Brew Science: The Iodine Test

This is in first of a series of articles about the science behind basic brewing processes.  These articles will explain how many of the procedures we use in brewing work – and hopefully provide sufficient information on how to make us of these tests/processes in your own brewing.

An iodine test [wikipedia] is used by brewers to test for conversion of unfermentable starches into the mixture of fermentable sugars and unfermentable dextrins which comprise the sweet wort we ferment to make beer.

The presence of starch in beer is very much unwanted.  Starches cannot be fermented, and induce an unpleasant appearance and feeling the the resulting wort.  In contrast, fermentable sugars (mostly glucose, or 2-3 glucose’s attached togeather – AKA maltose and maltriose) are consumed by yeast to make alcohol, and unfermentable dextrins (short chains of glucose) which impart a malty flavor and mouth-feel to the beer.

Left: the branching structure of starch.  The ‘…’ link to additional starches of glucose molecules (see below image).  A single starch molecule will be comrpised of many hundreds of glucose molecules.

Above: the structure of the long chains of glucose.

 A single molicule of gluocse.  Images from wikipedia.

The process of breaking down starch into sugars and dextrins is called saccrification (or conversion), and is driven by soaking grain in water at the desired temperature (generally 63-66C).  At these temps, enzymes called amylases break down starches into sugars and dextrins.  Many beginning brewers simply wait-and-hope for conversion to complete.

If only there were a better way…

…and there is — the iodine test.

First, a bit of biochemistry.  Starches are long chains of glucose, often linked in a way which forms a branching structure (see above images).  What is not obvious in the above pictures is that those long chains of glucose coil to form a tube-like structure.  Iodine ions fit near-perfectly in the ‘hole’ in this tube.  This complex forms a deep indigo, almost black, colour when formed:

Iodine ions (sphere’s with ‘I-‘) incorporated into the helical structure of a linear starch strand.

Image from wikipedia

This helical structure does not form in short starches (termed dextrins) below ~10 glucose in length.  Meaning, by adding iodine to small samples of wort, we can determine how well the conversion process has worked

How to do it:

CAUTION: iodine is highly toxic and should never be added to a brew.  The samples of wort used for this test must never be returned to the mash, and any equipment that comes in contact with iodine must be cleaned before being used for any other purpose.

What you need:

1) Tinicture of iodine (available at many pharmacies)
2) A white or clear spot-plate, saucer, or other non-absorbet surface
3) A clean eye dropper


Step 1: Add a small drop of iodine to the spot-plate.  You need only a tiny amount (i.e. the drop that sticks to the ‘dipper tube’ attached to the lid of most iodine) – approximately 0.25ml:
A single drop of iodine on the spot plate – note its light-brown colour

Step 2: Using a clean eye-dropper, pick up 1-2 drops worth of the liquid portion of the mash.  You must avoid picking up any solid material (husk, grit), as this can react with the iodine even if all the starch is gone.  Add this drop of starch to the iodine.

Check the colour quickly – with ~10 seconds.  If it turns dark-blue/black, there is still unconverted starch in the mash.  Do not wait too long, as slower reactions with substances other than starch can lead to mis-leading results:

A positive iodine test – the iodine goes from brown to dark-black.  Not obvious is a particulate-precipitate that also forms.  This is a picture taken ~2 seconds after adding wort.

Step 3: Completed conversion is indicated by a test where there is no formation of a dark-colour:

Comparison of iodine test results from an incomplete (left) and complete (right) conversion.  The exact colour of the complete reaction will vary depending on the colour of the wort, but what you will observe in all cases is a “dilution” of the colour of the iodine, without any darkening or formation of particulates.

In the above example, conversion was complete in 35 minutes.  However, the mash was continued for an additional 40 minutes to allow the amalyses further time to degrade dextrins into fermentables – thereby producing a lighter-bodied beer.

5 thoughts on “Brew Science: The Iodine Test

  • September 11, 2017 at 8:11 PM

    I know this is 18 months later, but I believe the iodine test works on a similar principle as the Fehlings Reaction ( If conversion has been completed, the long chain, non-reducable starches have been broken down into shorter chain sugars like Maltose, Glucose, and Fructose. The charged triiodide ion in the solution pulls away a hydrogen, oxidizing and opening the sugar ring. This oxidation has an effect on the pigment of the solution.

  • January 29, 2016 at 3:57 PM

    Thank you for the reply Bryan. If you come upon any resources that explain this, or a test used to determine conversion of whole cereal mashes i.e. not just runoff please let me know.

  • January 29, 2016 at 12:34 PM

    I'm not entirely certain, but there are a lot of non-starch carbohydrates in plant material (cellulose, lignin, etc) which are not soluble, nor are they converted to sugars by amylase. My guess would be that one of those materials are reacting with the iodine in a similar fashion to starch.

  • January 28, 2016 at 11:50 PM

    Could you expand on the mechanism that could cause a positive result if solid material is present. I've been trying to saccharify cereal porridges and keep getting positive results despite using excess amylase. Im wondering if the presence of no starch constituents is causing the false-positive.

  • October 25, 2013 at 10:49 AM

    Good job mate!
    Thank's from a Greek homebrewer !


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