Starch crops such as corn, wheat, potatoes and cassava make ethanol only after their starch is cooked and converted to sugar with malt, and they cost heavily in food: corn needs 2.5-2.7 kg of grain per litre of fuel and wheat 2.6-2.9 kg, each litre using about 3 person-days of food energy. Grain is stored food, so it is the last feedstock to turn into fuel, not the first.
Part of Ethanol Fuel Production
Cereal grains, potatoes and cassava as ethanol feedstocks: what they yield, what conversion they need and what the grain would otherwise feed.
Starch Must Be Broken Into Sugar Before Yeast Can Use It
Starch is long chains of glucose; yeast cannot ferment the chains. Cooking swells the starch granules so enzymes can reach them, and the enzymes in malted grain (Malting) cut the chains into fermentable sugar. The steps are on Mashing Starch Crops. The chemistry adds one water molecule per glucose unit, so 1 kg of starch becomes 1.11 kg of glucose (180.16 ÷ 162.14) and then at most 0.568 kg of ethanol, which is 0.72 litres.
The Table, Checked And Priced In Food
Working back from the parent’s litres per tonne, with 95% of the ideal conversion (0.568 × 0.95 = 0.54 kg ethanol per kg starch) and ethanol at 0.789 kg/L:
| Crop | Litres/tonne | Starch implied (kg/t) | Grain per litre | Food energy per litre | Fuel share of it |
|---|---|---|---|---|---|
| Corn | 370-400 | 541-585 | 2.5-2.7 kg | 9,100-9,800 kcal | 51-55% |
| Wheat | 340-380 | 497-556 | 2.6-2.9 kg | 8,800-9,900 kcal | 51-57% |
| Potatoes | 95-110 | 139-161 | 9.1-10.5 kg | 7,000-8,100 kcal | 62-72% |
Food energy uses the caloric guide’s 1,650 kcal/lb for corn, 1,520 for wheat and 350 for potatoes (per kg: 3,638, 3,351 and 772). The fuel column divides the 5,040 kcal in a litre of ethanol (21.1 MJ) by it. At 2,800 kcal a day that is 3.1-3.5 person-days for grain and 2.5-2.9 for potatoes, per litre.
The implied starch (about 500-585 kg per tonne of grain, 139-161 per tonne of potatoes) is in the range these crops hold, so the table is internally consistent. It is not a reason to plan on it: the per-hectare figures behind it are mechanised yields (see Yield per Hectare Comparison).
What The Grain Would Otherwise Do
A tonne of corn that becomes about 370-400 litres of ethanol holds 3.64 million kcal as food (1,000 kg × 3,638), which is 1,300 person-days at 2,800 kcal. The corpus plan for a community of 100 needs 25,500 lb of corn (about 11.6 tonnes) to cover the winter calorie gap if corn were the only store (Winter Calorie Gap Planning); a still that takes grain competes with exactly that store. Potatoes are the least wasteful of the three per litre (62-72% returned as fuel energy) but are heavy to move and store (Root Crops).
Cassava, listed in the parent for tropical regions at 3,500-4,000 L/ha, is a staple for the people who grow it; the same rule applies.
Common Mistakes
- Ordering cook and malt wrongly. Cooking happens first and hot; the malt goes in only after the mash has cooled to about 65°C or its enzymes are destroyed.
- Planning from litres per hectare. Use litres per tonne of what you weigh at harvest.
- Counting the feed return as free. Distillers’ grain keeps protein and fibre, not the starch (Distiller’s Grain).
What’s Next
- Learn next: Mashing Starch Crops, the cooking and conversion step
- Combine with: Grain Mash, the corpus’s alcohol-oriented guide to malting grain