Looking around the web there seems to be a lot of mention of the use of the iodine (starch) test at the end of your mash. Is it actually useful (i.e. is it reliable)? There have been times when I have done the test and it has continually returned a residual starch presence in my mash, even after having mashed for 60-90 minutes at the appropriate temperatures/mash thickness etc.

Is there some other test one can do, or do we just mash on ahead regardless?

  • I find it fun to taste the mash liquid periodically as I open the tun to stir or do a temp check. You can taste it getting sweeter as conversion happens during the first 20-30min or so.
    – GHP
    Commented Jan 7, 2014 at 14:10

3 Answers 3


IMO, yes it's pointless. First, it's pretty much impossible to not get conversion given a sound recipe and hitting your mash temps. Second, as you've noticed, it's more than possible to get a false reading. I haven't done an iodine test in the last 13 years and haven't felt like I needed to do one.

  • This is certainly what I'm inclined to think after hitting my target OG on a number of occasions, even though the iodine test suggested incomplete starch conversion. So, is there an alternative, or do we just trust in our mash temps & times?
    – Poshpaws
    Commented Jun 6, 2011 at 18:37
  • I've been doing all grain for the last 9 years and I have never done one. As a biochemist I think I had a lot more trust in the mash enzymes. The system is pretty forgiving with moderately fresh malt and conversion at some level is sure to happen. I've never had an issue with a batch that I thought was attributed to residual starch from the mash. Getting the right mash profile though is a but of a skill...one that the iodine test can't help you with.
    – brewchez
    Commented Jun 6, 2011 at 22:27
  • 1
    As long as you know that iodine tests for starch in solution, there's no ambiguity. But still, you can have starch in grain particles. Anyway, given sufficient mash time, you'd always get complete conversion, so in most cases is not necessary.
    – zgoda
    Commented Jun 7, 2011 at 7:25
  • So how do you ensure that you have a starch solution without suspended particulates?
    – Poshpaws
    Commented Jun 7, 2011 at 8:01

Seems strange to me that so many people are prepared to trust their own "untested" experience to know when a mash has got complete conversion. Kind of like an electrician saying I've never tested if circuits are live before working on them so testing is a waste of time.

I mean don't get me wrong, it is not the end of the the world if your mash did not achieve full conversion, but even with the freshest of malts and the right temps, I have still had the test fail now and again, an extra 10 - 15 minutes in the tun has sorted it out. BUT and here is the nub of it. If I had not bothered testing I would have just assumed that in my experience my mashes always completed on time every time.

When you consider how much time, effort and expense we put into a batch of beer, is the 20 seconds and a couple of pennies it would cost for the test that big of a deal that it is worth skipping?

If you have been regularly brewing using the same ingredients and same gear for years and never had a failed conversion test, then I can understand not bothering with the test every time, but in all honesty untested experience where you just assume something has been right all along, is not worth anything.

  • 1
    IMHO, if your conversion just required as extra 10min in the mash tun to be achieved, then you would have probably had full conversion anyway by the time you were done drawing off your first runnings. I hear what you are saying about measuring, but given that there's sometimes false-positives, and that the process seems almost universally successful for virtually everybody, I personally stopped doing the test a long time ago.
    – GHP
    Commented Jan 7, 2014 at 14:07
  • 1
    I hear what you are saying but it is still the difference between knowing an answer and taking a random guess. Your mash may possibly be complete or even probably but I don't have to guess, I know the answer which is always a good place to be especially for repeatability
    – Anigel
    Commented Jan 7, 2014 at 14:34

Its probably a good idea to do the test when you first go to all-grain using malted barley but once you are dialed it might be optional. HOWEVER, certain malted gluten free grains, such as millet and buckwheat for example, do not readily self convert. You can either do complex step mashes to achieve this, or do a simpler single infusion mash and add enzymes (alpha amylase) and in this case yes you want to know definitely if the conversion did indeed occur. So, no its not pointless either way you look at it but it might be optional if your experience makes it so.

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