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.
[The figure plots Temperature/Pressure against Extrusion cooking stages: it is low and flat in stage I, rises gently in stage II, rises steeply to a peak in stage III, and drops sharply in stage IV.]
Answer: C
The stages are I - Feeding, II - Kneading, III - Cooking, IV - Expansion.
In an extruder the material first enters the feed section, where it is merely conveyed and temperature and pressure stay low and flat (I). In the kneading (compression) section it is compacted and worked into a dough, so temperature and pressure begin to rise (II). In the cooking (metering) section intense shear and heat drive both to a maximum, gelatinising starch and denaturing protein (III). At the die the melt exits to atmospheric pressure, and the sudden drop lets superheated water flash to steam and expand the product (IV).
The other options place cooking before kneading or kneading before feeding, which contradicts the rising temperature and pressure profile.
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