Ethanol

Ethanol is probably the chemical with which humanity has had the longest and most ambivalent relationship. Its fermentation by yeasts (mainly Saccharomyces cerevisiae) from sugars goes back to at least -7000 in the Caucasus and China — archaeological records as old as human sedentarisation. The biotechnological route remains dominant: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ (ΔH = -68 kJ/mol), with a maximum yield of ~12-15 % alcohol before concentration inhibits the yeasts themselves. Distillation, refined by Arab alchemists in the 9th century (Jabir ibn Hayyan), allows concentration beyond this limit up to the water-ethanol azeotrope at 95.6 vol %.

Biologically, ethanol is metabolised by alcohol dehydrogenase (ADH) into acetaldehyde, then by aldehyde dehydrogenase (ALDH) into acetate. The ALDH genetic polymorphism explains "Asian flush": ~40 % of East Asians carry an ALDH2*2 variant whose activity is reduced, accumulating acetaldehyde and triggering flushing, tachycardia and nausea even at moderate intake. This same accumulation is responsible for most of the carcinogenic effects of chronic consumption (liver, ENT tract).

Industrially, ethanol splits its market between three uses: ~75 Mt/yr of fuel bioethanol (mainly Brazil and the United States, from sugarcane or corn), ~20 Mt/yr of synthetic industrial ethanol (from ethylene by hydration), and the rest for beverages and chemistry. The cellulosic-ethanol biorefinery route (from straw, miscanthus) remains limited by enzyme cost — the main technological bottleneck of the 21st century for decarbonising biofuels.