Physicochemical, Morphological, Thermal and Pasting Properties of Starches Isolated from Rice Cultivars Grown in India

Abstract

Physicochemical, morphological, thermal, and pasting properties of starches, isolated from basmati (HBC-19 and Bas-370) and non-basmati (Jaya, a coarse cultivar; P-44 and HKR-120, the medium cultivars and Sharbati, fine cultivar) rice cultivars grown in India were studied. The amylose content of starches from different cultivars ranged from 2.25 (Jaya) to 22.21 g/100 g of starch (HBC-19). Jaya, HKR-120, and P-44 cultivars showed soft gel consistency as 84, 73, and 69 mm, respectively, whereas Sharbati, Bas-370 and HBC-19 cultivars showed medium gel consistency as 54, 53, and 58 mm, respectively. Swelling power (at 95°C) indicated a significant positive correlation with amylopectin content (r = 0.828, p < 0.05) and gel consistency (r = 0.983, p < 0.01). Turbidity had a highly significant positive correlation with solubility (r = 0.919, p < 0.01) and amylose content (r = 0.945, p < 0.01). Starch form Jaya cultivar showed the presence of smallest size granules (2.4–5.7 μm) with an average size of 3.96 μm, whereas Bas-370 showed the presence of largest size granules (3.3–6.7 μm) with an average size of 5.0 μm. The transition temperatures, enthalpy of gelatinization (ΔH_gel), peak height index (PHI) and gelatinization range were determined using differential scanning calorimetry (DSC). The starch from Sharbati cultivar showed highest onset temperature (T_o), peak temperature (T_p), conclusion temperature (T_c), enthalpy of gelatinization and peak height index (PHI) of 68.8°C, 73.2°C, 79.0°C, 11.56 J/g and 2.63 respectively. Pasting temperature of rice starches varied from 68.9°C (Jaya) to 74.5°C (Sharbati). The peak viscosities observed were in the range of 2223 to 3297 cP, lowest for HBC-19 starch and highest for Jaya starch.

Keywords:

• Amylose
• Physicochemical
• Gel consistency
• Gelatinization
• Pasting properties

INTRODUCTION

Starch is the major constituent of milled rice and its characteristics differ widely among cultivars, as reflected in properties such as the amylose/amylopectin ratio and final gelatinization temperature.[1,2] Starch is composed of essentially linear amylose and highly branched amylopectin, normally 15–25 g amylose and amylopectin being 75–85 g/100 g of starch. It is well established that the amylose/amylopectin ratio is a major factor influencing the physicochemical and functional properties of starch. The close association of starch with proteins in rice makes it difficult to obtain starch with less than 0.5% protein. Alkali extraction appears to be the most effective in solubilizing the protein, as at least 80% of the protein is alkali-soluble glutelin (g/100 g of total protein). When the starch suspension is heated, the granules absorb water and swell due to dissolution of amylose molecules and destruction of the crystalline region. This gelatinization of starch is an important phenomenon occurring in different heat processed foods. Processes such as baking of bread and cakes, extrusion of cereal based products, thickening and gelling of sauces and pie fillings processes are dependent upon gelatinization of starch. The leached out amylose forms a three-dimensional network[3–5] and the swollen granules are embedded in the continuous matrix.[6,7] Swelling power and solubility provide evidence of the magnitude of interaction between starch chains within the amorphous and crystalline domains. Arisaka and Yoshi[8] revealed that the degree of gelatinization, swelling power and acid solubility of low-amylose starch paste were higher than those of high-amylose starch paste and the inferiority of high amylose rice for the processed products was due to its low degree of gelatinization. Cooking and eating qualities of milled rice are mainly influenced by starch properties like apparent amylose content (AC), final starch gelatinization temperature (GT) and gel consistency (GC), particularly among high amylose rice cultivars. The pasting properties are dependent on the rigidity of starch granules, which in turn affects the swelling potential of granules[9] and the amount of amylose leaching out in the solution.[10] The purpose for using starch in various applications is often related to an increased viscosity upon gelatinization, thus a thorough understanding of the basis for the increased viscosity is required. Knowledge of pasting properties is also an important indicator of the processing quality of foods and their constituents. For example, such knowledge can assist a processor in optimizing ingredient concentrations and temperature-pressure-shear limits to achieve a product of desired consistency.

The development of rice-based value added and other conventional foods depends upon thorough knowledge of physicochemical, thermal and pasting properties of rice starch. The Indian rice particularly basmati rice receives a good reputation in the market. The present study was undertaken with an objective to study the physicochemical, morphological, thermal and pasting properties of some Indian rice cultivars.

MATERIALS AND METHODS

Materials

Six paddy cultivars, i.e., four non-basmati (Jaya, a coarse cultivar; P- 44 and HKR- 120, the medium cultivars; Sharbati, fine cultivar) and two basmati (HBC-19 and Bas-370), were procured from the Rice Centre of CCS Haryana Agricultural University, Hisar situated at Kaul (Kaithal) Haryana, India. Paddy samples were dehusked and polished utilizing the locally available milling facilities. All the chemicals and reagents used were of analytical grade.

Isolation of Starch

Starch was isolated from various rice cultivars by alkali steeping method of Wang and Wang[11] with modifications. Milled rice was steeped with five volumes of sodium hydroxide solution (0.2% w/v) at 25°C for 24 h to soften the endosperms. The steep liquor was drained off and the rice grains were ground with pestle and mortar. The slurry was diluted to the original volume with sodium hydroxide (0.2% w/v). The mixture was stirred for 10 min and allowed to settle overnight. The cloudy supernatant was drained off and the sediment was diluted to the original volume with sodium hydroxide solution. The process was repeated until the supernatant became clear and gave a negative reaction to the biuret test for proteins. Starch was suspended in two-fold volumes of distilled water and centrifuged at 1400x g for 10 min. The washed starch was re-slurried with distilled water and the pH of slurry was adjusted to 6.5 with 1N HCl and centrifuged again. The starch was washed, dried in air oven at 45°C for 48 h, passed through a 150 mm sieve and stored in plastic jar at room temperature for further analysis.

Chemical Analysis

Moisture content was determined by drying samples in a hot air-oven at 105°C when a constant weight was achieved. Protein and fat contents were analyzed using approved methods of the Association of Official Analytical Chemists.[12] Amylose content of the isolated rice starch was determined using the method of Williams et al.[13] Amylopectin content was calculated by subtraction method.

Gel Consistency

Gel consistency of rice starch was determined according to the method of Cagampang et al.[14]

Swelling Power and Water Solubility

The swelling power (SP) and the water solubility (WS) of the rice starches were measured at different temperatures according to the method of Schoch[15] with some modifications.[16] Starch suspension (0.5 g, dry basis, to which 45 ml of distilled water was added) was heated to 55, 65, 75, 85, and 95°C, respectively, and was kept at that temperature for 30 min. The heated samples were cooled rapidly in ice water bath for 1 min, equilibrated at 25°C for 5 min, and then centrifuged at 3000x g for 20 min. The supernatants were drained into pretreated moisture dishes, evaporated to dryness in a hot air oven at 100°C and cooled to room temperature in a dessicator prior to reweighing. The swelling power was determined by measuring the sedimented paste weight and water solubility (%) as follows:

(1)

Turbidity

Turbidity of rice starches was measured as described by Perera and Hoover.[17] A starch suspension of rice starch (2 g/100 ml suspension, w/v) was heated in a boiling water bath for 1 h with continuous gentle stirring and then cooled for 1 h in a 25 ± 0.5°C water bath. The samples were stored for five days at 4 ± 0.5°C and the turbidity was determined by measuring absorbance at 640 nm against water blank.

Morphological Properties

Morphological properties of rice starch were studied using scanning electron microscope (Jeol JSM-6100, Jeol Ltd., Tokyo, Japan). Finely ground and ethanol dehydrated starch samples were placed on an aluminum stub and coated with a thin gold film with the help of Jeol, ion sputter (JFC-1100). An acceleration potential of 10KVA was used during micrography.

Thermal Properties

Thermal properties of isolated starches studied using a Differential Scanning Calorimeter (DSC-821^e, Mettler Toledo, Switzerland) equipped with a thermal analysis data station. The instrument was calibrated using purified, deionized distilled water as standard. Starch (3.5 mg, dwb) was added with distilled water with the help of micro syringe to achieve a starch/water suspension containing 70% water. The samples were sealed hermetically and allowed to stand for 1 h at room temperature before heating in DSC at a rate of 10°C/min from 25 to 100°C. Onset temperature (T_o), peak temperature (T_p), conclusion temperature (T_c), and enthalpy of gelatinization (ΔH_gel) were calculated automatically. The gelatinization range (R) was computed as (T_c-T_o).[18] Enthalpies were calculated on dry weight basis. The peak height index (PHI) was calculated by the ratio (ΔH_gel/T_p-T_o), as described by Krueger et al.[19]

Pasting Properties

Pasting properties of rice starches were determined using a rapid visco analyzer (RVA Starch Master TM, Newport Scientific, Warriewood, Australia). The test profile STD1 (Newport Scientific Method 1, Version 5, 1997) was used for determination of pasting characteristics. The sample (3.0 g of starch) was dispersed in water (25.0 ml) and stirred in an RVA container initially at 960 rpm for 10 sec and finally at 160 rpm for the remaining test. The temperature profile was started from 50°C for 1 min followed by ramping the temperature linearly to 95°C in 3 min and 42 sec, holding for 2 min and 30 sec, cooling the system to 50°C in 3 min and 48 sec and ending the process in 13 min. The pasting curves obtained were analyzed using an RVA Starch Master Software setup Tool (SMST) to obtain the characteristic parameters like pasting temperature (P_temp); peak viscosity (PV, maximum paste viscosity achieved in the heating stage of the profile); hot paste viscosity (HPV, minimum paste viscosity at 95°C); cool paste viscosity (CPV, final viscosity at 50°C); breakdown (BD = PV-HPV); set back (SB = CPV-PV) and consistency (CS = CPV-HPV).

Statistical Analysis

The data were analyzed statistically using one factor analysis of variance (ANOVA) in a complete randomized design (CRD) using Opstat. Pearson correlation coefficients (r) were calculated using SPSS 11.0 statistical software.

RESULTS AND DISCUSSION

Chemical Composition

The protein and fat contents of isolated starches from different rice cultivars were found in the range of 0.42 to 0.53 g and 0.52 to 0.72 g/100 g of starch, respectively. Moisture content varied from 9.92 to 11.80 g/100 g of starch. The amylopectin content as determined by subtraction method for different cultivars ranged from 64.98 to 86.32 % of starch (details of data not given). The amylose content from different cultivars varied significantly and ranged between 2.25 to 22.21 g/100 g of starch. Jaya showed lowest amylose content, whereas HBC-19 showed highest amylose content as shown in Table 1. Basmati cultivars were observed to contain higher amylose as also reported by Archana et al.[20]

Table 1 Physicochemical properties of isolated starches

Cultivar	Amylose (g/100 g starch)	Gel consistency (mm)	Swelling power*(g/g)	Solubility* (%)	#Turbidity**
Jaya	2.25 ± 0.09^a	84 ± 1.15^e	16.65 ± 0.07^e	7.90 ± 0.03^a	0.354 ± 0.004
HKR-120	4.32 ± 0.08 ^b	73 ± 0.88 ^d	15.30 ± 0.03^d	11.84 ± 0.05^b	0.372 ± 0.007
P-44	5.97 ± 0.09 ^c	69 ± 0.57 ^c	14.45 ± 0.06^c	13.61 ± 0.04^c	0.448 ± 0.012
Sharbati	7.10 ± 0.07 ^c	54 ± 0.88 ^a	13.15 ± 0.05^b	14.19 ± 0.04^d	0.481 ± 0.004
Bas-370	20.58 ± 0.06 ^d	53 ± 0.58 ^a	12.59 ± 0.07^a	15.88 ± 0.06^e	0.583 ± 0.005
HBC-19	22.21 ± 0.02^e	58 ± 1.45 ^b	12.80 ± 0.06^a	16.71 ± 0.06^f	0.576 ± 0.006
The values are mean± SD of three independent determinations at 5% level of significance. Values with similar superscripts in column do not differ significantly (p < 0.05). *Values at 95°C; **values (absorbance at 640 nm) at 4°C after 5 days; and # non-significant differences.

Gel Consistency

Gel consistency values differed significantly (p < 0.05) among various cultivars except for Sharbati and Bas-370 as shown in Table 1.Jaya, HKR-120 and P-44 cultivars showed soft gel consistency as 84, 73, and 69 mm, respectively, whereas Sharbati, Bas-370 and HBC-19 cultivars showed medium gel consistency as 54, 53, and 58 mm, respectively. It has been reported that the length of gel flow (long = soft gel, short = hard gel) had an inverse relation with amylose content.[14,21] Gel consistency measures the tendency of the gelatinized starch to retrograde on cooling. The amylose component of the starch retrogrades more readily than amylopectin due to its linear structure. The straight chain structure of amylose allows it to readily form hydrogen bonds between molecules, resulting in hard gels, which will be short-flowing.[22] Huaisan et al.[23] also observed hard gel consistency in the high amylose Thai rice cultivars which increased further upon addition of polysaccharides and frozen storage. Gel consistency together with the amylose content can be a good index of cooked rice texture.

Swelling Power and Solubility

Swelling power and water solubility of starches were assessed over temperatures of 55–95°C at 10°C intervals. Swelling power at 95°C varied from 12.59 g/g for Bas-370 to 16.65 g/g for Jaya. Water solubility at 95°C ranged from 7.90 to 16.71%, lowest for Jaya and highest for HBC-19 (Table 1). The extent of leaching of solubles mainly depends on the lipid content of the starch and the ability of the starch to form amylose-lipid complexes. The amylose involved in complex formation with lipids is prevented from leaching out.[5] Both swelling power and the water solubility of all the starches increased as the temperature was increased as shown in the swelling and solubility patterns of different starches with increasing temperature (Figs. 1 and 2, respectively). All the rice starches exhibited a rapid rise in swelling power from 65 to 75°C, where the gelatinization occurred. When the crystal region in the starch granule begins to melt, it enhanced the swelling power. At 55–95°C, Jaya starch exhibited the highest degree of swelling whereas, Bas-370 starch showed lowest swelling power supporting the idea that reduced amylose content relates to greater swelling. Gupta et al.[24] while studying properties of barley starch and its blend with corn, wheat and rice starch, reported amylose content to be negatively correlated with swelling power. Sodhi and Singh[25] also reported that low amylose cultivar starch (PR-103) showed the highest swelling power and lowest solubility, whereas high amylose cultivar starch (PR-113) showed the lowest swelling power. It has been reported that on the molecular level, the swelling power and solubility of the starch granule is influenced by many factors, including amylose-amylopectin ratio and contents, molecular mass of each fraction, degree of branching, conformation, length of outer branch of amylopectin, and the presence of other components such as lipids and proteins.[26] Lii et al.[27] also observed higher swelling power during heating for rice starch with lower amylose content. Because the swelling behaviour of cereals has been related to amylopectin,[5] the high swelling power suggested a less rigid granular structure of low-amylose rice[28] compared with that of high-amylose rice starches. Among the six rice starches, the lowest swelling power was shown by basmati cultivars (high in amylose content), suggesting that the difference in swelling power among starches is mainly affected by the amylose content acting as an inhibitor of swelling.[5,29]

Figure 1 Swelling power patterns of rice starches as a function of temperature.

Figure 2 Solubility patterns of rice starches as a function of temperature.

Turbidity

The turbidity values of gelatinized starch suspension prepared from starches separated from rice cultivars are shown in Table1. The turbidity values of Jaya starch suspension were significantly lower than the turbidity values of the starch suspensions from other rice cultivars. The lowest turbidity values of Jaya starch suspension may be attributed to its lower amylose content.[30] The pattern of turbidity of gelatinized rice starch suspensions stored at 4°C for five days is shown in Fig. 3. The increase in turbidity results from changes in density distribution due to phase separation during aging of gelatinized starch solutions.[31] Factors responsible for turbidity development in starches during storage have been previously identified by many researchers[30,32,33] and include aggregates made of leached amylose, amylose, and amylopectin chain lengths, intra or intermolecular bonding, granule swelling and granule remnants. Jaya cultivar developed a lower turbidity up to five days because the aggregation and slow crystallization of amylopectin were implicated in the long-term changes. In addition, its lower turbidity compared with others also could be attributed to its high swelling power and the absence of granule fragments.[17,34] Bas-370 cultivar developed the highest turbidity due to molecular association (especially involving amylose) occurring at the earlier stages of storage from the rapid cooling at low temperature. Furthermore, Perera and Hoover[17] also indicated that the increase in turbidity was mainly attributed to the rapid formation of double helical junction zones upon cooling, resulting from the continued interaction between leached amylose-amylopectin chains through hydrogen bonding.

Figure 3 Turbidity of gelatinised rice starch suspensions stored for 5 days at 4°C.

Correlation among Physicochemical Properties

Correlation coefficients were determined to examine the relationships among physicochemical properties of rice starches isolated from different cultivars (Table 2). Swelling power (at 95°C) indicated a significant positive correlation with amylopectin content (r = 0.828, p < 0.05) and gel consistency (r = 0.983, p < 0.01). Solubility (at 95°C) was negatively correlated with swelling power (r = −0.964, p < 0.01). A significant negative correlation was found between amylose content and swelling power (r = −0.820, p < 0.05). Hagenimana and Ding[35] have also reported a negative correlation between amylose content and swelling power. Amylose content had a negative effect on swelling power as also reported by Sasaki and Matsuki.[29] However, a significant positive correlation was found between amylose content and solubility (r = 0.835, p < 0.05).

Table 2 Pearson correlation coefficients between physicochemical properties of rice starches

	Starch	Amylose	GC	SP	WS	Turbidity
Amylose	−0.815
GC	0.641	−0.731
SP	0.673	−0.820*	0.983**
WS	−0.685	0.835*	−0.911*	−0.964**
Turbidity	−0.717	0.945**	−0.883*	−0.943**	0.919**
AP	0.792*	−0.998**	0.741	0.828*	−0.839*	−0.947**
*p < 0.05; **p < 0.01; GC: gel consistency; SP: swelling power, WS: water solubility; and AP: amylopectin.

Morphological Properties

The microstructure of starch granules from different cultivars as studied by SEM showed the presence of mainly polyhedral granules having size in the range of 2.4 to 6.7 μm (Fig. 4). Jaya showed the presence of smallest size granules (2.4–4.7 μm) with an average size of 3.96 μm, whereas Bas-370 showed the presence of largest size granules (3.3–6.7 μm) with an average size of 5.0 μm. HBC-19 and P-44 starch granules size ranged from 2.9 to 5.2 μm and 3.8 to 5.7 μm, respectively. The average size of starch granules of Sharbati and Bas-370 was observed to be similar (5.0 μm). HKR-120 showed the presence of more uniform size granules as compared to the starch granules of other cultivars.

Figure 4 Scanning electron micrographs of starches from different rice cultivars (A: Sharbati; B: Bas-370; C: HKR-120; D: Jaya; E: HBC-19; and F: P-44).

There was some variation observed in the starch granules of different cultivars. Sodhi and Singh[36] reported that a group of rice varieties grown in India had starch granules ranging from 2.4–5.4 μm in size. The starch granule average size from some waxy rice ranged from 4.9–5.7 μm in size.[37] Li and Yeh[38] reported an average granule size of 6.4 μm for Taiwan rice starch. Larger starch granules normally take more time to gelatinize and cook as heat and moisture take more time to penetrate to the centre of the granules.

Thermal Properties

Gelatinization describes the range of irreversible changes occurring when starch is heated with excess water. The gelatinization thermograms of starches isolated from different rice cultivars are shown in Fig. 5. The transition temperatures (T_o, T_p and T_c) and enthalpy of gelatinization (ΔH_gel) from various rice cultivars differed significantly (P < 0.05) as shown in Table 3. Sharbati starch showed highest T_p followed by HBC-19, Bas-370, P-44, HKR-120 and Jaya starch, respectively. T_o, T_p and T_c did not differ significantly in starches separated from Jaya and HKR-120 cultivars (P < 0.05). T_o, T_p, T_c and ΔH_gel of starch isolated from sharbati were significantly higher than the starches separated from other rice cultivars. ΔH_gel was observed to be highest (11.56 J/g) for Sharbati, whereas HKR-120 starch showed lowest value (7.79 J/g). T_o, T_p, T_c and ΔH_{gel of} Jaya starch were significantly lower than the starches separated from other rice cultivars except HKR-120 (P < 0.05). The enthalpy of gelatinization and the transition temperature are believed to be increased with the crystallinity of the granules, which is governed mainly by the amylopectin. However, Cooke and Gidley[39] demonstrated that ΔH values of gelatinization primarily reflect the loss of double helical order rather than the loss of crystallinity. It has been reported that the low T_o, T_p and ΔH are associated with a greater amount of short-chain amylopectin (AP) and a smaller amount of long-chain amylopectin molecules.[40,41] Higher values of T_o, T_p and T_c for Sharbati starch may be attributed to the compact nature of small starch granules and higher degree of molecular order of granules.[19] The transition temperature and enthalpies observed for rice starches in the present study were found to fall within a range similar to those reported in the literature.[27,42] The gelatinization range (R) of HBC-19 starch was the highest followed by Bas-370, Sharbati, P-44, Jaya and HKR-120 starch, respectively. These differences may be attributed to variation in size and number of uniform starch granules among the various starches. The long grain cultivars such as Sharbati, Bas-370 and HBC-19 were found to show higher values of T_o, T_p, T_c, and ΔH_gel in comparison to short grain cultivars such as Jaya and medium grain cultivars such as HKR-120 and P-44, suggesting that the long grain cultivars gelatinized at higher temperature than the short and medium grain types. The micellar structure of the molecules in the granule might be the main factor involved in the varietal difference in the gelatinization temperature. Thus, gelatinization temperature reflects the degree of orderly arrangement of the molecules in the granule and perhaps of the whole endosperm.

Figure 5 DSC endotherms of starches from different rice cultivars(A: Jaya; B: HKR-120; C: P-44; D: Sharbati; E: Bas-370; and F: HBC-19).

Table 3 Thermal properties of starch isolated from rice cultivars

Cultivar	To (°C)	TP(°C)	TC (°C)	ΔHgel (J/g)	PHI	R
Jaya	61.8 ± 0.40^a	65.3 ± 0.29 ^a	69.7 ± 0.35 ^a	7.98 ± 0.23 ^a	2.27	7.91
HKR-120	62.6 ± 0.24 ^a	65.7 ± 0.40 ^a	69.7 ± 0.12 ^a	7.79 ± 0.16 ^a	2.52	7.12
P-44	63.0 ± 0.46 ^a	67.2 ± 0.40^b	71.9 ± 0.40^b	8.93 ± 0.22^b	2.10	8.94
Sharbati	68.8 ± 0.40^d	73.2 ± 0.58^e	79.0 ± 0.19^e	11.56 ± 0.19^d	2.63	10.14
Bas-370	64.2 ± 0.42^b	69.5 ± 0.22^c	75.5 ± 0.38^c	10.12 ± 0.13^c	1.93	11.29
HBC-19	65.6 ± 0.23^c	71.6 ± 0.28^d	77.5 ± 0.16^d	9.96 ± 0.06^c	1.64	11.92
The values are mean± SD of two independent determinations at 5% level of significance. Values with similar superscripts in column do not differ significantly (p < 0.05).To: onset temperature; TP: peak temperature; Tc: conclusion temperature; ΔHgel: enthalpy of gelatinization; PHI: peak height index; ΔHgel/(TP- To); and R: gelatinization range Tc- To.

Pasting Properties

The pasting properties of isolated starches as analyzed by RVA are summarized in Table 4. Pasting temperature of rice starches varied from 68.9°C for Jaya to 74.5°C for Sharbati cultivar. The peak viscosities observed were in the range of 2223 to 3297 cP, lowest for HBC-19 starch and highest for Jaya starch. Peak viscosity is indicative of water binding capacity and ease with which starch granules are disintegrated and it is often correlated with final product quality.[43,44] It can be affected by the molecular structure of amylopectin,[45] starch water concentration, lipids, residual proteins,[46] granule size,[47] and operating conditions of the instrument.[48] The hot paste viscosity (HPV) ranged from 2053 cP for Bas-370 to 2699 cP for HKR-120, while the cool paste viscosity (CPV) varied from 3901 cP for Jaya to 4479 cP for HKR-120 starch. HPV is influenced by the rate of amylose exudation, amylose-lipid complex formation, granule swelling and competition between exudated amylose and remaining granules for free water, while CPV is largely determined by the retrogradation tendency of the soluble amylose upon cooling.[49]

Table 4 Pasting properties of starch isolated from rice cultivars

Cultivar	Ptemp (°C)	PV (cP)	HPV (cP)	CPV (cP)	BD (cP)	SB (cP)	CS (cP)
Jaya	68.9^a	3297^d	2129^b	3901^a	1168^e	604^a	1772^a
HKR-120	69.2^a	3198^c	2699^d	4479^d	499 ^b	1281^c	1780^a
P-44	70.8^b	3279^d	2401^c	4238^c	878^d	959 ^b	1837^a
Sharbati	74.5^d	3208^c	2658^d	4470^d	550^c	1262^c	1812^a
Bas-370	73.4^c	2644^b	2053^a	4128^b	591^c	1484^d	2075^b
HBC-19	74.3^d	2223^a	2077^a	4191^c	146^a	1968^e	2114^b
The values are mean± SD of two independent determinations at 5% level of significance. Values with similar superscripts in column do not differ significantly (p < 0.05). Ptemp: pasting temperature; PV: peak viscosity; HPV: hot paste viscosity; CPV: cool paste viscosity; BD: breakdown (PV-HPV); SB: setback (CPV-PV); and CS: consistency (CPV-HPV).

The breakdown viscosities differed significantly in the starches from different rice cultivars and ranged from 146 to 1168 cP. The breakdown is caused by the disintegration of gelatinized starch granule structure during continued stirring and heating.[46] The differences among rice starches in breakdown viscosities are related to differences in rigidity of swollen granules.[50,51] The highest setback viscosity (CPV-HPV) was recorded for HBC-19 (1968 cP) and the lowest for Jaya (604 cP). The higher the setback, the more syneresis is likely to take place and this also indicates a higher retrogradation tendency.[35] High setback is also an indication of the amount of swelling power of the starch and is usually related to the amylose content of the starch. The amylose component of the starch retrogrades more readily than amylopectin due to its essentially linear structure. The straight chain structure of amylose allows it to readily form hydrogen bonds between molecules, resulting in rigid gels.[22]

Jaya (waxy rice) exhibited a pasting profile typical of a low-amylose starch with the lowest pasting temperature (PT), the highest peak viscosity (PV) and lower setback. Kuno et al.[52] also reported that the values of peak viscosity were higher and those of setback were lower for low-amylose cultivars than for normal amylose cultivars. Starch from HBC-19, which had a higher amylose content, as expected had a lowest peak viscosity and higher setback value than the lower amylose starches of other cultivars. Chau Dang and Copeland[53] reported similar results for starch from Doongara cultivar. Amylose content is believed to have a marked influence on the breakdown viscosity (a measure of susceptibility of cooked starch granule to disintegration) and the setback viscosity (which is measure of recrystallization of gelatinized starch during cooling).[54] High amylose content has also been suggested as the major factor contributing to the nonexistence of a peak, a high stability during heating, and a high setback during cooling.[55,56]

Correlation Among Thermal and Pasting Properties

Correlation coefficients were determined to study the relationships among thermal and pasting properties of starches separated from different cultivars (Table 5). Peak temperature (T_p) had a highly significant positive correlation with onset temperature (T_o) (r = 0.954, p < 0.01), conclusion temperature (T_c) (r = 0.994, p < 0.01) and enthalpy of gelatinization (ΔH_gel) (r = 0.961, p < 0.01). ΔH_gel had a highly significant positive correlation with T_o (r = 0.941, p < 0.01), T_p (r = 0.961, p < 0.01) and T_c (r = 0.965, p < 0.01). Pasting temperature had a highly significant positive correlation with T_p (r = 0.971, p < 0.01), and ΔH_gel (r = 0.940, p < 0.01). Peak viscosity (PV) was negatively correlated with gelatinization temperature and ΔH_gel. Peak viscosity indicated a positive correlation with BD, HPV and CPV. The present results are in accordance with those reported earlier by Hagenimana and Ding.[35] Amylose content appears to play a critical role in determining pasting properties of starch by Brabender amylograph or RVA, as amylose suppresses starch swelling. Lower amylose content was associated with higher peak viscosity.[56] From the present data, a highly significant negative correlation was found between the amylose content (AC) and PV (r = −0.957, p < 0.01), whereas a significant positive correlation was found between AC and setback (SB) (r = 0.855, p < 0.05) Amylose content had a positive correlation with T_o (r = 0.320), T_p (r = 0.569), T_c (r = 0.635) and ΔH_gel (r = 0.485). Tang et al.[58] have also reported a positive correlation between gelatinization temperature and amylose content.

Table 5 Pearson correlation coefficients between thermal and pasting properties of rice starch isolated from different cultivars

	T0	TP	TC	ΔHgel	PHI	R	Ptemp	PV	HPV	CPV	BD
TP	0.954**
TC	0.925**	0.994**
ΔHgel	0.941**	0.961**	0.965**
PHI	0.142	−0.16	−0.23	−0.056
R	0.599	0.807	0.858*	0.762	−0.676
Ptemp	0.867*	0.971**	0.989**	0.940**	−0.34	0.913*
PV	−0.255	−0.506	−0.558	−0.351	0.824*	−0.831*	−0.642
HPV	0.254	−0.004	−0.093	0.012	0.838*	−0.539	−0.171	0.614
CPV	0.505	0.348	0.278	0.309	0.515	−0.096	0.247	0.147	0.858*
BD	−0.536	−0.636	−0.627	−0.455	0.339	−0.598	−0.669	0.749	−0.063	−0.534
SB	0.482	0.646	0.663	0.481	−0.542	0.747	0.729	−0.884*	−0.18	0.333	−0.967**
*P < 0.05; **P < 0.01; TO: onset temperature; TP: peak temperature; TC: conclusion temperature; ΔHgel: enthalpy of gelatinization; PHI: peak height index; R: gelatinization range; Ptemp: pasting temperature; PV: peak viscosity; HPV: hot paste viscosity; CPV: cool paste viscosity; BD: breakdown; SB: setback; and CS: consistency.

CONCLUSION

Amylose/amylopectin content, gel consistency, swelling power, water solubility, and turbidity form the basis of cooked rice quality and end product uses of rice. The observed values for the amylose suggest that there was a wide variation in the amylose content and hence, the end product uses of the Indian rice cultivars. Gel consistency together with the amylose content can be a good index of cooked rice texture. Low amylose cultivars such as Jaya, HKR-120 and P-44 can be preferred for breakfast cereals and baby foods. Their low amylose starch produced relatively stable gel, which tended to harden slowly during storage. Intermediate amylose rice cultivars such as Bas-370 and HBC-19 can be used in canned soups and in dry soup mixes as intermediate amylose and medium gel consistency characteristics of their starch can result in optimum volume expansion on steaming providing soft texture. The swelling behaviour of rice starch is mainly a property of their amylopectin content and amylose acts as both a diluent and inhibitor of swelling. Low-amylose (Jaya, HKR-120, P-44, Sharbati) cultivars showed high swelling power, low water solubility and low turbidity than intermediate-amylose (Bas-370, HBC-19) cultivars. The rice starches having large average granular size showed more amylose content (basmati cultivars) than those with smaller average granular size. Sharbati, a non-basmati cultivar having long grains and low amylose content (7.10%) showed the higher values for transition temperatures and enthalpy of gelatinization in comparison to basmati cultivars. However, the other non-basmati cultivars having low amylose contents showed lower values of the transition temperatures and enthalpy of gelatinization as compared to basmati cultivars. These findings conclude that not only the amylose/amylopectin composition but the micelle structure of molecules in the granules also decides the transition temperatures of the starch. The long grain cultivars such as Sharbati, Bas-370 and HBC-19 were found to show higher values of T_o, T_p, T_c and ΔH_gel in comparison to short grain cultivars such as Jaya and medium grain cultivars such as HKR-120 and P-44. The low gelatinization temperatures of short and medium grain varieties should be of interest to brewers and cereal manufacturers using diastatic digestion in the processing. A low gelatinization temperature allows complete liquefaction of starch before thermal inactivation of the enzymes can occur.