Biology Reference
In-Depth Information
a
-amylase activity during seed development and germination. Most amylase
inhibitors from plant are active against animal amylases. A natural amylase
inhibitor has been isolated from the germ fraction of wheat, which appeared
very labile. It is indeed destroyed by heating treatment and by passage
through the roller mill (
Snow & O'Dea, 1981
)
. Various commercial
starch-blocking products have been manufactured since early 1940s, but
many of the clinical studies have proved that they do not work
in vivo
,
whereas some studies have reported their positive role during
in vitro
exper-
iments.
-Amylase inhibitors have been found to be unstable in the stomach
and active only after preincubation with amylase in the absence of starch
Dietary fiber has been suggested as the primary factor influencing the
slower rate of glucose release in foods through its high viscosity, which slows
down gastric emptying absorption of digested products in the small intestine.
The rate of starch digestion, however, cannot be explained by the amount of
fiber alone since some legumes (e.g., lentils) have similar fiber contents as
certain cereal products (e.g., wholemeal bread) and yet are digested at very
centration of antinutrients, such as phytic acid, lectins, and enzyme inhibi-
tors in legumes, may also play a role in starch digestibility. Enzyme inhibitors
and lectins have also been shown to produce hypoglycemia and to decrease
growth rates in rat (
Thorne, Thompson, & Jenkins, 1983
)
.
Phytic acid is the most important phosphate reserve compound in many
plants. It can form a complex with proteins and/or metal ions, reducing their
biological availability.
Yoon, Thompson, and Jenkins (1983)
have showed
that the addition of phytic acid after preincubation with saliva decreased the
sugar liberation significantly while little effect has been seen on simultaneous
addition. The phytic acid may affect the starch digestibility through inter-
action with amylase protein and/or binding with salivary minerals such as
calcium which is known to catalyze amylase activity. The effects of
processing on antinutrients (phytic acid, condensed tannins, polyphenolics,
a
a
-amylase inhibitor) acting on starch digestibility are also important to con-
sider (
Alonso, Aguirre, & Marzo, 2000; Rehman & Shah, 2005
)
. For
instance, the most efficient treatment for phytic acid reduction found was
germination for 72 h. Extrusion cooking also causes a significant reduction
in phytic acid and in condensed tannins and polyphenolics as well. Thermal
processing methods have been reported (
Rehman & Shah, 2005
)
to act by
thermal degradation of the antinutrient molecules, changes in their chemical
reactivity, and formation of insoluble complexes.
