To mash a starch crop for ethanol, crush it, cook it in water until the starch swells (boiling for 30-60 minutes for corn), cool the mash to about 65°C, stir in ground malt and hold 60-65°C for 1-2 hours, then cool to 30°C before adding yeast. The temperatures matter more than the times: malt enzymes work at 55-75°C and are destroyed above about 78°C, so adding malt to a mash that is still hot wastes it.
Part of Ethanol Fuel Production
Mashing is cooking a starchy crop and converting its starch to fermentable sugar with the enzymes in malted grain.
Cook First, Then Convert, And Never The Other Way Round
Raw starch granules resist enzymes; cooking opens them. The malt enzymes then cut starch to sugar, together working across 55-75°C: alpha-amylase (best at 65-75°C) chops chains into fragments, and beta-amylase (best at 55-65°C) makes fermentable maltose. The corpus’s alcohol-oriented grain guide sets the windows: 55-62°C gives a highly fermentable wash, 62-68°C balances both enzymes, and above 78°C both are destroyed (Grain Mash). For distilling, the lower end (60-65°C) gives the most fermentable sugar.
The parent says to cook at 60-70°C. That is enough for softer starches such as potato, but the grain-mash guide boils corn for 30-60 minutes, and whole or coarsely cracked corn needs that. Follow the grain guide for corn. (This page takes the 60-70°C figure as the parent’s, not as proven for corn.)
What You Need
- Crushed or cracked grain, or chopped potatoes
- Malt: ground sprouted barley (Malting), or salvaged amylase enzyme
- A pot or drum you can heat and stir, and a thermometer or the hand-test from the grain guide
- Plenty of water and a way to cool the mash quickly
Method
Step 1: Cook. Stir the crushed grain into water and bring to the cooking temperature, boiling 30-60 minutes for corn, stirring so it does not scorch. Potatoes cook until soft, then mash.
Step 2: Cool to 65°C. Stand the pot in cold water or add cold water in stages. Do not add malt above about 70°C: beta-amylase is destroyed above 70°C and both enzymes by about 78°C.
Step 3: Add malt and hold. Stir in the ground malt and hold at 65°C for 1-2 hours, stirring regularly and keeping the temperature within the 60-65°C band by insulating the pot or adding hot water. A mash that gets thinner and sweeter-tasting as it sits is converting.
Step 4: Test. If you have iodine, a drop on a spoonful of mash turns blue-black while starch remains and stays brown when conversion is complete. Without iodine, taste: finished mash is clearly sweet and no longer starchy or pasty.
Step 5: Cool to 30°C and transfer to the fermenter (Fermentation Vessel Design).
Worked Example: 10 Kg Of Corn
The implied starch in the parent’s corn yield is 541-585 kg per tonne (Starch Crops), so 10 kg of corn holds 5.4-5.9 kg of starch. Each starch unit gains a water molecule on conversion (× 1.111), giving 6.0-6.5 kg of fermentable sugar. At 15-20% sugar by weight that dilutes to 6.0 ÷ 0.20 = 30 kg and 6.5 ÷ 0.15 = 43 kg of wash, which is 30-43 litres, so about 3-4.3 litres of water per kilogram of grain. At 9-12% alcohol that holds 3.7-4.0 litres of ethanol, matching the parent’s 370-400 litres per tonne (10 kg × 0.370-0.400 L/kg).
Common Mistakes
- Malt added hot. The enzymes are dead, the mash stays starchy, the wash ferments little and the still produces almost nothing.
- Scorching. Burnt grain on the pot bottom gives a bitter wash and a pot that is hard to clean; stir and use a water bath or low heat.
- Cooling too slowly. A mash left between 30 and 50°C for hours picks up bacteria and sours before the yeast starts.
Hot mash scalds
A pot of boiling grain porridge spits and sticks to skin, causing deep burns. Use a stable pot, long-handled paddle, and keep children away. Carry nothing hot down a step.
What’s Next
- Learn next: Fermentation, which takes the converted mash
- Combine with: Mash Preparation, the corpus’s alcohol-oriented overview of preparing mash