3.2. Evolution of granules morphology (scanning electron microscopy, SEM)
SEM micrographies chosen are representative of the predominant
state of starch granules at the target times. Fig. 2(a–b) shows native
waxy maize starch granules which still have not been exposed to
−amylase action. Waxy maize starch granules show a polyhedral
shape with smooth surfaces. The analysis of the size distribution
evidenced granules diameters in the range of 3–21 m, with an
average diameter of 11 m. Numerous pinholes were observed on
some of the waxy maize starch granules (Fig. 2(b)), which have
been proposed to be openings to channels/pores which provide
access to hydrolysing enzymes to the granule interior (Huber &
BeMiller, 2000). Oates (1997) reported thatthe entry of hydrolysing
enzymes and other large molecules into the interior of starch granules
is restricted and only possible through pores or channels,
which are either naturally occurring features of the granule, or
the consequences of damages that may have taken place during
granules extraction. The evolution of starch granules morphology
during hydrolysis (Fig. 2(c–h)) may be explained by the current
knowledge of the action of -amylases over starch granules. Enzymatic
reaction with insoluble substrates such as starch granules
occurs via three steps: diffusion to the solid surface, adsorption,
and finally catalysis. Once that the enzyme is adsorbed onto the
granules surface, starch hydrolysis initiates as a superficial erosion
phenomenon as illustrated in Fig. 2 (c). Following the initial attack,
hydrolytic enzymes find access to the interior of the granules via
channels, leaving holes that enlarge with hydrolysis time, as illustrated
in Fig. 2(d–e). Subsequent enzyme attack proceeds from the
interior to the outwards of the granules.