Food Microstructure and Starch Digestion - Advances in Food and Nutrition Research - page 145

cooked potato parenchyma in some cultivars also showed disintegrated
structures ( Fig. 4.1 B and H). Cooked potato parenchyma showed separated
cells containing gelled mass formed by the gelatinization of starch, as
observed using CLSM ( Fig. 4.2 ) .
Transmission electron microscopy is a very helpful technique in order
to understand the changes taking place in the starch and cell wall material
during cooking ( Figs. 4.4 and 4.5 ).Thecellwallofrawtuberparen-
chyma cells was observed to be made up of middle lamella and the
primary cell wall ( Fig. 4.4 A and D). The middle lamella is mainly com-
posed of pectic substances, whereas the primary cell wall has been
reported to be made of cellulose molecules, arranged into thin hair-like
strands called microfibrils. The microfibrils are arranged in a meshwork
pattern along with other components such as hemicellulose, glycans,
and pectins, which link them together and help strengthen the cell wall
( Raven, Evert, & Eichhorn, 2005 ) . Many cytoplasmic organelles were
observed in the raw potato parenchyma cells, such as starch granules,
mitochondria, golgi apparatus, amyloplasts, generative cells, and lipid
droplets ( Fig. 4.4 A). Plasmodesmata and the pit fields were clearly
observed in the cell walls and some densely stained material was also
observed along the tonoplast ( Fig. 4.4 B and C). Some of the large starch
granules showed electron dense radial “channels” around them, which in
some cases cross the whole granule ( Fig. 4.4 A). Some starch granules
also showed broken amyloplast membrane around their surface (figure
not shown). Similar starch granules have been reported for other plant
sources demonstrating starch degradation ( Appenroth, Keresztes,
Krzysztofiwicz,&Gabrys,2011 ) .
Upon processing of potatoes, the cell wall material degraded partially,
resulting in loosening of the microfibrils. The cell wall of parenchyma cells
decreased in thickness after cooking, probably due to the loss of primary cell
wall to a greater extent. Middle lamella and some remains of primary cell
wall were still observed ( Fig. 4.5 A-C and E). The remains of the primary
cell wall along with some electron dense granular structures were observed
floating in the cytoplasmic starchy matrix ( Fig. 4.5 D). Pectic material has
been reported to degrade during cooking and partly solubilized into the
cooking media ( Hughes, Faulks, & Grant, 1975a,1975b; van Marle,
Recourt, et al., 1997 ) . This degradation greatly influences intercellular
adhesion and the structure of the remaining cell walls, which are both
important texture parameters ( van Marle, Clerkx, & Boekestein, 1992;
van Marle, Recourt, et al., 1997 ).