8 min readConcept sheetOften a 10-mark answer (both together) or a 5-mark note on each

Spray drying and drum drying

Liquids are dried by spraying fine droplets into hot air (milk powder, instant coffee) or by spreading a thin film on a hot rotating drum (baby-food flakes, potato flakes).

Spray dryer airVery hot inlet air (Fellows: about 150-300 °C), yet droplets stay much cooler

Spray drying timeA few seconds

Drum dryerSteam-heated drum dries a thin film in one turn; a blade scrapes it off

Typical productsSpray: milk powder, coffee, egg. Drum: cereal flakes, potato flakes

Spray drying, step by step

1
Concentrate the liquid first
e.g. milk in a vacuum evaporator; saves energy
2★
Atomise it into a fine mist
spinning-disc (rotary) atomiser or high-pressure nozzle
3★
Mix the mist with hot air in a large drying chamber
evaporation keeps droplets far cooler than the air
4
Water evaporates in seconds; droplets become powder particles
5
Separate the powder from the air
most falls to the chamber bottom; fines are caught in cyclones or bag filters
6
Cool, agglomerate for 'instant' powder if needed, and pack airtight
whole-milk powder is often nitrogen-flushed

★ one-way, controls the pace

Drum (roller) drying

A drum dryer has one or two hollow metal drums, heated inside by steam, turning slowly. A thin film of liquid or paste (milk, cereal slurry, mashed potato, fruit pulp) is applied to the hot surface by dipping, spraying or feeding between two drums. The film dries in less than one turn and is scraped off by a fixed knife (doctor blade) as sheets or flakes, which may then be milled. Types: single drum, double drum and vacuum drum (lower temperature for heat-sensitive foods).

Spray drying vs drum drying

Point	Spray drying	Drum drying
Feed	Liquids that can be pumped and atomised	Liquids, purees and pastes
Heat transfer	Hot air (convection); droplets stay cool	Hot metal surface (conduction)
Product	Fine, free-flowing powder that dissolves well	Flakes or sheets; may be milled to powder
Heat damage	Low; suits heat-sensitive foods	More: cooked flavour, browning, lower solubility
Cost and scale	High capital cost; big plant; large volumes	Lower cost; compact; simple
Examples	Milk powder, infant formula, instant coffee and tea, egg powder, fruit juice and tomato powders	Infant cereal flakes, instant potato flakes, roller-dried milk, soup mixes

Advantages and disadvantages

Spray dryingAdvantages: very fast; continuous and large-scale; good colour, flavour and nutrient retention; free-flowing, readily soluble powder; particle size can be controlled. Disadvantages: high capital and energy cost; liquids only; large building; low bulk density; fatty powders stick; needs moisture-proof packing.

Drum dryingAdvantages: simple and cheap; high drying rate; handles thick pastes and purees; suits small plants. Disadvantages: heat damage, cooked flavour and browning; poorer solubility of milk powder; some lysine loss; only feeds that form a film.

In food and nutrition

Spray-dried milk powder and infant formula keep most of their protein, calcium and vitamins and matter where fresh milk is scarce. Drum-dried cereal flakes make quick weaning foods; drum heat lowers available lysine a little, so pair them with milk or pulses.

Exam angle

2 marks: “Name two foods made by spray drying and two by drum drying.” Spray: milk powder, instant coffee. Drum: infant cereal flakes, instant potato flakes.

5 marks: “Write a note on spray drying.” Principle; atomiser, drying chamber, cyclone; steps; why droplets stay cool; products; advantages and disadvantages; diagram.

10 marks: “Describe spray drying and drum drying with their merits and demerits.” Principle and diagram of each; steps; products; comparison table; advantages and disadvantages.

Remember it

Spray = mist in hot air → powder. Drum = film on hot roller → flakes.