Biology Reference
In-Depth Information
b
cooking. Cell wall xyloglucans are composed of
-(1-4)-linked
D
-glucose
molecules, an identical primary structure to cellulose but with additional
xylosyl units attached to the O-6 position of the glucosyl units. These
xyloglucans occur at relatively high levels in legumes, which might be
the reason for the extremely high rigidity of cotyledon cells of navy beans
when cooked (
Carpita & Gibeaut, 1993
). In the dry state, the cell walls
are very brittle and are easily disrupted during milling.
3.2.2 Starch digestion in vitro and microstructure of digesta
Cells observed under the light microscope in a sample of freshly prepared
bean paste were found to be generally intact, with little evidence of disrupted
cells. The starch granules present in the cotyledon cells also showed birefrin-
gence when viewed in polarized light suggesting incomplete gelatinization
in cooked whole navy beans, which is presumably due to restrictions to
water uptake imposed by the thick cell walls
(
Fig. 4.11
).
Figure 4.12
gives
an overview of the kinetics of starch hydrolysis during the
in vitro
digestion
process. Like mentioned earlier, the first 30 min of hydrolysis represented
simulated gastric conditions having low pH, whereas the next 120 min rep-
resented the simulation of small intestinal conditions at neutral pH. In the
gastric step, constant levels of glucose in a very low range (2.89
0.54%)
were detected. This glucose may be derived from sugars or dextrins initially
present in navy beans. The constant level of glucose throughout the
in vitro
Figure 4.11 Light microscopy picture (polarized light) showing cotyledon cells in navy
bean paste before in vitro digestion. Reproduced from
Berg et al. 2012
with permission
from Elsevier.
