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What it does :
Tests with iodine are used to determine when mashing is complete. As we all remember, the goal of mashing is to turn the starches in the grain to sugars that the yeast can then convert to alcohol through fermentation.
What it is:
Iodine is a chemical used to test for the presence of starch.
How it works:
Iodine solution reacts with starch amylose molecules to produce a deep blue-black color. By withdrawing a sample of water from the mash, and adding some iodine solution, you can determine if the starch conversion is accomplished.
Iodine is used during the hot side of brewing (e.g. wort preparation). However, it does not come into contact with the wort, and is disposable. So, simply keep it handy for use.