In food extrusion cooking (e.g., expanded snack foods), product expansion (puffing) at the die exit is primarily driven by:
Options:
(A) Instantaneous flash evaporation of superheated moisture as pressure drops from barrel pressure (gt; 5\text{ MPa}$) to atmospheric pressure
(B) Thermal decomposition of starch into gaseous carbon dioxide
(C) Rapid enzymatic fermentation occurring inside the metering zone
(D) Sublimation of frozen ice crystals
Answer Key:
A
Step-by-Step Solution & Explanation
Inside the extruder barrel, high temperature and pressure keep water liquid in a superheated state within molten viscoelastic dough. As the melt exits through the die, pressure suddenly drops to atmospheric; water flash-vaporizes, creating steam bubbles that expand the matrix before it cools below its glass transition temperature ($T_g$).