6 min readPathway sheetUsually a 5-mark answer

Extrusion cooking

Extrusion cooking pushes starchy or protein dough through a hot screw barrel and a shaped die; heat, pressure and shear cook it in seconds into puffed, shelf-stable products.

Type of processHigh-temperature short-time (HTST) and continuous

ProductsPuffed snacks, breakfast cereals, soya chunks, fortified rice kernels, weaning mixes

Syllabus wording'Extraction cooking' in some copies means extrusion cooking

What it is

An extruder is a long barrel with one or two rotating screws. Moist flour or grits (cereal, pulse or soya) is fed in; the screw pushes it forward, compressing, mixing, shearing and heating it, often well above 100 °C, from steam jackets and friction. Starch gelatinises and protein denatures into a hot, plastic mass that is forced through a small shaped opening, the die. As it leaves the die the sudden fall in pressure flashes the water into steam, so the product puffs up, and a rotating knife cuts it to size. The cooking takes well under a minute (HTST).

Extrusion cooking step by step

1
Mix raw materials: cereal, pulse or soya flour, water, salt, vitamins
moisture adjusted to suit the product
2
Feed into the barrel, often after steam preconditioning
3
Screw conveys, compresses and kneads the mass
single- or twin-screw extruder
4★
Cook under heat, pressure and shear
starch gelatinises; enzymes and antinutrients are destroyed
5
Force through the die
rings, balls, curls, chunks or rice-shaped kernels
6★
Expand and cut
pressure drop flashes water to steam; product puffs
7
Dry, cool, coat with oil, flavour or sugar, and pack
low moisture keeps it shelf-stable

★ one-way, controls the pace

Products of extrusion

Product	Example
Puffed snacks	Corn curls, puffed rings and balls sold as namkeen
Ready-to-eat breakfast cereals	Puffed and shaped cereal pieces
Texturised vegetable protein (TVP)	Soya chunks and granules from defatted soya flour
Fortified rice kernels	Rice flour with iron, folic acid and vitamin B12, shaped like rice grains and blended with ordinary rice
Weaning and supplementary foods	Instant cereal-pulse mixes for young children
Cold extrusion (shaping only, no cooking)	Pasta, vermicelli

Advantages and limitations

AdvantagesVersatile (many shapes from one machine), continuous and high output, HTST so less nutrient loss than long cooking, low labour, low-moisture shelf-stable products; destroys antinutritional factors such as trypsin inhibitors in soya, and microbes; makes starch more digestible.

LimitationsHigh machine cost and energy use; the Maillard reaction lowers available lysine; some loss of thiamine and vitamin C; needs skilled control; coated snacks may be high in fat and salt.

In food and nutrition

Extrusion gives soya chunks (a cheap, high-protein meat substitute), fortified rice kernels for public feeding programmes and instant weaning mixes, and destroys trypsin inhibitors in soya. Watch the oil and salt in coated extruded snacks.

Exam angle

2 marks: “What is extrusion cooking?” A high-temperature short-time process in which moist starchy or protein material is cooked by heat, pressure and shear in a screw barrel and forced through a die, giving puffed, shaped products.

5 marks: “Explain extrusion cooking with its advantages.” Principle; parts (feeder, screw, barrel, die, cutter); steps; products (snacks, cereals, TVP, fortified rice); advantages and limitations.

Remember it

Extruder = Feed, Screw, Heat, Die, Puff, Cut.