Introduction

Rice is a staple food in many countries. Degree of milling is one of the factors that influence consumers’ perception of quality (Chrastil, 1989). Paddy rice is dehulled to obtain brown rice, which is milled before consumption. The primary purpose of milling is to remove the germ and bran layer from the kernel endosperm. The quantity of bran removed is referred to as the “degree of milling” and the intended use of rice dictates the level of bran removal (Terry and Meullenet, 2004). Milling degree influences the chemical composition (eg lipid content) and functional properties of the rice grains.

Brabender Viscoamylograph has being replaced with the RVA (Newport Scientific, Warriewood, Australia) to measure the viscosity properties of starch (Blakeney et al, 1991). The texture parameters from Texture Profile Analysis (TPA) can be correlated with sensory ratings (Szczeniak et al., 1963). This study shows the effect of milling degree on phenolic composition, pasting and textural properties of four rice cultivars.

Material and methods

Rice

Samples of four rice cultivars (Amaroo, Koshihikari, Kyeema, and Langi) were obtained from Sunrice for use in this study. These samples were grown in the 2004/2005 growing season. Paddy rice from each variety was dehulled using a THU35A Test Husker (Satake) and milled for 0s, 20s 40s, 60s, 120s, or 180s using a McGill No.2 Mill. Milled grains were then ground through a Retdch grinder (MODEL, Zm 100, 0.10mm screen).

Phenolic compound analysis

Rice flour (1.0 g dry basis) was slurried with distilled water (5 mL). A suspension of alpha-amylase (from Bacillus Licheniformis; 43 units) and a solution of protocatechuic acid, used as an internal standard (in 50% methanol, 1% acetic acid; 200 mg/L, 2 mL), were added and maintained at room temperature for 15 min. The mixture was hydrolysed in a boiling water bath for 2 min. The hydrolysate was cooled rapidly under a stream of running water for 3 minutes while blanketed by N₂ and then aqueous NaOH (4M; 20mL) was added. The mixture was stirred for 5 min.

The extracts were acidified to pH 1.5-2.5 by gradual addition of ice-cold 6M HCl and extracted three times with ethyl acetate (3x70 mL). The ethyl acetate fraction was dried by addition of anhydrous sodium sulphate and evaporated to dryness using a rotary vacuum evaporator at 35ºC. The residue was redissolved in aqueous methanol (50% v/v; 4 mL), filtered through a 0.45 μm nylon filter and stored in the dark prior to analysis by HPLC.

An aliquot (20 μL) of the hydrolysed extract was separated using HPLC (Alliance Waters 1690 Separations Module). Peaks were detected with a variable wavelength UV-VIS detector (Waters 2996 Photodiode Array Detector) operated at 280 nm. Separations were achieved on a column LiChrospher 100 RP-18. Gradient elution was performed with a gradient of A (water : acetic acid, 100:1, v/v) and B (methanol : acetonitrile : acetic acid, 95:5:1, v/v/v) as follows: 0-2 min, 5% B; 2-10 min, 5-25% B; 10-20 min, 25-40% B; 20-30 min, 40-50% B; 30-40 min, 50-100% B; 40-45 min, 100% B; 45-55 min, 100-5% B. Solvent flow rate was 1.0 mL.min^-1. The temperature of the samples was maintained at 15ºC and the column was maintained at 35ºC. Peak identities were confirmed by spiking of extracts with authentic standards.

RVA Analysis

The pasting properties from each variety of rice flour, which had been milled to different degrees, was determined using a Rapid Visco Analyser (Newport Scientific Model 3D, Warriewood, Australia) using the Newport Scientific RVA rice method. Each canister contained 3g of rice flour and was made up to 28g using de-ionized water. Peak viscosity, Final Viscosity, Breakdown and Setback values were determined in duplicate.

Texture analysis

Textural properties of the rice flour gels formed after RVA analysis were determined using a TA XT2 textural analyser (Stable Microsystems, Surrey, Great Britain). Rice pastes were sealed with paraffin film to prevent moisture loss and stored overnight at 4ºC. The hardness of the rice flour gels was determined using a standard two cycle compression, force-versus-distance procedure in duplicate.

Results and discussion

Phenolic compound analysis

Coumaric acid and ferulic acid contents both decreasing with increasing milling degree but coumaric acid level appeared to decrease more rapidly than ferulic acid level (Figure 1).

Figure 1: Concentrations of coumaric acid and ferulic acid in Kyeema samples milled to different degrees.

RVA analysis

The pasting properties of all rice cultivars were significantly affected by milling. The peak viscosity (PV), breakdown (BD) and final viscosity (FV) values increased with higher degrees of milling. The Setback (SB) for Amaroo and Langi increased, whereas Kyeema SB decreased and Koshihikari was unstable with increasing degrees of milling (Figure 2).

Figure 2: Effect of milling on peak viscosity, breakdown, setback and final viscosity.

Texture analysis

The hardness of rice flour gels generally increased with increasing degree of milling (Figure 3.).

Figure 3: Effect of milling on hardness of rice flour gels.

Conclusion

The results from this study showed that degree of milling has a significant effect on chemical composition, texture and pasting characteristics of rice. Coumaric and ferulic acids were shown to be distributed differently in rice grains. The coumaric acid appears to be found almost entirely in the outer surface of the bran, whereas ferulic acid appears to be distributed throughout the bran layer.

References

Blakeney, A.B., Welsh, L.A. and Banon, D.R. (1991). In: Cereals International. (D.J. Martin and C. W. Wrigley eds.) Royal Australia Chemistry Institute. Melbourne. pp 180-182

Chrastil, J. (1989). Journal of Cereal Science. 11, 71-85.

Champagne, E.T., Bett, K. L., Vinyard, B.T., McClung, A. M., Barton II, F. E., Moldenhauer, K., Linscombe, S. and Mc Kenzie, K. (1999). Cereal Chemistry 76,764-771.

Szczesniak, A.S., Brandt, M. A. and Friedman, H. H. (1963). Journal of Food Science. 28, 397-403.

Terry J. S & Meullenet J, (2004). In: Rice Chemistry and Technology ( E.T. Champagne ed). St. Paul, Minnesota: American Association of Cereal Chemists. pp. 301-325.