THESIS

milled rice (Juliano and Bechtel, 1985). Starch granules are relatively dense, insoluble,
and swell only slightly in cold water. Rice starch has the smallest particle size and the
whitest color of all the commercial starches. The average rice starch granule is
between 2 - 8 microns in mature grain (Figure 2 A). These granules are polyhedral
and constitute approximately 90% of milled rice (dry weight). Starch granules consist
of alternating semi-crystalline and amorphous (Myer et al. 2001). However, rice
contains only compound granules in amorphous rings. Dang and Copeland (2003)
suggested that the growth rings in rice are approximately 400 nm apart. Based on
scanning electron microscopy observations, one block-let in the semi-crystalline
growth ring contains several amorphous and crystalline lamellae (Figure 2 B). Donald
et al.(1997) reported that the cross striations within the growth rings of rice starch (as
observed by AFM) correspond to the blocklet of amorphous and crystalline lamellae.
These blocklet have an average size of 100 nm in diameter and are proposed to
contain 280 amylopectin side chain clusters. Amorphous lamellae contain branch
points of the amylopectin side chains and possibly some amylose, whereas semi-
crystalline lamellae are constituted of amylopectin double helices (Figure 2 C).
Figure 2 Granule structure of rice starch and lamellar structure of a starch granule
Source: Donald et al.(1997) and Dang and Copeland (2003)
17