28
more slowly than gelatinized starch; since gelatinization has lost the crystalline
structure of starch, allowing greater access to enzymes without the obstructions
caused by α-glucan associations, such as double helices or by amylose-lipid complexes in cereal starches (Tester et al. 2002). Other studies claimed that the dispersed, amylopectin fine structures with high branch density, either long or short
internal chains as well as short terminal non reducing ends, leads to the slow digestion property, because of the inherent structure of amylopectin molecules (Hamaker
et al. 2007; Zhang et al. 2008). The plasma glucose response after consuming raw
maize starch was slow and sustained, which is characteristic of SDS (Seal et al.
2003). The structure of SDS is composed of imperfect crystallites and amylopectin
with a high branching pattern and density, and this is most likely the cause of slow
digestion property.
Heat and Moisture
Heat and moisture content are important factors for development of SDS and
RS. When native starch is heated in excess water, the starch granules undergo gelatinization. The extent of gelatinization depends on the temperature, time, water content and degree of shear during the process. As described previously, native starch
(A-type) is an ideal SDS and the slow digestibility changes during cooking or processing. Incomplete gelatinization can be achieved by lowering the temperature,
decreasing the moisture content, or shortening the heating time. In this way, low GI
benefits of SDS and RS may be retained. When partially gelatinized waxy rice
starch was heated at different temperatures (60, 65, or 70
°
C) for 5 min, they showed
different digestibility rates after retrogradation (Chung et al. 2006). The amounts of
SDS and RS positively correlated with the relative enthalpy of the partially gelatinized starches. In cereal products, such as parboiled rice, barley porridges, biscuits
and pasta, the degree of gelatinization or limited swelling of starch, which is determined mainly by the cooking time and temperature, moisture level, largely influences the formation of SDS and RS (Wolever et al. 1986a, b; Holm et al. 1992;
Granfeldt et al. 1994; Garsetti et al. 2005). Heat-moisture treatment usually refers
to the incubation of starch at low moisture content (<35% w/w) for a certain period
of time at a temperature below the gelatinization temperature, but above the glass
transition temperature, while as annealing is performed in excess water or at an
intermediate water level (≥40% w/w) (Jacobs and Delcour 1998; Tester and Debon
2000). Heat–moisture treatment does not destroy structure of starch granules, but it
alters its crystalline packing; for example, the B type of starch can be converted to
the A or C type, whereas the annealing technique can modify the binding forces
between the crystalline and the amorphous matrix (Stute 1992). Therefore, hydrothermal treatment can be used as a method to form SDS and RS. Anderson et al.
(2002) adjusted both nonwaxy and waxy rice starches to 20% moisture, after that
heated them to their melting temperature (Tm) in a differential scanning calorimeter
(DSC), and held them there for 60 min. They observed that these starches were
B. A. Ashwar et al.
more slowly than gelatinized starch; since gelatinization has lost the crystalline
structure of starch, allowing greater access to enzymes without the obstructions
caused by α-glucan associations, such as double helices or by amylose-lipid complexes in cereal starches (Tester et al. 2002). Other studies claimed that the dispersed, amylopectin fine structures with high branch density, either long or short
internal chains as well as short terminal non reducing ends, leads to the slow digestion property, because of the inherent structure of amylopectin molecules (Hamaker
et al. 2007; Zhang et al. 2008). The plasma glucose response after consuming raw
maize starch was slow and sustained, which is characteristic of SDS (Seal et al.
2003). The structure of SDS is composed of imperfect crystallites and amylopectin
with a high branching pattern and density, and this is most likely the cause of slow
digestion property.
Heat and Moisture
Heat and moisture content are important factors for development of SDS and
RS. When native starch is heated in excess water, the starch granules undergo gelatinization. The extent of gelatinization depends on the temperature, time, water content and degree of shear during the process. As described previously, native starch
(A-type) is an ideal SDS and the slow digestibility changes during cooking or processing. Incomplete gelatinization can be achieved by lowering the temperature,
decreasing the moisture content, or shortening the heating time. In this way, low GI
benefits of SDS and RS may be retained. When partially gelatinized waxy rice
starch was heated at different temperatures (60, 65, or 70
°
C) for 5 min, they showed
different digestibility rates after retrogradation (Chung et al. 2006). The amounts of
SDS and RS positively correlated with the relative enthalpy of the partially gelatinized starches. In cereal products, such as parboiled rice, barley porridges, biscuits
and pasta, the degree of gelatinization or limited swelling of starch, which is determined mainly by the cooking time and temperature, moisture level, largely influences the formation of SDS and RS (Wolever et al. 1986a, b; Holm et al. 1992;
Granfeldt et al. 1994; Garsetti et al. 2005). Heat-moisture treatment usually refers
to the incubation of starch at low moisture content (<35% w/w) for a certain period
of time at a temperature below the gelatinization temperature, but above the glass
transition temperature, while as annealing is performed in excess water or at an
intermediate water level (≥40% w/w) (Jacobs and Delcour 1998; Tester and Debon
2000). Heat–moisture treatment does not destroy structure of starch granules, but it
alters its crystalline packing; for example, the B type of starch can be converted to
the A or C type, whereas the annealing technique can modify the binding forces
between the crystalline and the amorphous matrix (Stute 1992). Therefore, hydrothermal treatment can be used as a method to form SDS and RS. Anderson et al.
(2002) adjusted both nonwaxy and waxy rice starches to 20% moisture, after that
heated them to their melting temperature (Tm) in a differential scanning calorimeter
(DSC), and held them there for 60 min. They observed that these starches were
B. A. Ashwar et al.