Effect of Extrusion Cooking on X‐Ray Diffraction Characteristics of Rice and Rice–Legume Blends

Abstract

Raw rice cultivars, Basmati‐370 (Fine) and Jaya (Coarse) exhibited essentially A‐type diffraction patterns, however coarse variety showed somewhat more crystalline than fine one. Blending of Jaya rice flour with different grams (bengal, black, and green gram) did not cause any change in d‐spacings and hence blends were characterized for A‐type patterns. Extrusion cooking disorganized the crystalline structure of starch component of different blends and resulted in the conversion of A‐type pattern to V‐hydrate (less crystalline) pattern. The presence of V‐amylose structure in extruded blends indicated complexing of amylose with fatty acids of legumes during extrusion cooking.

Keywords:

• Rice
• Grams
• Extrusion cooking
• X‐ray diffraction patterns
• Starch
• Crystalline

Introduction

Extrusion cooking of rice–legume blends seemed to disorganize the crystalline structure of starch and results in the conversion of A‐type (crystalline) pattern to V‐hydrate (less crystalline) pattern.[1] The X‐ray patterns of normal corn and wheat starches extruded at a temperature between 65 and 70°C showed disorganized structure.[2] Presence of V‐amylose structure during extrusion cooking of manoic starch has been observed by workers.[3] Studies have been conducted to evaluate changes in starch crystallinity in parboiled rice and reported that A‐type X‐ray diffraction spectra of raw rice were weakened upon mild parboiling and completely disappeared in moderate to severe parboiled rice.[4] The authors further reported that V‐pattern of lipid amylose complex appeared in its place. Limited information is available on the changes in legume starch during extrusion cooking. Therefore, the present study was aimed to study changes in starch component of rice–legume blends as function of extrusion cooking using X‐ray diffraction technique. The observations of X‐ray diffraction analysis can be applied to understand cold paste viscosity and water absorption characteristics of cereal–legume blends as the function of extrusion cooking.

Materials and Methods

Samples of paddy (Orizae sativa cultivar, Jaya, and Basmati‐370), chick pea (Cicaer arientinum), black gram (Phaseolus mungo), green gram (Phaseolus aureus), and soybean (Glycine max) were procured from the department of Plant Breeding of this university. Paddy was milled on Satake rice mill and legumes on dehuller (designed at G.B. Pant University of Agriculture & Technology) to obtain raw milled rice and cotyledons, respectively. Raw milled rice and cotyledons of legumes were ground separately to pass through 80 mesh (180 micron) using pin mill (Gansons Co., India). Blends of Jaya rice flour with each legume flour were prepared (75:25) and sifted through 60 mesh (250 micron) sieve to obtain uniform mixing.

Extrusion Cooking of Blends

Each blend was extruded in WengerX‐5 extruder (Wenger Manufacturing Co., Sabetha, KS) at an extruder exit temperature (EET) of 90°C ± 2°C; feed rate of 27.2 kg/hr, extruder barrel speed of 300 rpm; water flow rate of 6.71 L/hr, die size of 5/16 an inch; steam pressure at 40 psi. These extruder parameters are based on the results of preliminary optimization trials. The samples of extrudate collected after the process acquired steady state, were tray dried at 50°C to moisture content of about 5–6%. After drying, each extrudate was ground to fine powder (75 micron) and equilibrated (43% RH) before subjecting to X‐ray diffraction analysis.

X‐Ray Diffraction Analysis

Phillips X‐ray diffractometer (Model LZ5) equipped with Geiger‐Muller counter and Nickle filtered Cu K_α radiation was used. The equilibrated ground sample was loaded to sample holder which was then placed in the space provided for it. The following settings were used to perform X‐ray diffraction analysis:

Table

Range cps	2 × 10^2
Goniometer	2θ per min
Time constant	1 sec
Chart speed	10 mm/min

In the diffractometer analysis, the scattered X‐rays from the sample are received by a G.M. counter tube. The output of which is amplified and feed to a chart recorder where the X‐ray diffraction patterns are recorded automatically in the form of peaks. The table for conversion of X‐ray diffraction angles to interplanar spacings was used for calculating the d‐spacings for various angles obtained from the X‐ray patterns. X‐ray patterns were designed according to the d‐spacings and intensities as given by earlier workers.[5],[6]

Results and Discussions

The 2θ d‐spacing and relative intensities of the peaks identified for Jaya rice(coarse) and Basmati rice (fine) are given in Table 1. Raw rice exhibited essentially the A‐type diffraction patterns characterized by the strong d‐spacings at 3.85, 4.86, 5.15, and 5.78 Å and 3.85, 4.86, 5.31, 5.78 Å, respectively, for Basmati‐370 and Jaya. However, the starch granule of Jaya rice showed somewhat more crystallinity than that of Basmati‐370 as indicated by the larger numbers of discernible peaks. This difference in crystallinity may be attributed to the differences in moisture content and also to the period of maturity.[6] But both the rice varieties exhibited sharper, and narrower peaks with relatively stronger intensities suggesting the greater degree of crystallinity.

Table 1. Effect of extrusion cooking on X‐ray diffraction patterns of Basmati‐370 and Jaya rice

	Basmati‐370			Jaya
Treatments	2θ	d‐Spacings (Å)	RI	2θ	d‐Spacings(Å)	RI
Raw (unprocessed)	28.6	3.11	W^+	26.2	3.39	W^+
	23.0	3.85	S	23.0	3.85	S
	20.0	4.43	M	20.0	4.43	M
	18.2	4.86	S	18.2	4.86	S
	17.2	5.15	S	16.8	5.31	S
	15.3	5.78	S	15.3	5.38	S
	9.8	9.01	W	11.5	7.68	W
	—	—	—	10.1	8.74	W
Extruded	25.0	3.55	W	21.8	4.07	W
	23.8	3.73	W	20.0	4.43	S
	22.8	3.97	W^+	18.0	4.92	W
	19.8	4.47	S	9.4	9.39	W
	18.4	4.81	W	—	—	—
	16.7	5.30	W	—	—	—
	13.2	6.72	S	—	—	—

Key: RI, relative intensity scale: S, strong; M, medium; W, weak; (−) less than; (+) more than.

The results of d‐spacing and their relative intensities for unprocessed rice–legume blends are summarized in Table 2. The X‐ray patterns of the unprocessed blends were more or less similar to those of rice alone. This suggested that either the addition of 75% rice flour superimposed the X‐ray patterns of legumes or the legume also exhibited the A‐type X‐ray pattern, which is characteristics of cereal starches. The presence of strong lines at 5.15 Å, and 5.78 Å was also an indication of B and C‐type patterns, respectively, besides the overwhelming evidence of A‐type pattern. However, due to the low moisture contents of all the samples (8–10%), only the strong intensity lines could be received clearly. The addition of bengal gram, black gram, and green gram had similar effect on intensities contrary to the extent by the full fat soy flour.

Table 2. Effect of extrusion cooking on the X‐ray diffraction patterns of rice–legume blends (75:25)

	Blends rice +
	Bengal gram			Black gram			Green gram			Soy flour
Treatment	2θ	d‐Spacing (Å)	RI	2θ	d‐Spacing (Å)	RI	2θ	d‐Spacing (Å)	RI	2θ	d‐Spacing (Å)	RI
Raw (Unprocessed)	30.8	3.80	W	23.0	3.86	S	30.8	2.89	W	27.0	3.30	W
	26.6	3.34	W^+	19.5	4.54	W	26.2	3.39	W^+	23.4	3.79	S
	23.2	3.82	S	18.2	4.86	S	22.8	3.89	S	18.2	4.86	S
	20.8	4.20	W	17.2	5.15	S	21.3	4.16	M	15.3	5.78	S
	19.8	4.47	W^+	15.3	5.78	S	20.3	4.37	M	—	—	—
	18.2	4.86	S	9.6	9.29	W	18.0	4.92	S	—	—	—
	17.2	5.15	S	—	—	—	16.8	5.27	S	—	—	—
	15.3	5.78	S	—	—	—	15.0	5.89	S	—	—	—
	9.8	9.01	W	—	—	—	9.4	9.39	W	—	—	—
Extruded	27.0	3.29	W	22.2	4.0	W	25.4	3.50	W	25.4	3.50	W
	26.5	3.57	W	19.7	4.50	S	22.6	3.93	W^+	22.0	4.03	W^+
	25.8	3.44	W	17.2	5.15	W	21.0	4.43	S	20.4	4.34	S
	19.6	4.52	S	13.3	6.64	W	19.0	4.46	W	18.4	4.81	W
	16.8	5.27	W	11.4	7.75	W	13.8	6.40	W	15.5	5.71	W^+
	13.0	6.80	S	—	—	—	11.8	7.45	W	—	—	—
	9.4	9.39	W	—	—	—	9.0	9.81	W^−	—	—	—

Key: RI, relative intensity scale: S, strong; M, medium; W, weak; (−) less than; (+) more than.

In Tables 1 and 2 are given the results of d‐spacings with relative intensities of various extruded blends. Extrusion cooking of the various blends at 95°C with a water content of about 26% and a feed rate of 27.2 kg/hr, disorganized the crystalline structure of starch. A V‐hydrate (less crystalline) superimposed pattern over A‐type was shown by the extrusion cooked blends. A strong d‐spacing at 4.43 Å, characteristics of V‐hydrate structure was obtained in almost all the extruded products. The “Vh” structure was fully supported by the d‐spacings of 3.93 Å, 4.80 Å, 3.35 Å, and 7.75 Å. Five to six folds increase in water absorption of the products has been reported[7] which supported the presence of V‐hydrate structure, characteristics of decreased crystallinity. A V‐amylose structure was also noticed but that was superimposed by the “Vh” structure. The presence of V‐amylose structure may be due to the formation of amylose–fatty acid complexes during extrusion cooking as suggested by earlier worker[3] on the basis of X‐ray diffraction structures in extrusion cooked manioc starch with fatty acids. The presence of V‐type structure in various extruded starches was reported by earlier worker.[2] The X‐ray diffraction patterns of extruded products showed that the starch turned to have less crystalline structure (Vh) as a result of temperature and shear stress forces exerted by the screw of the extruder during its operation. A typical X‐ray pattern is presented in Fig. 1.

Figure 1. A typical X‐ray diffraction pattern.

Conclusion

From this study it can be concluded that the starch component of coarse rice had more crystallinity than that of fine one. Blends of rice with different grams showed A‐type pattern suggesting that legumes starches also possess similar X‐ray pattern to that of rice‐starch. Extrusion cooking disorganized the structure of starch and converted more crystalline to less crystalline structure indicating the higher water absorption capacity of extruded blends as compared to raw blends.