How to Run the Iodine Mash Test in 5 Easy Steps

The iodine mash test gives you X-ray vision into your mash. One drop of iodine reveals whether your starch has surrendered to enzymes or is still hiding in husks. 

No guesswork. No wasted time. 

You stand at the crossroads of body and fermentability and this simple color change guides your next move.

Who needs guesswork?

Mash conversion isn’t a guarantee.

Temperature drift, pH swings and enzyme limits can stall breakdown.

Unconverted starch steals fermentable sugars, clogs filters and muddies foam. The iodine test rings the alarm so you can tweak rest time or temperature, lock in yield, clarity and consistent body.

The Science Behind It

Grain starch is stored in microscopic granules of two polymers, linear amylose and highly branched amylopectin. Beta-amylase cleaves maltose units from chain ends.

Alpha-amylase attacks internal bonds to yield dextrins that add mouthfeel. Iodine slips into the 1.5-nanometer-wide amylose helix, forming a polyiodide complex that absorbs light around 580 nanometers and turns deep blue or black.

 Converted sugars and broken chains cannot bind iodine so the solution stays amber brown.

That instant color shift proves enzyme action. 

Enzyme kinetics follow Michaelis-Menten dynamics with a Km around 0.5 mg/mL and a Vmax tied to temperature and pH. Beta-amylase peaks near 62 °C at pH 5.2 to 5.4 and alpha-amylase peaks near 68 °C at pH 5.6 to 5.8. 

Slide outside those windows and reaction rates plummet or enzymes denature.

Gear You Need

• A clean clear glass or ceramic dish, no plastic residue to trap starch
• A sanitized pipette or calibrated micropipette for precision
• Food-grade iodine tincture or lab-grade Lugol’s solution, 5 g iodine and 10 g potassium iodide per liter
• A digital thermometer accurate to plus or minus 0.5 °C for your enzyme map
• A reliable pH meter or narrow-range pH strips from 5.0 to 6.0
• Sampling spoon, turkey baster or small beaker to avoid contamination
• Nitrile gloves and splash-proof goggles since iodine stains and irritates

Step by Step Process

Crush grain to a medium-fine crush that leaves husks intact but exposes endosperm. 

Dough in with strike water heated to your saccharification rest, typically 63 °C for a dry crisp finish or up to 67 °C for fuller body. 

Stir to eliminate dough balls.

Hold temperature within plus or minus 1 °C for 45 to 60 minutes, stirring or recirculating every 15 minutes for uniform conversion. At the end of your rest, draw 5 to 10 mL of wort from mid-mash. Cool the sample below 40 °C since iodine reacts unpredictably above that. Transfer to your test dish.

Running the Test

Using your micropipette, add exactly 0.05 mL of iodine solution. Observe up to 30 seconds. Blue or black means leftover starch. 

Amber brown means conversion is complete. If starch remains, raise mash temperature by 2 °C or extend the rest by 10 minutes and retest. Adjust until you hit the amber brown endpoint. 

You can return the sample to the mash if sanitation allows or discard it and continue your sparge.

Advanced Brewing Insight

Step mashes and decoctions change granule gelatinization to give more control over conversion. An early cereal mash at 50 to 55 °C activates beta glucanase to break cell walls and prevent stuck mashes. A brief protein rest around 50 °C can improve clarity and head retention later. 

If you push high-adjunct bills, the iodine test becomes critical to confirm that barley enzymes handle rice, corn or unmalted grains. 

Enzyme supplementation, especially granular or liquid amylase, can rescue underperforming mashes but only if you know conversion stalled in the first place. Use the iodine test to validate supplement dosages and mash schedules.

Troubleshooting Common Issues

Persistent blue or black? 

Inspect your crush.

Uneven particle size traps starch behind husks. 

Adjust your mill gap by 0.1 mm increments. Next, verify mash pH with a calibrated meter and adjust with food-grade lactic or phosphoric acid to hit 5.2 to 5.6. Ensure strike water calculations and sparge rates keep pH stable. 

Confirm thermometer accuracy with ice-water and boiling-point checks. Examine mash agitation since dead zones or channeling can leave pockets of raw starch. Consider gentle recirculation or pulse-sparging. 

Finally check water hardness since excess calcium stabilizes enzymes but too little and activity wanes.

Safety and Clean Up

Iodine stains like crude oil on denim and irritates skin and mucous membranes. Wear gloves and goggles and work over stainless steel or a disposable tray.

Rinse utensils immediately with hot water since starch residue can lead to false positives on the next brew. 

Store iodine in an amber-glass dropper bottle away from light and securely out of reach. Dispose of test samples responsibly and do not pour iodine into your mash tun or fermenter.

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