The scanning electron micrographs of rice grains during puffing
showed the extensive morphological difference (Fig. 3). The raw
rice grain showed the compact internal structure with cracks was
observed in some parts. The parboiling treatment leads to the
swelling and fusion of starch granules resulting in the healing of
rice crack and in the alteration of crystalline structure of amylopectin
molecules in starch granules into amorphous form. Thus the
endosperm portion of parboiled grain was seen as fused mass.
The coherent mass of gelatinised starch together with disrupted
protein bodies in the endosperm provide the hardness to the rice
grain due to hydrothermal treatment. However, during expansion
the compact rice grain changed into highly porous matrix with
numerous big and small cavities of various sizes. The polyhedralstructure of starch granules changes into a coherent mass and the
gelatinised starch along with the disrupted protein bodies occupied
the air spaces in the endosperm (Chandrasekhar &
Chattopadhyay, 1990). The expansion makes the significant
increase in the size of rice grain as indicated by its dimensions.
The high temperature provided to parboiled rice from all sides during
puffing causes rapid evaporation of moisture inside the grains
resulting in the formation of numerous void spaces inside the grain
due to the high pressure of steam formed inside forcing the retrograded
starch to expand. The peripheral thin layer formed during
parboiling treatment of rice enveloped the grain and prevented it
from bursting during puffing process