THESIS

1.2.9 Starch hydrolysis rate and estimated glycemic index
In vitro starch hydrolysis and glycemic index were determined
according to Goňī, et al. (1997). RS III samples (50 mg) were incubated with 1 mg
pepsin in 10 ml HCl-KCL buffer (pH 1.5) at 40 o C for 60 min in a shaking water
bath. The digest was diluted to 25 ml by adding Tris maleate buffer (pH 6.9), and then
5 ml of α-amylase solution, containing 2.6 IU of α-amylase in Tris maleate buffer,
were added. The samples were incubated at 37 o C in a shaking water bath. 0.1 ml
sample was taken from each flask every 30 min from 0 to 3-hr and boiled for 15 min
to inactivate the enzyme. Sodium acetate buffer (1 ml 0.4 M, pH 4.75) was added and
the residual starch digested to glucose by adding 30 µl amyloglucosidase and
incubating at 60 o C for 45 min. Glucose concentration was determined by using a
glucose oxidase-peroxidase kit. The rate of starch digestion was expressed as the
percentage of starch hydrolyzed at different times.
An equation: C=C∞ (1-e -kt ) was used to described the kinetics of
starch hydrolysis, where C, C∞ and k were the concentration at time t, the equilibrium
concentration and the kinetic constant, respectively. The area under the hydrolysis
curve (AUC) was calculated using the equation:
AUC
=
C
α
C α
− k ( t f − t 0
( t − t ) − ( 1 − e )
f
0
k
Where, C∝ corresponds to the concentration at equilibrium (t180). tf is
the final time (180 min), t0 is the initial time (0 min) and k is the kinetic constant.
A hydrolysis index (HI) was calculated by comparison with the
AUC of a reference food (white bread). By using the in vitro starch HI values, the GI
was estimated by the following equation established by Goňī, et al. (1997):
GI = 39.71 + (0.549 × HI).
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