3.4. Scanning electron microscopy
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 polyhedral structure 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 (Chandrasekhar and Chattopadhyay, 1990 and Chinnaswamy and Bhattacharya, 1983).