Ethanol Production from Cereal Grains

Fuel-grade ethanol is a nearly water-free alcohol suitable for blending with petrol. In cereal-based production, grain starch is first converted into fermentable sugar through starch hydrolysis, because yeast cannot directly ferment starch. The sugar is then fermented, purified and dehydrated to obtain fuel-grade ethanol.

Preparation and conversion of starch

Cereals such as maize contain starch, along with protein, fibre, oil and minerals. Industrial plants commonly process grain through dry milling, while maize may also be processed through wet milling.

• In dry milling, cleaned grain is ground and mixed with water to form a slurry.
• Heating gelatinises the starch; alpha-amylase liquefies it into shorter carbohydrate chains.
• During saccharification, glucoamylase converts these chains mainly into glucose.
• Liquefaction, saccharification and fermentation may be arranged as separate or partly overlapping stages.

Fermentation and fuel finishing

The glucose-rich mash is converted into ethanol and carbon dioxide under controlled conditions. The resulting fermented liquid contains ethanol mixed with water and other components, so further separation is essential.

• Yeast, commonly Saccharomyces cerevisiae, carries out alcoholic fermentation under oxygen-limited conditions.
• Distillation and rectification concentrate ethanol by separating it from the fermented mash.
• Residual water is removed through dehydration, commonly using molecular sieves, to produce anhydrous ethanol suitable for petrol blending.
• The finished ethanol is stored and handled under prescribed fuel-quality and safety requirements.

Processing routes and co-products

The choice of milling route determines the co-product stream and plant economics. Dry milling processes the whole grain together, whereas wet milling first separates major grain components before processing the starch fraction.

• Dry milling produces distillers dried grains with solubles, a protein-rich co-product commonly used as animal feed when it meets quality requirements.
• Wet milling can separately recover germ, fibre and gluten-rich fractions in addition to the starch stream.
• The carbon dioxide generated during fermentation may also be captured and purified.
• Overall sustainability depends on grain sourcing, water use, process energy, transport and productive use of co-products.