Concept:
Parboiling is a hydrothermal process applied to paddy before milling.
Step 1: Paddy parboiling generally involves soaking, steaming and drying.
Step 2: During soaking, moisture enters the grain.
Step 3: During steaming, heat is applied to the soaked paddy.
Step 4: This heat causes starch granules inside the grain to gelatinize.
Step 5: Starch gelatinization improves milling quality and reduces breakage of rice grains.
\[
\boxed{\text{Starch gelatinization}}
\]