Comparative Study on Functional, Rheological, Thermal, and Morphological Properties of Native and Modified Cereal Flours

Abstract

Effect of extrusion on the functional, rheological, thermal, and morphological properties of the modified cereal flours from different cereals was assessed. Rice flour, wheat flour, and flour, in combination (rice: wheat, 50:50) were passed through twin screw extruder to obtain modified cereal flours at variable conditions (barrel temperature: 175 and 190°C, feed moisture: 14 and 16% and screw speed: 500 rpm). Functional properties (water absorption index, water solubility index, swelling index, and viscosity) improved in modified cereal flours as compared to the unmodified flours. Modified flours showed lower paste viscosity as compared to unmodified flours, which was a desirable property for modified flours to be utilized as a functionality ingredient in food products. Processed flours recorded higher onset (T_o), peak (T_p), and endset (T_e) temperature and showed higher enthalpy change (∆H) than the raw cereal flours. Degree of gelatinization was higher in flours processed at higher barrel temperature and feed moisture. The morphological pattern of modified flours was determined by scanning electron microscopy. The starch surface of cereal flours (modified and unmodified) differed from each other with respect to their morphological pattern.

Keywords:

• Extrusion
• Modified flour
• Functional properties
• Rheological properties
• Thermal properties
• Morphology

INTRODUCTION

Cereal starches possess a wide application in the food industry as an additive but the native starch is not widely used in food industry due to its poor functional properties. Modified starches and flours have become important in processed foods because the functional properties of the starches and flours are improved over the native starches and cereal flours.^[1] Chemical modifications such as crosslinking and acetylation, and physical treatments, such as high pressure treatment,^[2] microwave heating,^[3] heat-shear extrusion processing,^[4,5] γ-irradiation,^[6] and warm-water treatment,^[7] are widely used to change starch properties. Heat-moisture treatment (HMT; heating at a restricted moisture level) is one method used to physically modify starches and cereal flours. Heat-moisture treated starches and cereal flours are considered safe and have a preferred image with many consumers.

High-temperature extrusion cooking is used extensively by many food industries to produce various food products with unique texture and flavor characteristics. Desirable properties in the end product are obtained by varying the processing conditions as well as the composition of the raw material. It is recognized that the addition of ingredients such as lipids, proteins, sugar, and salt alter the physical and chemical properties of the extruded foods. Changes in the properties of starchy foods caused by the addition of lipids are attributed to the formation of complexes between amylose and lipids. Twin screw extrusion studies have been conducted with cereal flours and grits that contain protein, which also binds lipids.

Differential scanning calorimetry (DSC) has commonly been used for investigating thermal behavior of starches. Gelatinization as well as retrogradation are two important thermal behaviors. DSC data can provide a quantitative measure of gelatinization,^[8] retrogradation,^[9] and the phase transitions on maize and wheat starches, as well as on rice. Therefore, an endothermic transition could be observed by DSC. It was indicated that rice starch was thermally degraded and starch structural changes were caused by high-pressure steaming.

A proper understanding of the starch phase transitions is extremely important in food processing. Native starch is not widely used in the food industry due to its poor functional properties, therefore, most starches currently incorporated into foods are modified. The mean thermal transition of the starch is gelatinization, it is used to describe the molecular behavior of starch related with heat and moisture content. In this process the starch changes its semi-crystalline phase to an amorphous phase. In excess of water, the hydrogen bridges are broken allowing water be associated with the free hydroxyl groups. This change, in turn, facilitates its molecular mobility in the amorphous regions and allowing the swelling of the grains. The most important parameter in the gelatinization study is the temperature. In the gelatinization process, T_o is defined as the initial temperature, T_p is the begin of gelatinization or crystal melting and T_e is the final temperature. The energy necessary to complete the process or gelatinization enthalpy ∆H are also important. The starch is composed by amylose and amylopectin, both with a semi-crystalline phase. Scanning electron microscopy (SEM) plays an important role in increasing understanding the granular structure of modified flours. It is used to detect structural changes caused by modification.

MATERIALS AND METHODS

Materials

Wheat flour and rice flour were procured from commercial market and a combination of both flours was prepared in the ratio of 1:1.

Extrusion Processing

Extrusion experiments were performed on a co-rotating intermeshing twin screw extruder (Clextral, Firminy, France). The barrel diameter and its length to diameter (L/D) ratio were 2.5 mm and 16:1, respectively. Temperatures of the first, second, and third zone was maintained at 40, 70, and 100°C, respectively, throughout the experiments, while the temperature at the fourth zone was varied according to the experimental design. The die plate had one circular hole with 3.0 mm diameter. The extruder was powered by 8.5 kW motor with speeds variable from 0 to 682 rpm. The extruder was equipped with a torque indicator, which showed percent of torque in proportion to the current drawn by the drive motor. Raw materials were metered into the extruder with a single screw volumetric feeder (D, S & M, Modena, Italy). The feed rate was varied for optimum fill according to screw speed. The moisture content of the feed was adjusted by injecting water into the extruder with a pump. A variable speed die face cutter with four bladed knives was used to cut the extrudates. Thermal processing variables used were: Barrel temperature: 175; 190°C; moisture content: 14; 16%; screw speed: 500 rpm.

Grinding and Sieving

The grinding of Extruded Modified product was done up to a uniform particle size using cemotac mill (Foss, Hoganas, Sweden) having setting at no. 1. The ground material was sieved through 48 mesh size to obtain uniform particle size and stored in pearlpet jars for subsequent study.

Methods

Water absorption index (WAI)

WAI was determined by method outlined by Anderson et al.^[10] Two and one-half grams of ground sample was suspended in 30 mL of distilled water at 30°C in a 50 mL tarred centrifuge tube. The contents were stirred intermittently over 30 min period and centrifuged at 3000 × g for 10 min. The supernatant liquid was poured carefully into tarred evaporating dish. The remaining gel was weighed and WAI was calculated as the grams of gel obtained per gram of solid.

$\mathrm{W}\mathrm{A}\mathrm{I}\left(\mathrm{g}/\mathrm{g}\right)\hspace{0.17em}=\frac{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}}{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{s}}$

Water solubility index (WSI)

WSI was determined from the amount of dried solids recovered by evaporating the supernatant from the WAI test described above.^[10] WSI was expressed as:

$\frac{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{i}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{v}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{n}\mathrm{s}\mathrm{u}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}}{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{d}\mathrm{s}}\times 100$

Swelling power

Swelling power was measured using a method reported by Tester and Morrison.^[11] One-half of a gram of sample was dispersed in 15 mL of distilled water. The suspension was heated at 90°C in a water bath for 30 min with vigorous shaking. The starch gel was then centrifuged at 3000 rpm for 15 min. the weight of sediment was used to calculate the swelling power. Swelling power was calculated as follow:

$\mathrm{S}\mathrm{w}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{p}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}\left(\mathrm{\%}\right)\hspace{0.17em}=\frac{\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{w}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\left(\mathrm{g}\right)}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{m}\mathrm{a}\mathrm{s}\mathrm{s}\left(\mathrm{g}\right)}$

Viscosity

Viscosity was measured using Brookfield viscometer at room temperature with a spindle no. 3 at a speed of 30 rpm. Slurry was prepared by mixing 50 g of modified flour with distilled water to make a volume up to 500 mL.^[12]

Rheological Properties

A rapid visco analyzer (RVA) model starch Master (Newport Scientific, Warrie Wood, Australia) was used to determine the pasting properties of blends using the following procedure: Switch on the RVA and allow it to warm up for 30 min prior to the experiment. Weigh 3.0 g (14% moisture basis) of flour in a canister. Place the paddle into the canister and vigorously jog the blade through the sample up and down 10 times or until it mixes uniformly. Insert the canister into the pre-adjusted instrument. Initiate the measurement cycle by depressing the motor tower of the instrument. Remove the canister on completion of test and discard.

Thermal properties

It was done by using DSC (Mettler). Heating rate was 5°C/min. from 20–150°C. Samples (110–120 mg) were mixed with water (1:2, w/w) and kept at 4°C overnight to allow a uniform distribution of water in the flours. Samples were sealed in stainless crucibles and reweighed before the DSC analysis. Onset temperature (T_o), peak temperature (T_p), endset temperature (T_e), and enthalpy change (∆H) were assessed.

Degree of gelatinization

It was estimated by using a DSC. It was calculated as follows:

$\mathrm{D}\mathrm{G}\mathrm{\%}=\frac{\mathrm{\Delta }{H}_{raw}-\hspace{0.17em}\mathrm{\Delta }{H}_{sample}}{\mathrm{\Delta }{H}_{raw}}$

Morphology of Starch Granules

Morphology of modified flours was studied by SEM. Samples were sputter coated with Au using a vacuum evaporator and examined using SEM (Hitachi S 3400 N) at 15 kV accelerating voltage using the secondary electron technique.

Statistical Analysis

Proximate composition was expressed at 14% moisture. Values are mean of triplicates. Data collected from the aforesaid experiments was subjected to statistical analysis for least significant difference (LSD) test and standard error (SE).^[13]

RESULTS AND DISCUSSION

WAI

The WAI (g/g) measures the volume occupied by granule or starch polymer after swelling in excess water. WAI of raw and modified cereal flours is given in Table 1. WAI of raw cereal flours was 4.56 g/g (rice flour), 3.12 g/g (wheat flour), and 3.98 g/g (flour in combination) which almost doubled on processing. Extrusion processing significantly altered the WAI of cereal flours, as it increased on modification of cereal flours. Maximum WAI (7.20 g/g) was obtained for modified flour, in combination (rice and wheat) extruded at barrel temperature of 190°C. Kebede et al.^[14] reported a consistent increase in WAI with increase in temperature for white tef extrudates. This increase was attributed to the higher proportion of gelatinized starch granules. However, when temperature increased, amylose and amylopectin chains formed an expandable matrix that translated into a higher water retention capacity.^[15,16] Comer and Fry^[17] documented that the water holding capacity of treated starches were more than the raw starch because the starch granules get bruised by pregelatinization process including heating and drying treatment and the starch get reduced into dextrins.

TABLE 1 Effect of modification on the functional properties of cereal flours

Type of flour	Functional properties
	WAI (g/g)	WSI (%)	Swelling power (%)	Viscosity(cP)
Rice flour
Raw	4.56 ± 0.01	14.7 ± 0.002	8.65 ± 0.01	3521 ± 4
Modified* A	6.64 ± 0.02	17.9 ± 0.003	9.25 ± 0.03	2201 ± 2
B	6.71 ± 0.02	20.4 ± 0.001	11.52 ± 0.01	1782 ± 3
Wheat flour
Raw	3.12 ± 0.03	12.3 ± 0.002	5.31 ± 0.02	1154 ± 2
Modified* A	6.64 ± 0.04	16.4 ± 0.002	8.81 ± 0.04	874 ± 2
B	6.85 ± 0.02	20.6 ± 0.001	9.25 ± 0.01	782 ± 3
Flour in combination
Raw	3.98 ± 0.03	13.5 ± 0.004	7.63 ± 0.02	2476 ± 1
Modified* A	6.89 ± 0.04	17.2 ± 0.002	9.31 ± 0.03	1897 ± 2
B	7.20 ± 0.03	20.9 ± 0.003	10.72 ± 0.02	1809 ± 3
LSD (p<0.05)	1.47	0.95	1.19	7.13
Modified flour	WAI (g/g)	WSI (%)	Swelling power (%)	Viscosity (cP)
Rice flour	6.68 ± 0.03	19.3 ± 0.02	10.39 ± 0.02	1991 ± 3
Wheat flour	6.75 ± 0.02	18.5 ± 0.01	9.03 ± 0.01	828 ± 4
Flour in combination	7.05 ± 0.03	19.1 ± 0.04	10.02 ± 0.03	1853 ± 3

*A-175°C, 16%, 500 rpm; B: 190°C, 14%, 500 rpm Mean ± SE

WSI

WSI (%) determines the amount of free polysaccharides or polysaccharide released from the granule after addition of excess water.^[18] WSI of native and modified cereal flours differed significantly (Table 1). WSI (%) for different raw flours ranged between 12.3–14.7%, least for wheat flour and maximum for rice flour. Mean WSI for modified rice flour and flour in combination were at par (19.3 and 19.1%), whereas for modified wheat flour it was 18.5%. Increasing temperature contributed an increasing effect on solubility of cereal flours. Zeng et al.^[19] found a significant difference in WSI of native and extruded flours and concluded that the solubility index of extruded flour was relatively higher than that of raw flour. During extrusion, starch structures get disrupted and crystalline regions get melted. Due to this melting under high temperature and high shear conditions molecular fragmentation of starch occurred which improved the solubility index. Higher barrel temperature increases the WSI of HMT treated rice starch due to the retropolymerization of starch and other macro molecules present in the mixture.^[20]

Swelling Power

Table 1 shows the data pertaining to swelling power of unprocessed and processed cereal flours. Native rice flour had maximum swelling power 8.65% followed by flour in combination (7.63%) and wheat flour (5.31%). Swelling power on modification increased to 11.52, 9.25, and 10.72% for processed rice flour, wheat flour, and flour in combination, respectively, when extruded at conditions of 190°C, 14% and 500 rpm. Temperature has a significant effect on swelling power as starch granules swell more on increasing temperature. This may be because of higher absorption of starch on increasing temperature. Wheat flour showed least swelling as compared to other cereal flours because of the reason that amylose-amylopectin ratio is less in wheat flour. Lawal et al.^[21] reported an increase in swelling and solubility indices in oxidized and thinned hybrid maize starch which was a result of increased mobility of starch molecules, which facilitated easy percolation of water.

Viscosity

Viscosity profile of native and extruded cereal flours is presented in Table 1. Significant differences were obtained in the viscosity among native and extruded cereal flours. Raw cereal flours were more viscous than their relative processed cereal flours. Native rice flour had maximum viscosity (3521 cp) relative to other flours. Wheat flour had least viscosity (1154 cp) because of the difference in amylose-amylopectin ratio, which is more in wheat as compared to rice. On modification of the cereal flours, mean viscosity reduced and followed the order rice flour (1991 cp) > flour in combination (1853 cp) > wheat flour (828 cp), irrespective to the extrusion variables. Increase in barrel temperature from 175 to 190°C decreased the flour viscosity. High temperature caused starch degradation, thereby lowering the viscosity. It is inferred from the data that flour processed at 190°C had significant lower viscosity relative to the flour processed at 175°C. Similar observations were reported for pregelatinized wheat starch produced by a single drum-drier^[22] and for pregelatinized maize starch produced by twin drum drier.^[23] They indicated that degradation of starch macromolecules may result to the reduction of the viscosity.

Pasting Properties of Cereal Flours

The pasting properties of a material reflect its structure. During gelatinization, starch granules swell to several times to their initial volume. Swelling is accompanied by leaching of granule constituents; predominately amylose and the formation of a three dimensional network.^[24] These changes are responsible for the pasting characteristics exhibited by starch suspensions during heating and shearing. The pasting behavior of raw and modified cereal flours was measured using a RVA and is presented in Table 2. Significant difference (p ≤ 0.05) was observed among the pasting properties of native and high temperature short time (HTST) modified cereal flours. Variable extrusion conditions represented the significant influence with regard to pasting behavior of native and modified flours. Temperature required for the flour paste to attain the maximum viscosity was 77.2°C (rice flour), 94.8°C (wheat flour), and 90.2°C (flour in combination). Modified cereal flours did not show the pasting temperature, might be due to the pregelatinization of starch which had occurred during extrusion processing of cereal flours. Peak viscosity occurs at the equilibrium point between swelling and polymer leaching which caused an increase in viscosity. Peak viscosity for unmodified rice flours was 2338 cp and that for wheat flour and flour in combination was 1709 and 1975 cp, respectively, which decreased as the cereal flours undergo modification depicting that flours processed at conditions of high temperature were relatively less viscous. Final viscosity for different cereal flours differed significantly for unprocessed and processed cereal flours. The lower final viscosity of HTST processed cereal flours was the result of starch liquification during processing at higher temperature. Our results are in accordance with Zeng et al.,^[19] who also reported in his studies that the extruded flour had significantly reduced RVA viscosity parameters and pasting temperature relative to raw flour, which were influenced by molecular degradation of starch during extrusion. When the starch is cooled in water, its cohesive forces within the swollen granule get weakened, and the viscosity of the paste gets decreased as the integrity of the granule is lost. The peak did not form in the dextrinized cereal starch and crosslinked corn starch.^[25] The reason of this might be that dextrinized modified flour had very low viscosity. These results were similar to the findings of Luallen^[26] who reported that dextrin had a very low viscosity and very high dissociation value. Breakdown viscosity of raw cereal flours was significantly higher than the modified cereal flours. Modified rice flour, wheat flour, and flour in combination showed viscosity of 1381, 870, and 1237 cp, respectively, on breakdown. Native rice flour had maximum breakdown viscosity (2011 cp), followed by flour in combination (1408 cp), and wheat flour (923 cp). Lai^[27] documented that peak viscosity, final viscosity, and breakdown viscosity of treated rice flour were lesser than that of raw rice flour. During setback, cooling occurs and reassociation between starch molecules, especially amylose resulted in formulation of gel structure and, therefore, viscosity increased during this phase. This phase is generally related to retrogradation and reordering of starch granules. Native cereal flours showed higher reordering of starch granules as compared to the modified cereal flours in the order rice flour (1821 cp) > wheat (1485 cp) > flour in combination (1534 cp). Flours processed at higher temperature (190°C) were less viscous than the flours extruded at 175°C.

TABLE 2 Effect of modification on the pasting properties of cereal flours

Type of flour	Pasting properties
	Pasting temperature (°C)	Peak viscosity (cP)	Final viscosity (cP)	Breakdown viscosity (cP)	Setback viscosity (cP)
Rice flour
Raw	77.2 ± 3	2338 ± 2	3148 ± 1	2011 ± 3	1821 ± 1
Modified* A	NR	1634 ± 4	1571 ± 2	1537 ± 4	1474 ± 3
B	NR	1393 ± 4	1529 ± 4	1226 ± 2	1269 ± 2
Wheat flour
Raw	94.8 ± 1	1709 ± 2	2471 ± 2	923 ± 2	1485 ± 2
Modified* A	NR	1299 ± 2	1490 ± 2	893 ± 1	1034 ± 3
B	NR	1070 ± 3	1493 ± 2	847 ± 2	1270 ± 3
Flour in combination
Raw	90.2 ± 4	1975 ± 4	2801 ± 2	1408 ± 6	1534 ± 3
Modified* A	NR	1301 ± 1	1549 ± 1	1357 ± 1	1405 ± 3
B	NR	1411 ± 2	1522 ± 3	1117 ± 4	1428 ± 3
LSD (p < 0.05)	1.33	3.11	4.15	3.56	3.54
Modified flour	Peak viscosity (cP)	Final viscosity (cP)	Breakdown viscosity (cP)	Setback viscosity (cP)
Rice flour	1513 ± 2	1550 ± 3	1381 ± 2	1371 ± 4
Wheat flour	1184 ± 1	1491 ± 5	870 ± 1	1152 ± 3
Flour in combination	1356 ± 4	1356 ± 3	1237 ± 2	1416 ± 1

*A: 175°C, 16%, 500 rpm; B: 190°C, 14%, 500 rpm Mean ± SE

Thermal Properties and Degree of Gelatinization of Cereal Flours

Native starches are not used in the food industry due to its poor functional properties; therefore, most starches incorporated into foods are in modified form. The mean thermal transition of starch is used to describe the molecular behavior of starch related to heat and moisture content. In this process, the starch changes its semi-crystalline phase to an amorphous phase. In excess water, the hydrogen bridges are broken allowing water to be associated with the free hydroxyl groups. This facilitates the molecular mobility in the amorphous regions and allowing the swelling of the grains.^[28] The data presented in Table 3 highlighted the effect of modification on the thermal properties and degree of gelatinization of different cereal flours. Onset temperature (T_o), peak temperature (T_p), and endset temperature (T_e) varied significantly for raw and processed cereal flours. Unprocessed cereal flours had higher onset, peak and endset temperature as compared to the HTST processed cereal flours. This may be attributed to the reason that processed cereal flours had undergone through extrusion process making the starch in pregelatinized form. There was a loss of birefringence indicating the disorder of the starch molecules. Hence, the modified cereal flours got gelatinized at lower temperature as compared to the raw cereal flours.

TABLE 3 Effect of modification on the thermal properties and degree of gelatinization of cereal flours

Type of flour	Thermal properties
	To (°C)	Tp (°C)	Te (°C)	∆H (J/gm)	DG (%)
Rice flour
Raw	85.42 ± 0.04	87.73 ± 0.02	93.46 ± 0.03	18.46 ± 0.01	NA
Modified* A	77.54 ± 0.05	79.48 ± 0.02	81.53 ± 0.03	3.50 ± 0.04	81.2 ± 0.01
B	80.48 ± 0.01	83.59 ± 0.03	87.52 ± 0.01	2.30 ± 0.04	87.5 ± 0.03
Wheat flour
Raw	91.78 ± 0.02	94.14 ± 0.03	97.53 ± 0.03	15.40 ± 0.01	NA
Modified* A	82.67 ± 0.03	85.75 ± 0.02	88.42 ± 0.03	5.20 ± 0.04	66.5 ± 0.02
B	87.87 ± 0.01	89.32 ± 0.03	92.47 ± 0.03	4.70 ± 0.03	69.4 ± 0.02
Flour in combination
Raw	89.32 ± 0.04	93.82 ± 0.01	96.84 ± 0.01	16.60 ± 0.02	NA
Modified* A	77.57 ± 0.01	79.52 ± 0.04	84.72 ± 0.04	4.44 ± 0.01	72.5 ± 0.03
B	82.72 ± 0.02	84.83 ± 0.02	88.69 ± 0.01	3.66 ± 0.03	77.3 ± 0.03
LSD (p < 0.05)	1.13	1.03	1.02	1.01	1.20
Modified flour	To (°C)	Tp (°C)	Te (°C)	∆H (J/gm)	DG (%)
Rice flour	79.01 ± 0.02	81.54 ± 0.01	84.53 ± 0.02	4.23 ± 0.03	84.35 ± 0.01
Wheat flour	85.27 ± 0.03	87.54 ± 0.02	90.45 ± 0.02	4.95 ± 0.02	67.95 ± 0.02
Flour in combination	80.15 ± 0.03	82.18 ± 0.04	86.71 ± 0.01	4.05 ± 0.03	74.9 ± 0.04

*A: 175°C, 16%, 500 rpm; B: 190°C, 14%, 500 rpm Mean ± SE

T_o required for raw wheat flour was maximum (91.78°C) followed by flour in combination (89.32°C) and rice flour (85.42°C). Similarly, modified wheat flour had maximum mean T_o (85.27°C) relative to flour in combination and rice flour. Similar pattern was observed for T_p and T_e in all cereal flours. Native wheat flour had higher T_p and T_e (94.14 and 97.53°C) as compared to rice flour and flour in combination. Wheat flour required higher temperature as compared to rice flour and flour in combination which was due to the difference in the shape and structure of the wheat starch granules which were round in shape and usually are 2–10 nm in size as compared to rice flour having polygonal starch granules and 3–8 nm granule size which required higher temperature to get gelatinize. It was found that the flour obtained at 175°C had a low T_o, T_p, and T_e while those extruded at higher temperature (190°C) had a higher T_o, T_p, and T_e. This may be due to the increase in time, shear, and pressure during extrusion which increased the rate of gelatinization. Karaoglu et al.^[29] reported that the starch granule get unfolded and degraded during pregelatinization process. Because of this, the temperature of gelatinization increased. Similar values for T_o, T_p, and T_e were obtained by Semesaka et al.^[30] T_o of extrudates ranged from 43.99–96.17°C, T_p (51.27–99.17°C), and T_e (58.09–99.92°C). It was found that the products prepared at low temperature had lower T_o, T_p, and T_e than those prepared at higher temperature. Zavareze et al.^[31] also documented that HMT processing increased T_o, T_p, and T_e of rice starches. The effect of HMT on thermal parameters was dependent on the moisture level and the temperature of the treatment. These HMT induced effects became more intense with the increasing temperature and moisture of the hydrothermal treatment.

As inferred from tabulated data, modified cereal flours showed lower ΔH than the native cereal flours. Among the different native cereal flours, the order of ΔH was rice flour (18.46 J/gm) > flour in combination (16.60 J/gm) > wheat flour (15.40 J/gm). The enthalpy change (ΔH) is the energy required for the transition from an ordered to disordered state on melting of starch crystallites. As raw flour contains more percent of ordered starch granules as compared to the modified cereal flour, it requires more energy for this transition, hence having higher ΔH than processed cereal flours. Mean ΔH of modified cereal flours was at par 4.23 J/gm (rice flour), 4.95 J/gm (wheat flour), and 4.05 J/gm (flour in combination). Flours extruded at higher barrel temperature of 190°C had lesser ΔH than those extruded at 175°C This may again be attributed to the reason that as the temperature of barrel during extrusion increased, the energy required for phase transition decreased. Zavareze et al.^[31] analyzed that HMT decreased the enthalpy of gelatinization (ΔH) in rice starches. This reduction in ΔH by HMT processing may be due to the disruption of the crystallites, which are less stable or smaller in size, producing a lower degree of crystallinity and thus requiring less energy for the disruption of these crystals. According to Chung et al.,^[32] the reduction in ΔH by HMT suggests that the high temperature during HMT may increase the mobility of double helices forming the crystalline structure, resulting in disruption of the hydrogen bonds in the double helices and between adjacent double helices. In addition, Hormdok and Noomhorm^[33] reported the reduction of ΔH after hydrothermal treatment may be due to the partial gelatinization of amylose and amylopectin molecules, both of which are not very stable during heating. A similar decrease in the enthalpy value after acid treatment has been shown by Muhr et al.,^[34] who attributed this decrease to the loss of some degree of order of amorphous regions prior to gelatinization.

Degree of gelatinized starch gives the extent of modification occurred in the starch granules of cereal flours during extrusion. Starch gelatinization is the disruption of molecular order within the starch granule manifested to irreversible changes in properties such as granular swelling, native crystalline melting, loss of birefringence, and starch solubilization. The data presented in Table 3 highlighted the effect of modification on the degree of gelatinization. It varied significantly for different modified cereal flours whereas extrusion variables had an effective role in achieving the derived degree of gelatinization in case of modified cereal flours. Among the processed cereal flours, rice flour showed highest degree of gelatinization. Mean degree of gelatinization for rice flour was 84.35%, followed by flour in combination (74.9%), and wheat flour (67.95%). This difference in degree of gelatinization between rice and wheat flour may be because of the compositional difference and the role of amylose in achieving the gelatinization. The disappearance of all the ordering in the granule is needed to increase the mobility of the amylose chains within the granule to enable the release of amylose. More leaching of amylose contribute to higher gelatinization. As rice flour has higher amylose content than wheat, it achieved higher degree of gelatinization. The effect of extrusion on gelatinization of cereal flours was dependent on temperature and moisture. Higher temperature and moisture favored the degree of gelatinization. These results are in agreement with the findings of Chiang and Johnson^[35] who reported that starch gelatinization increased sharply with increasing temperature and moisture content.

Morphology of Cereal Flours

Starch modification involves physical, chemical, and biochemical alterations on the surface of contacting phases. SEM plays an important role in understanding the granular structure of modified starches. It has been used to detect the structural changes caused by various modifications and the most substituted regions in starch granules.^[36,37] Most of the structural changes upon modification take place at the relatively less organized central core region of the starch granule.

SEM pictures (Fig. 1) signifies the effect of extrusion processing on the external structure of the individual starch granules and also signifies the comparative structural pattern between unmodified and modified cereal flours. Different morphological pattern was observed in raw and processed cereal flours. Figure 1 depicts the differences among the different raw cereal flours. Scanning electron micrographs of isolated rice starch granules illustrated that the granule shape was polygonal whereas lenticular and round shape was observed in wheat starches. A mixture of both types of granule shapes (polygonal and round) was observed in starch of flour in combination (rice and wheat flour). McPherson and Jane^[38] also documented similar morphological results for starch of native cereal flours to be polyhedric in shape and had compact crystalline starch granules. The granule surface of starches were smooth with no evidence of any fissures in case of native cereal flours, whereas the starch granules of HTST modified cereal flours lost their individuality and smoothness after modification. SEM illustrated that the modified starch granules under different processing conditions (barrel temperature, feed moisture, and screw speed), differed in shape from that of unmodified cereal flours. Various grooves and fissures on the exterior surface of the granules were noticed in the processed cereal flours. Discontinuous structure of processed cereal flours was probably due to the higher extent of gelatinization at higher temperature which caused more irregularity in the structure of starch granules. SEM of modified rice flour showed several protrusions on the surface of starch granules. As compared to its native flour, the granules lost its smoothness and the polygonal structure of starch also got disrupted. Similarly, many upfoldings and discontinuity was seen in processed wheat starch granules, whereas starch in modified flour in combination had many irregularities as starch granules got fragmented during extrusion at higher temperature conditions. Majzoobi et al.^[39] revealed that in case of pregelatinized starches, a sheet like structure was observed in which air bubbles were randomly distributed inside the continuous solid. This may indicate that all the starch granules underwent gelatinization during high temperature. Similar observations were also reported by Anastasiades et al.^[23] for pregelatinized maize starch. The high amylose starch treated at 25% HMT presented a more agglomerated surface than the native starch. Studies indicate that heat moisture treatment decreases the relative crystallinity with an increase in the moisture of HMT starches.^[31] Kawabata et al.^[40] observed the formation of cracks on the surface of treated maize and potato starches and hollowing of the granules. The starch granules lost their individuality and smoothness after acetylation.^[41] As the DS of starch increases with acetylation, there was an increasing destruction on the surface of the starch granules and many of them appeared to be wrinkled and distorted. When the starch acetate reached the higher DS (DS = 2.82) the starch almost fell into pieces and a number of holes were appeared on some granule cluster surface. Some fragmentation of the granules would have taken place during the increased DS of starch acetates, which revealed that the acetylation not only happened on the surface but also the inner structure of the starch. Once the reaction began, the starch became looser and more porous, which accelerated the acetylation to make the starch acetate granules smaller.^[42] Jayakody and Hoover^[43] postulated that the pores in the surface of the granules allow the direct access of acids to the interior of the granule. This behavior corroborated the higher starch damage found in the samples of jicama starch.

FIGURE 1 Effect of modification on the morphology of cereal flours (a: native rice flour, b: modified rice flour; c: native wheat flour, d: modified wheat flour; e: native flour, in combination, f: modified flour, in combination).

CONCLUSION

The functional, rheological, thermal, and morphological properties of modified cereal flours (rice, wheat, and flour, in combination) prepared by extrusion processing were studied and they varied significantly relative to their native flours. The developed modified cereal flour had the requisite functional and thermal properties over the native cereal flours to be used as a versatile additive in food products. Extrusion had a profound effect on thermal properties viz T_o, T_p, T_e, ∆H and degree of gelatinization and rheological properties. SEM pictures signified the effect of extrusion processing on the external structure of the individual starch granules and also signified the comparative structural pattern between unmodified and modified cereal flours.