Biology Reference
In-Depth Information
and after cooking by autoclaving. A high level of cell integrity was observed
in all samples. The cotyledon cells appeared to have shrunk slightly and
showed indentation and wrinkles on their surface. The swelling of starch
granules during autoclaving resulted in the enlargement of cotyledon cells
due to absorption of water during cooking. This was evident in the case
of undigested samples (
Fig. 4.13
). A magnified view of the cotyledon outer
cell wall of the undigested sample showed less wrinkles compared to the
samples subjected to intestinal digestion. It might be the case that hydrolysis
of starch during
in vitro
digestion and removal of water left a space in which
the cell wall could be folded in during freeze drying, causing an even more
wrinkled surface and indentation. This phenomenon was especially distinc-
tive in cells that underwent the whole simulated digestive process (120 min).
However, the possibility of some artifacts produced due to freeze-drying
process could not be ruled out. The development of holes in the cotyledon
cell walls during the
in vitro
digestion is nevertheless unlikely because the cell
wall is composed of
-(1-4)-linked
D
-glucose molecules and enzymes capa-
ble of hydrolyzing these linkages are not present in the simulated digestive
fluids. The cells stayed mainly intact during the enzymatic action.
The
in vitro
digestion of starch therefore has to at least partially take place
inside the bean cells, implying permeability of the cell wall to digestive
enzymes. The presence of proteins and other components in the digestion
matrix has been reported to decrease the starch hydrolysis (
Singh et al.,
2010
). Cooking or processing sometimes may reduce the starch digestibility
as the conformational changes in proteins may occur that could facilitate the
formation of disulphide-linked polymers
(
Oria, Hamaker, & Shull, 1995
).
b
4. RHEOLOGY OF FOOD MATRIX AND
STARCH DIGESTION
Food matrix viscosity has been reported to be one of the major factors
affecting enzymatic digestibility of starch and glycemic response (
Dartois
Figure 4.13 Scanning electron micrographs collected during in vitro gastric and intes-
tinal digestion of starch in cooked navy beans: (A) undigested sample taken at 0 min,
(a) showing cotyledon cells and (b) magnified view of cotyledon cell wall; (B) sample
taken after 30 min of gastric digestion, (a) cotyledon cells in navy bean digesta and
(b) magnified view of cotyledon cell wall; (C) sample taken after 10 min of intestinal
digestion, (a) cotyledon cells in navy bean digesta and (b) magnified view of cotyledon
cell wall; and (D) sample taken after 120 min of intestinal digestion, (a) cotyledon cells in
navy bean digesta and (b) magnified view of cotyledon cell wall. Reproduced from
Berg
et al. (2012)
with permission from Elsevier.
