How to test for mash conversion

If you mash crushed malted barley at around 153°F (67°C) for an hour, your malt starches will convert to fermentable sugar. You’ll get awesome beer.

On your first couple of batches of partial mash brewing or full mash brewing, you may want to test that you have converted your mash.

You can test conversion though two ways…

The duck test

Converted mashes will be darker, clearer and smell wonderful.

Converted mashes will be darker, clearer and smell wonderful.

If it looks like a duck, swims like a duck and quacks like a duck, then it probably is a duck.

Likewise we can check conversion through simple observation:

  • Converted mash liquid will be clear. Unconverted mashes are cloudy with starch.

  • Converted mashes will darken. They will look more like beer, with nice amber hues.

  • Converted mashes smell great. They’ll smell sweet and malty. The whole house may smell noticeably malty (and delicious!).

If you hit all three signs, your mash is most likely converted your starches to sugars.

Starch conversion Iodine test

You can test conversion with iodine. It turns dark purple when exposed to starch. To test:

  • With a spoon carefully take a sample of mash liquid (no grains – they give a false-positive reading).

  • Spread on a white saucer and drop a few specks of iodine.

  • Stir the iodine through roughly.

If it stays light brown, conversion has happened. Purple or black means you aren’t there yet.

To test for conversion, take a sample free of grain particles and pour on a white saucer.

To test for conversion, take a sample free of grain particles and pour on a white saucer.

If iodine turns dark purple or black, starch is present and mash conversion is incomplete.

If iodine turns dark purple or black, starch is present and mash conversion is incomplete.

What to do if your mash isn’t converting

If you have trouble converting after mashing at 153°F (67°C) for an hour, you probably have pH issues or using old malt (the enzymes degrade over time).

Check your temperature, adjust as needed and leave for another half hour. It will probably convert over.

If not, check your mash pH. It should be between 5 and 6, ideally 5.3.

Further issues may need to be managed by adjusting your pH with gypsum.


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