Influences of temperature-cycled storage on retrogradation and in vitro digestibility of waxy maize starch gel

1. A presentation by Atheer Jasim Mohammed – Ph.D. Student - 05352409136
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2.  Starch gelatinization is a process that disrupts the native molecular
orders of starch granules and typically occurs during thermal
processing in the presence of water.
 Tends to re-associate to form an ordered gel-like structure which
ultimately induces changes in the physical behavior of the starch.
Starch often changes from an amorphous state to a crystalline state.
 Retrogradation of starch during the storage of starch foods affects
the digestibility of starch and often decreases consumer acceptance
of starch-based foods such as bread, noodles or cooked rice.
 Amylopectin contributes to the retrogradation occurring in long
term rheological and structural changes, whereas amylose is usually
responsible for the short-term, rapid changes in food texture.
 Crystallization occurs in three consecutive steps: nucleation
(formation of critical nuclei), propagation (growth of crystals from
the nuclei formed) and maturation (crystal perfection or continuing
slow growth), all of which are temperature-dependent.
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3.  Starch retrogradation increases the resistance of starch to digestive
enzymes and consequently reduces the GI value.
 At a temperature near the starch glass transition, the nucleation rate of
starch crystallization is high and the propagation rate is low, whereas
the nucleation rate is low and the propagation rate is high when the
temperature is close to crystal melting.
 Starch can be classified into three groups according to the rate of
glucose release and its absorption in the gastrointestinal tract: rapidly
digestible starch (RDS), slowly digestible starch (SDS) and resistant
starch (RS). RDS is the group of starches that can be rapidly
hydrolyzed by digestive enzymes; SDS is the group of starches that are
digested at a relatively slow rate and RS is the group of starches that
are not digested by digestive enzymes and are consequently transferred
into the colon.
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4. • waxy maize starch gels were retrograded under isothermal (4C) or
temperature cycles of 4 and 30C.
• We examined the starch recrystallization, crystalline structure and gel
texture, and digestibility of waxy maize starch gels to determine the
influences of storage conditions including the duration and
temperature cycle on starch retrogradation.
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 The onset temperature of melting retrograded starch molecules increased from 30.6C to 34.4C
as the storage time at the constant 4C increased from 2 days to 16 days.
 The peak temperature of the DSC endotherms gradually decreased with extended storage time.
 The melting enthalpy increased as the storage time increased, indicating continuous
propagation of starch recrystallization.
 The gels stored under the temperature cycles of 4 and 30C showed substantially greater onset
temperature of melting retrograded starch, this result may indicate that the homogeneity of
retrograded starch increases under temperature-cycled storage conditions.

9.  The extended storage of waxy starch gel up to 16 days under cycled temperature
conditions led to an increased peak temperature.
 A decreased melting enthalpy was also observed with waxy maize starch after a
treatment at cycled temperatures of 6 and 40C. During storage at 30C, possibly
some of the unstable crystals that melt at a relatively low temperature
transformed to more stable crystals.
 Starch gels stored at 4C showed a large increase in the enthalpy of melting
retrograded starch in the first 4 days of storage and leveled off with further
storage, whereas the melting enthalpy of the retrograded starch in gels stored
under the cycled temperatures continuously increased.
 The cycled temperature storage appears to induce the formation of more perfect
and stable crystalsin the waxy starch gel
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10.  The glass transition temperature Tg of the freeze-concentrated
liquid phase in the starch gel increased and the enthalpy of ice
melting decreased during storage for 16 days.
 The retrogradation of starch gel occurs through the
association of starch molecules during storage, reduces the
mobility of water molecules and decreases the amount of
effective plasticizing water. This change induces an increase in
Tg and the amount of freezable water.
 Re-associations of starch molecules in the amorphous regions
occur primarily during the initial stage of storage where long
branches of amylopectin mainly contribute. This was verified
by the significant change in glass transition temperature in the
early stage of starch retrogradation.
 Ice melting enthalpy was negatively related with Tg of the
freeze-concentrated liquid phase; an increase in Tg results in
decreased enthalpy for ice melting.
 The starch gels stored under the cycled temperatures exhibited
a comparable increase in Tg and a much smaller decrease in
ice melting enthalpy than those stored under the 4C constant
temperature
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11.  Storage of gel for 2 days induced no notable changes in X-ray
diffraction patterns, while a small increase in crystallinity was
observed after longer than 4 days of storage (data not shown).
 The peak at 17° (2) indicates that the starch crystals formed in the
retrograded starch gel had the B-type configuration.
 The B-type configuration is typical for retrograded starch. There
were no significant differences in X-ray patterns among starch gels
stored for different periods of time and temperature profiles.
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12.  The hardness of the retrograded starch gel
increased during storage at a constant 4C for 16
days. However, relatively few changes in hardness
were observed when the gel was stored under cycled
temperatures of 4 and 30C for 16 days.
 The hardness of starch gel stored at 4C increased
rapidly in the first 2 days of storage and the
increase slowed with further storage time. When
stored isothermally at 4C, the amorphous matrices
of starch gel may be continually transformed to
more rigid structures.
 under cycled temperature storage, the amorphous
matrices formed at 4C were easily disrupted during
subsequent storage at 30C, preventing the
hardening of starch gel during storage.
 When the starch gel was stored at 4C, the flexibility
increased in the first 2 days, and then decreased
with extended storage time despite the continuous
increase in hardness, the starch gels stored under
cycled temperatures were always softer than those
stored under the constant temperature of 4C.4/29/2015

13.  The starch gelatinized by autoclaving was fully
digested and its SDS and RDS contents were
18.91% and 81.09%, respectively.
 With 2 days of storage at 4C, a substantial
decrease in the proportion of RDS of starch gel
and a concurrent increase in the proportion of
SDS were observed.
 A significant decrease in the proportion of RDS
and an increase in the proportion of SDS were
observed with storage under a cycle of 4C for 2
days and 30C for 2 days.
 The RS proportion consistently increased with
extended storage time for 16 days at the cycled
temperature conditions, along with decreases in
the proportions of SDS and RDS.
 The starch gel stored for 16 days at the cycled
temperatures of 4 and 30C contained 48.2% RS,
whereas the gel stored for 16 days at constant
4C had 10.3% RS.
 Although the starch retrograded during storage
at 4C displayed a higher melting enthalpy than
those stored under the cycled conditions (Table
1), its enzyme susceptibility was much greater
(Fig. 3).
 The degree of starch crystallinity estimated by
DSC and X-ray diffraction may not effectively
explain the variations in the digestibility of the
retrograded starch gels.
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14.  Both the kinetic constant (k) and the in vitro
glycemic index (GI) of retrograded starch gel
decreased as storage time increased.
 The kinetic constant (k value) of starch digestion
sharply dropped in the first 4 days of storage and
exhibited no changes afterward in both starch gels
with different storage conditions, indicating that a
large portion of starch gel becomes retrograded and
indigestible.
 The starch gels stored under the cycled temperatures
displayed lower k values than the gels stored at 4C.
(Fig. 3).
 It was observed that the decrease in in vitro GI was
more substantial when the starch gel was stored at
the cycled temperatures, and the decrease continued
during storage for 16 days (Fig. 4). The continued
reduction in in vitro GI of the starch gel coincided
with the increased proportion of RS as the
temperature cycles were repeated.
 Storage under the cycled temperatures of 4 and 30C
would be an effective way to reduce the digestion rate
and in vitro GI of starch-based foods.
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15.  Although crystal formation was relatively slower when the temperature cycles were
applied during storage, it continued over an extended period.
 The differences in retrogradation behavior of starch molecules during storage under
the cycled temperatures resulted in a greater amount of RS content and lower value of
glycemic index.
 The increase in gel hardness, which often results from the retrogradation of starch in
gels during storage, was effectively reduced when the gel was stored under the cycled
temperatures of 4 and 30C.
 At conditions 4 , 30C, the starch crystals formed in the gel was melted at a higher
onset temperature with a narrower endothermic peak and a lower enthalpy than
those formed under constant temperature storage at 4C.
 The glass transition temperature Tg increased and the ice melting enthalpy decreased
throughout the retrogradation process regardless of the temperature cycle.
 The starch gel stored under the cycled temperature conditions shows slightly lower Tg
and greater ice melting enthalpy than those stored under the constant 4C.
 The starch gel retrograded at the cycled temperature conditions remained softer than
those stored under constant temperature with similar flexibility, On the other hand, it
was a greater amount of resistant starch and reduced the in vitro glycemic index more
effectively than the isothermal storage condition.
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