when I was mashing at 3.5qt/lb for no-sparge.  As I recall it took nearly 2hr to reach completion.  I agree about the iodine test, the refractometer is simple.  I don't usually do this for mashes though, because you saw for yourself only a 4% difference between 60 and 90min.  Not really noticeable in the grand scheme of things.

[nateo]

I use the highly calibrated instrument that is my tongue. The mash darkens and tastes sweeter as it converts. I did iodine tests for a while, but I could never get the hang of it. After one wheat mash where I got a positive iodine results for two hours, I decided either the test was unreliable, or I couldn't competently perform it. Either way, it wasn't working for me. 

[denny]

I've been saying for years that the iodine test is useless.

[jds357]

There is also another way you can check for conversion.  This method was discussed in the book "Brewing Better Beer" by Gordon Strong.  It's not a scientific way of doing it nor does it require equipment, only experience and your eyes.  During the early stages of the mash, the liquid is hazy or cloudy.  This is because of the presence of starch.  After some time, you will notice extreme clarity in the liquid.  This indicates that all available starches have been converted. 

I've never heard the way you've described but if it works, it works.  I only use iodine when I teach my friends home brewing. 

Jonathan

[denny]

I've started using Kai's conversion efficiency chart.  That's a far better way to check for complete conversion IMO.

[The Professor]

I've been saying for years that the iodine test is useless.

Same here.
I haven't bothered with it  for probably  the last 22 years or more.

[tschmidlin]

Add me to the list of people who has not bothered to do an iodine test in a very long time.

[joe6pack]

Thanks all for confirming what I was thinking.  I've also noticed the clarity of the mash near the end, but I was never confident I was converting all of the starches.  I've been trying Kai's efficiency analysis spreadsheet, and on this particular brew it said I achieved 90% conversion efficiency, not bad I guess.   I think that even though the liquid is clear, it doesn't mean there aren't starches going into solution and then being converted.  I just want a way of knowing when the mash really was done.

[nateo]

I just want a way of knowing when the mash really was done.

But, does it matter? Does 85% (or 80%, or 75%) conversion efficiency make worse beer? How low is too low?

[malzig]

I think that even though the liquid is clear, it doesn't mean there aren't starches going into solution and then being converted.  I just want a way of knowing when the mash really was done.

Late in the mash, it appears that starch gelatinization becomes rate limiting, instead of amylase earlier.  That's why the wort will appear clear, and an iodine test even negative, while starch is still being converted.

My concern with a poorly converted mash is that I could mechanically extract starch during the sparge that might not convert due to the dilute amylase and high temperature.  If you are having incomplete conversion, but your beer is coming out crystal clear, then it probably just comes down to whether you can afford the couple dollars it adds to your brew day.

[rightasrain]

I never bothered with the iodine test because I never could find out what to do if it failed the test. But from what is presented on this topic, i'm guessing waiting longer will convert the starch.