EXTRUSION BEHAVIOUR AND PRODUCT CHARACTERISTICS OF BROWN AND MILLED RICE GRITS

ABSTRACT

Extrusion behaviour of milled and brown rice grits obtained from six rice cultivars (Jaya, IR-8, PR-103, PR-106, Pusa No.1 and Basmati 385) was investigated using Brabender single screw extruder. Feed moisture of the grits was adjusted to 16%, and extrusion cooking was done at barrel temperature of 150°C and screw speed of 100 rpm. The milled and brown rice grits from different rice cultivars differed significantly with respect to extrusion behaviour and extrudate characteristi cs. Extrudates obtained from milled rice grits showed higher die pressure, expansion, water absorption index (WAI) and water solubility index (WSI) as compared to those from brown rice. WSI of extrudates varied from 3.38 to 10.5%, WAI from 5.20 to 6.90, die pressure from 6068 to 9325 kPa and expansion from 7.52 to 11.30 respectively. The grits from cultivars which had lower length-breadth ratio and mealy endosperm resulted into more expanded extrudates and hence were found to be more suitable from extrusion.

INTRODUCTION

Extrusion cooking is used world wide for the production of expanded snack foods, modified starch, ready to eat cereal foods, pet foods and porridge.1-3 Extrusion variables, composition of feed material, particle size distribution and additives significantly affect extrusion parameters and product properties.4-9 Bhattachar ya[10] investigated the effect of extrusion process variables on extrusion characteristics and extrudate properties during the extrusion cooking of a 1:1 rice-green gram (mung beans) blend using co-rotating twin screw extruder. The effect of the addition of Na₂HPO₄ and Na₂H₂PO₄ with a change of process variables on the extrusion behaviour rice grits was studied. [9] The effect of incorporating various hydrocolloids and processing variables on the extrusion behaviour and product characteristics of rice grits were studied.[11] Sekhon et al.[12] studied the effect of incorpora-ting rice bran in baked and extruded snack foods. The effect of substituting rice with extrusion cooked rice at different levels on the quality of idli was investigated.[13]

Guha et al.[14] studied the effect of extrusion variables on extrusion system parameters and product attributes during the extrusion cooking of rice flour. Juliano[15] reported that rice cultivars differ significantly with respect to endosperm texture (vitreous and mealy). The objective of the present study was to investigate the extrusion behaviour of grits from different rice cultivars and effect of milling on the extrusion behaviour and extrudates characteristics.

MATERIAL AND METHODS

Paddy Samples

Six different paddy samples Jaya, IR-8, PR-103, PR-106, Pusa No. 1 and Basmati-385 were collected from Punjab Agriculture University Rice Research Station from 1999 harvest.

Preparation of Rice for Extrusion

The different paddy cultivars were dehusked in McGill rice sheller. One batch of brown rice was ground in a coffee grinder to produce brown rice grits. The other batch of brown rice was milled in a McGill rice miller to 7% degree of milling.[16] The length breadth ratio was determined by dividing the length of ten kernels by the breadth of ten kernels. The milled rice was ground in a coffee grinder to milled rice grits. Both the brown and milled rice grits obtained were conditioned to 16% moisture content. The samples were packed in polyethylene bags and placed in a refrigerator overnight to equilibrate the moisture. The samples were brought to room temperature before extrusion cooking.

Extrusion Cooking

Extrusion cooking of brown and milled rice grits was carried out in a single screw extruder (C.W. Brabender, Hackensack, NJ) fitted with a die nozzle having 3.75 mm diameter with the specification as described earlier.[9] The feed zone temperature was maintained at 75°C while the compression and die zone temperature was kept at 150°C.

Die Pressure

Die pressure was measured using Dynisco pressure transducer (Dynisco Ltd. USA) just before the discharge orifice. Readings were recorded every 10 s for at least 2 min and average was reported as kPa.

Expansion

The diameter of extrudates was measured as described earlier[17] as the mean of 10 random measurements using a vernier calliper.

Water Solubility Index (WSI) and Water Absorption Index (WAI)

WSI and WAI were determined by the method of Anderson et al.[18] The extrudates were ground to pass through 60-mesh sieve. Extrudate sample (2.5 g) was dispersed in 25 mL distilled water, using a glass rod to break up any lumps. After stirring for 30 min the dispersions were rinsed into tarred centrifuge tubes and then centrifuged at 3000 g for 10 min. The supernatant was decanted for determination of its solids content and the sediments were weighed. WSI and WAI were calculated by the following equations.

Statistical Analysis

The date on length breadth ratio, die pressure, WAI, WSI and expansion was subjected to analysis of variance (Minitab Statistical Software) to determine the significant effect of milling and cultivars on the length breadth ratio, extrusion parameters and extrudate characteristics.

RESULTS AND DISCUSSION

The brown and milled rice were ground to obtain grits and the particle size of the grit was such that all of it passed through 20 mesh sieve and 99% was retained by 60 mesh sieve. The feed moisture, extrusion temperature and screw speed were kept constant at 16% 150°C and 100 rpm, respectively. The six rice cultivars selected for the study had length breadth ratio between 2.69 and 4.20 (Fig. 1). The milling of brown rice to the extend of 7% lowered the length breadth ratio by 0.01, 1.11, 3.79, 1.86, 6.01 and 3.33% respectively, in Jaya, IR-8, PR-103, PR-106, Pusa No.1 and Basmati 385. Cultivars with higher length breadth ratio showed higher decrease in the length breadth ratio at same degree of milling. The analysis of variance of data in Fig. 1 is given in Table 1. The cultivars differed more significant with respect to length breadth ratio (p<0.0005) as compared to the effect of milling on length breadth ratio (p<0.05).

Figure 1. Effect of milling on length breadth ratio of different rice cultivars.

Table 1. Analysis of Variance of Data on Length Breadth Ratio

Source of Variation	Degrees of Freedom	Sum of Squares	Mean Sum of Squares	F Value	P Value
Cultivar	5	3.705	0.741	179.18	0.000012
Milling	1	0.03	0.03	7.36	0.42
Error	5	0.02	0.004
Total	11	3.75

Die Pressure

Figure 2 illustrates the die pressure of the extruder for extruding brown and milled rice grits from different cultivars. Brown rice grits showed significantly lower die pressure as compared to the grits from their counter part milled rice in all the cultivars. Milling lowered the die pressure by 5.24, 15.72, 48.29, 48.56, 23.71 and 14.5% respectively in Jaya, IR-8, PR-103, PR-106, Pusa No.1 and Basmati 385. The lower die pressure in case of brown rice grits may be attributed to their higher fat and fibre content and lower starch content. Effects of addition of rice bran on die pressure of extruder have been reported earlier.[12] The analysis of variance of data in Fig. 2 is given in Table 2. The die pressure was more significantly affected by milling (p<0.05) and less affected by different cultivars.

Figure 2. Effect of milling on the die pressure during extrusion of different rice cultivars.

Table 2. Analysis of Variance of Data on Die Pressure

Source of Variation	Degrees of Freedom	Sum of Squares	Mean Sum of Squares	F Value	P Value
Cultivar	5	1115502	223100.4	0.382	0.842
Milling	1	8502180	8502180	14.55	0.012
Error	5	2919785	583957
Total	11	12537467

Expansion

Figure 3 illustrates the expansion of extrudates from brown and milled rice grits. Milled rice grits from different cultivars showed significantly higher expansion as compared to brown rice grits. These differences may be attributed to differences in chemical composition between milled rice grits. The brown rice grit has been reported to have higher amount of protein, minerals, fat and fibre content.[15] These constituents have been reported to have a negative effect on expansion of cereals.[7] Brown and milled rice grits from Jaya and IR-8 cultivars showed highest expansion while those from Pusa No. 1 and Basmati 385 showed lowest expansion (Fig. 3). Milling increased the expansion ratio by 14.83, 18.98, 30.51,12.34, 28.85 and 15.88% respectively in Jaya, IR-8, PR-103, PR-106, Pusa No.1 and Basmati 385. The analysis of variance of data in Fig. 3 is given in Table 3. The analysis revealed that milling had a greater effect on expansion of extrudates as compared to the effect of different cultivars.

Figure 3. Effect of milling on the expansion of extrudates from different rice cultivars.

Table 3. Analysis of Variance of Data on Expansion

Source of Variation	Degrees of Freedom	Sum of Squares	Mean Sum of Squares	F Value	P Value
Cultivar	5	6.875	1.375	10.29	0.011
Milling	1	8.744	8.744	65.48	0.0004
Error	5	0.667	0.133
Total	11	16.28

Water Solubility Index

Figure 4 shows WSI of extrudates from both brown and milled rice grits. The analysis of variance of data in Fig. 4 is given in Table 4. The statistic al analysis indicated that milling had a more significant effect on WSI as compared to the effect of different cultivars. Extrudates from milled rice grits showed significantly higher WSI as compared to those from brown rice grits. The increase in WSI on milling was 162.24, 122.92, 16.80, 18.34, 8.35 and 17.85% respectively in Jaya, IR-8, PR-103, PR-106, Pusa No.1 and Basmati 385. These differences may be attributed to differences in structure of endosperm in different cultivars. The lower WSI of extrudate s from brown rice grits may be attributed to formation of amylose lipid complex[19] since brown rice has been reported to have higher fat content as compared to milled rice.[15]

Figure 4. Effect of milling on the WSI of extrudates from different rice cultivars.

Table 4. Analysis of Variance of Data on WSI

Source of Variation	Degrees of Freedom	Sum of Squares	Mean Sum of Squares	F Value	P Value
Cultivar	5	24.78	4.95	1.405	0.358
Milling	1	15.68	15.68	4.448	0.088
Error	5	17.63	3.52
Total	11	58.10

Water Absorption Index

Figure 5 shows WAI of extrudates from both brown and milled rice grits from different cultivars. Extrudates from milled rice grits showed higher WAI as compared to their counterpart extrudates from brown rice grits. Milling of brown rice resulted in an increase in the WAI by 5.87, 2.85, 18.66, 10.74, 19.77 and 17.38% respectively, in Jaya, IR-8, PR-103, PR-106, Pusa No. 1 and Basmati 385. The higher WAI of extrudates from milled rice grits may be attributed to their higher starch content. The analysis of variance of data in Fig. 5 is given in Table 5. The statistical analysis showed that milling had a more significant effect (p<0.005) on WAI as compared to the effect of cultivars (p<0.05).

Figure 5. Effect of milling on the WAI of extrudates from different rice cultivars.

Table 5. Analysis of Variance of Data on WAI

Source of Variation	Degrees of Freedom	Sum of Squares	Mean Sum of Squares	F Value	P Value
Cultivar	5	2.723	0.544	9.068	0.015
Milling	1	1.372	1.372	22.84	0.004
Error	5	0.3003	0.06
Total	11	4.396

CONCLUSION

All the cultivars differ significantly with respect to extrusion behaviour and product characteristics. Die pressure of extruder, expansion, water solubility and water absorption indices increased with the milling of brown rice. Brown and milled rice obtained from varieties with lower length-breadth ratio (Jaya and IR-8) showed higher expansion as compared to their counterpart brown and milled rice obtained from varieties with higher length-breadth ratio (PR-103, PR-106, Pusa No. 1 and Basmati-385). The study revealed that cultivars (Jaya and IR-8) with a mealy opaque endosperm and low length-breadth ratio resulted into extrudates with greater expansion and water absorption indices.

Acknowledgments