Discussion
Extrusion cooking is accomplished in four different stages, which are indicated as I, II, III and IV in the figure given below. Choose the correct option representing the name of each stage.
Answer: C
The stages are I - Feeding, II - Kneading, III - Cooking, IV - Expansion.
In the extruder the raw material first enters and is conveyed at low temperature and pressure (feeding, I, the flat start of the curve). The screw then compresses and works it into a dough, raising temperature and pressure gently (kneading, II). Next, intense shear and barrel heat raise both steeply, gelatinizing the starch and denaturing proteins (cooking, III, ending at the peak just before the die). Finally the hot, pressurized melt leaves the die, the pressure drops suddenly and water flashes to steam, puffing the product (expansion, IV, the sharp fall).
Options putting cooking before kneading, or cooking first, do not fit the gradual-then-steep rise, and kneading cannot come before the material is fed.
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