How is ethanol produced by fermentation and why is it considered a green fuel?

In one line

Fuel ethanol is produced by yeast fermentation of sugars from molasses, sugarcane juice, grains or biomass, followed by distillation, and it is a renewable, cleaner-burning fuel usually blended with petrol.

Explained simply

Yeast "eats" sugar and releases ethanol and CO₂, the same process used in bread and brewing. India produces lots of sugarcane, and molasses (the thick leftover from sugar making) is a cheap sugar source. The ethanol is then distilled and dried so it can be mixed with petrol. Because the sugar came from plants that absorbed CO₂, burning the ethanol largely returns CO₂ that was recently taken from air.

How it works

1. From molasses (about 50% sugars): dilute to roughly 8–10% sugar, add a little acid (pH about 4–5) and ammonium salts as yeast nutrient.
2. Add yeast; ferment at about 30–35 °C for 2–3 days.
3. Enzymes from yeast act:
   • Invertase: sucrose + water → glucose + fructose
   • Zymase: glucose/fructose → ethanol + CO₂
4. The fermented liquid ("wash") contains about 8–10% ethanol (yeast is killed at higher concentrations).
5. Fractional distillation gives rectified spirit (about 95.6% ethanol, an azeotrope with water).
6. Dehydration (azeotropic distillation with benzene/cyclohexane, or molecular sieves) gives absolute (anhydrous) ethanol, needed for blending with petrol.
7. From starch (grains, potatoes): diastase (from malt) converts starch to maltose; maltase converts maltose to glucose; then zymase as above. Second-generation ethanol uses cellulose from crop residues after acid or enzymatic hydrolysis.

Key formula

\( \mathrm{C_{12}H_{22}O_{11} + H_2O \xrightarrow{\text{invertase}} C_6H_{12}O_6\ (glucose) + C_6H_{12}O_6\ (fructose)} \)

\( \mathrm{C_6H_{12}O_6 \xrightarrow{\text{zymase}} 2C_2H_5OH + 2CO_2} \)

Starch route: \( \mathrm{2(C_6H_{10}O_5)_n + nH_2O \xrightarrow{\text{diastase}} nC_{12}H_{22}O_{11}} \) (maltose), then \( \mathrm{C_{12}H_{22}O_{11} + H_2O \xrightarrow{\text{maltase}} 2C_6H_{12}O_6} \).

Quick example

Advantages of ethanol as fuel:

• Renewable; reduces import of crude oil.
• High octane number, so it improves anti-knock quality of petrol blends.
• Contains oxygen (about 35% by mass), so it burns more completely with lower CO and hydrocarbon emissions; nearly no sulphur.
• Biodegradable and less toxic than petrol components.
• Uses agricultural by-products (molasses, crop residue), supporting farmers.
• Blends such as E10 and E20 (10% and 20% ethanol in petrol) are used in India; a petrol–ethanol blend is called power alcohol.

Limitations: lower calorific value than petrol (about two-thirds per litre), absorbs water (phase separation in blends), can corrode some rubber and metal parts, and food-versus-fuel concerns.

Common mistakes

• Writing 100% ethanol directly from distillation; ordinary distillation stops at about 95.6% (azeotrope).
• Giving zymase as the enzyme that splits sucrose (that is invertase).
• Writing CO as the gas in fermentation; it is CO₂.

How MU asks it

"Explain the synthesis of ethanol from molasses with reactions. State its advantages as a fuel" (5 marks) or "Write a note on power alcohol" (3 marks). For full marks: conditions, both enzyme reactions, distillation to rectified spirit and absolute alcohol, and 5–6 advantages with 1–2 limitations.