04/15/2023

INFLUENCES OF TEMPERATURE-CYCLED STORAGE ON RETROGRADATION AND IN VITRO DIGESTIBILITY

OF WAXY MAIZE STARCH GEL

A presentation by Atheer Jasim Mohammed – Ph.D. Student - 05352409136

04/15/2023

SHORT INTRODUCTION 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.

04/15/2023

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.

SHORT INTRODUCTION

04/15/2023

• 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.

THE AIM OF STUDY

04/15/2023

MATERIALS AND METHODS

04/15/2023

MATERIALS AND METHODS

04/15/2023

MATERIALS AND METHODS

04/15/2023

RETROGRADATION CHARACTERISTICS OF STARCH

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.

04/15/2023

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

RETROGRADATION CHARACTERISTICS OF STARCH

04/15/2023

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

GLASS TRANSITION AND ICE MELTING PROPERTIES

04/15/2023

X-RAY DIFFRACTION PATTERN 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.

04/15/2023

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.

TEXTURAL PROPERTIES OF STARCH GELS

04/15/2023

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.

IN VITRO DIGESTIBILITY OF STARCH GELS

04/15/2023

K VALUE AND IN VITRO GLYCEMIC INDEX OF STARCH GELS 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.

04/15/2023

CONCLUSIONS

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.

04/15/2023