Starch source Apparent amylose content
Unit-chain length distribution DP 6
–8 DP 9–12 DP 13–24 DP 25–30
A-type Elephant yam
25.2 5.6
26.8 60.5
7.1 New cocoyam 22.5
7.4 27.3
58.7 6.6
Sweet potato 19.8
11.0 27.9
54.1 7.0
Kudzu 19.6
9.8 26.4
55.8 8.0
Arrowroot 20.0
4.0 27.7
58.4 9.9
Sago 21.9
9.0 28.1
56.2 6.7
Taro 16.3
7.4 28.9
57.3 6.4
Yam bean 18.4
10.3 30.5
52.1 7.1
Cassava 17.9
9.9 36.3
48.3 5.5
Corn 23.4
5.1 31.4
56.7 6.8
Rice 13.2
8.0 34.5
52.1 5.4
B-type Edible canna
27.9 7.2
21.5 63.4
7.9 Water yam
20.8 3.9
18.9 67.5
9.7 Potato
18.0 10.2
23.5 58.9
7.4 C-type
Lesser yam 14.2
11.6 24.9
56.2 7.3
3.2. Branch chain-length distribution of amylopectin
To investigate the unit-chain length distribution of amylopectin, unit-chains of debranched starch were labeled with APTS and subjected to fluorophore-assisted capillary electrophoresis. The
distribution profile and relative molar distribution were specific to the starch botanical origin
Fig. 2 ,
Table 2 . The profiles showed a maximum at DP 11
–12 for A- and C-type starches and at DP 13 for B-type starches. Note that, a wide range of distribution patterns and relative
abundances of very short chains with DP 6 –8 was observed among botanical sources, and this
was in agreement with the results of previous reports Hanashiro et al., 1996
and Jane et al.,
1999 .
Table 2 clearly shows that A-type starches contained higher proportions of chains with
DP 9 –12 26.4–36.3 than B- and C-type starches 18.9–24.9. The proportions of longer
chains with DP 13 –24 of elephant yam, new cocoyam and arrowroot starches 58.4–60.5 were
comparable to those of B-type starches 58.9 –67.5. According to Hizukuris revised cluster
model of amylopectin Hanashiro et al., 1996
and Hizukuri, 1986
, short chains with DP 6 –24
comprise A and B1 chains. These short chains, which are arranged in double helices, are mostly located in the crystalline region of granules. Accordingly, the difference in distribution of unit-
chains between DP 6 and 24 would very likely affect the crystalline structures of starches.
Full-size image 57K
Fig. 2. Unit-chain length distribution of amylopectin analyzed by fluorophore-assisted capillary electrophoresis. Mean areas of peaks between DP 6 and DP 30 were summed and the relative
molar distribution of individual peak was expressed as a fraction of this sum.
3.3. Thermal properties
Table 3 presents the gelatinization temperatures of native starches, namely, onset temperature
T
oG
, peak temperature T
pG
and conclusion temperature T
cG
as well as gelatinization enthalpy on a dry starch basis ΔH
G
. Gelatinization temperature is a measurement of perfectness of starch crystallites
Tester Morrison, 1990 . High gelatinization temperatures were noticed in starches
of elephant yam, new cocoyam, arrowroot, taro, water yam and lesser yam T
oG
, 71.9 –78.2 °C;
T
pG
, 74.8 –81.0 °C; T
cG
, 82.4 –91.4 °C. Cassava, corn, rice and potato starches were gelatinized
by low temperatures T
oG
, 59.3 –62.6 °C; T
pG
, 65.5 –67.3 °C; T
cG
, 77.7 –81.3 °C. The wide range
of ΔH
G
values 13.7 –19.8 Jg among starches might be due to differences in the overall degree
of granular crystallinity Tester Morrison, 1990
. The results of DSC study further support the large variation in the crystalline nature of each starch.
Table 3. Thermal properties of starches
Starch source
Gelatinization
a
Retrogradation
b
T
oG
°C T
pG
°C T
cG
°C ΔH
G
Jg T
oR
°C T
pR
°C T
cR
°C ΔH
R
Jg
A-type starch Elephant yam 77.9
80.2 89.9
19.7 53.8
63.2 72.0
9.2 New
cocoyam 74.3
77.2 87.3
13.7 53.0
62.1 70.5
9.2 Sweet potato
66.7 74.0
86.6 18.4
53.9 63.3
70.2 7.5
Kudzu 68.4
74.8 88.2
17.4 53.7
63.3 70.8
8.1 Arrowroot
73.5 75.9
86.8 17.5
53.6 63.3
72.8 8.5
Sago 65.4
70.4 81.9
17.0 52.5
61.5 69.5
7.8 Taro
74.2 77.4
86.4 16.2
53.6 62.5
69.5 7.7
Yam bean 66.4
70.9 81.5
16.0 53.2
61.7 69.2
6.8 Cassava
59.3 65.7
79.6 17.6
53.1 61.5
67.1 3.1
Corn 62.6
66.7 81.3
16.9 53.8
61.0 67.8
5.6 Rice
61.6 67.3
80.0 18.7
53.5 60.3
65.4 2.9
B-type starch Edible canna
67.4 70.0
78.9 18.7
55.2 65.8
76.2 8.7
Water yam 78.2
81.0 91.4
19.4 55.8
66.6 77.9
11.3 Potato
61.4 65.5
77.7 19.8
54.4 65.1
75.0 9.1
C-type starch Lesser yam
71.9 74.8
82.4 14.3
54.0 64.6
73.5 11.1
a
Onset temperature T
oG
, peak temperature T
pG
, conclusion temperature T
cG
and enthalpy change
ΔH
G
of native
starch.
b
Onset temperature T
oR
, peak temperature T
pR
, conclusion temperature T
cR
and enthalpy change ΔH
R
of retrograded starch.
The DSC parameters of retrograded starches are presented in Table 3
. The gelatinization temperatures of dissociating retrograded starch were lower than those of native starch. This
might result from improper alignment of the starch chains during re-association, which causes formation of less ordered andor less stable crystalline structures than those existing in native
starch. Among the parameters, the range of T
oR
values 52.5 –55.8 °C was narrower than those of
T
pR
60.3 –66.6 °C and T
cR
65.4 –77.9 °C. According to DSC parameters, the re-crystallization
degree of water yam was the greatest whereas those of cassava and rice starches were very small. The T
oR
, T
pR
and T
cR
of A-type starches were lower than those of B-type starches. Similar findings were previously reported
Jane et al., 1999 and
Kalichevsky et al., 1990 . It was found
that T
pR
and T
cR
were positively correlated with T
pG
and T
cG
, respectively P0.05 data not shown, except for those of edible canna and potato starches.
3.4. Digestibility by porcine pancreatic α-amylase
