RICE QUALITY AND PROCESSING. A Comparison of Laboratory-Milled to Commercially-Milled Rice

Transcription

1 RICE QUALITY AND PROCESSING A Comparison of Laboratory-Milled to Commercially-Milled Rice A.M. Graves, T.J. Siebenmorgen, and M.I. Saleh ABSTRACT The degree of similarity between rice milled in a McGill #2 laboratory mill and in commercial milling processes was evaluated using eight physicochemical and enduse properties. Twenty-nine rough rice, and accompanying milled rice, samples were collected from commercial rice mills in Arkansas, California, Louisiana, Mississippi, and Missouri. The rough rice samples were milled to the same degree of milling (DOM) as the corresponding commercially-milled (CM) sample. There was no statistical difference between rice milled in the two systems with respect to color parameters L* and a*, final viscosity, texture, and end-use cooking properties (α = 0.05). Overall, the kernel dimensions of length, width, and thickness were less in the laboratory-milled (LM) rice than the same rice milled commercially. The incidence of bran streaks and peak viscosity values were each greater when rice was milled commercially in twentyseven, and twenty-eight, respectively, of the 29 samples by means comparison. As the DOM increased and surface lipid content (SLC) decreased, L* increased, and a*, b*, and the incidence of bran streaks decreased for both milling systems. INTRODUCTION Results from research using laboratory mills are often questioned due to the lack of evidence that laboratory mills adequately represent commercial milling processes. The objective of this study was to quantify the degree of similarity between commerciallymilled (CM) and McGill #2 laboratory-milled rice across a range of physicochemical properties. 264

2 B.R. Wells Rice Research Studies 2008 METHODS AND MATERIALS Twenty-nine samples, comprising two medium-grain samples and 27 long-grain samples, with each sample comprising rough and milled rice sub-samples, were gathered from four rice processors, totaling seven commercial milling sites in Arkansas (3), California (1), Louisiana (1), Mississippi (1), and Missouri (1). Collection of the milled rice sub-samples was timed to match the milling completion of the respectivelycollected rough rice sub-samples (Fig. 1). Samples were collected over a five-month period from 20 June 2006 to 4 October 2006, during which rice from the 2005 growing season was being milled. Once collected, the samples were stored at 4 C for 5 to 12 months prior to testing. Because the SLC, as a quantifiable measurement of DOM, affects physicochemical and end-use properties, it was necessary to mill samples in the McGill #2 laboratory mill to the same SLC values as those of the CM samples. To match the SLC of the LM samples to the CM rice samples, milling curves were produced for each of the 29 rough rice sub-samples using a laboratory mill as described by Cooper and Siebenmorgen (2007). Duplicate 150-g rough rice samples were dehulled in a laboratory huller (THU- 35, Satake, Hiroshima, Japan). The resulting brown rice was milled with a laboratory mill (RAPSCO, Brookshire, Texas) for a duration that produced the same SLC as the respective CM sample. The samples were separated into head rice and broken rice using a shaker table. Approximately 100 g of each CM rice sample was separated into head rice and broken rice in the same manner. The head rice masses of the CM and LM samples were expressed, and subsequently compared, as a percentage of the milled rice mass, not the rough rice mass, because the exact mass of the corresponding rough rice from the CM sub-sample was unknown. Milled rice SLC values were determined using the method developed by Matsler and Siebenmorgen (2005). The SLC was the mass of extractable lipids expressed as a percentage of the original 5-g head rice sample. Color was quantified using a color meter (ColorFlex, Hunter Associates Laboratory, Reston, Va.) which utilizes the International Commission on Illumination (CIE) L* a* b* system. A color reading was taken and then the sample was rotated, approximately 120 in order to obtain two color readings for every 50 g. The length, width, and thickness of 200 head rice kernels from each duplicate of the CM and LM samples were taken using an image analyzer [Rice Image Analyzer (RIA 1A), Satake Corp., Hiroshima, Japan]. The number of bran streaks was quantified, using the method of Bhattacharya and Sowbhagya (1976), by staining 100 head rice kernels from each duplicate of CM and LM samples in a 3:1 mixture of ethyl alcohol to 2% potassium hydroxide. A kernel was counted as having a bran streak when 50% or more of the bran was present on the dorsal rim of the kernel. Peak and final viscosities were taken with a viscometer (RVA-4 Rapid Visco Analyzer, Foss North America, Eden Prairie, Minn.) as described in Perdon et al. (2001). The methods for measuring the textural analysis traits of firmness and stickiness were those used by Saleh and Meullenet (2007). Water uptake and volumetric change, both calculated on a wet-weight basis, were determined by cooking 5 g of head rice in excess water for 20 min. 265

3 AAES Research Series 571 RESULTS AND DISCUSSION Surface lipid contents ranged from 0.23 to 0.93 (average 0.43) percent for the CM rice and 0.15 to 0.85 (average 0.42) percent for the LM samples. The difference in mean SLC between paired CM and LM samples ranged from to (average 0.015) percentage points. The mean head rice percentage, averaged across the 29 lots, was statistically different between the CM and LM samples. The CM rice samples and the LM samples had mean head rice percentages of 86.8%, and 80.2%, respectively (Fig. 2). The magnitude of the difference in head rice percentages between the CM and LM samples was associated with the rice processor. Two of the four rice processors contributed CM rice subsamples that were 8.8 to 19.0 percentage points greater than the rough rice sub-sample milled in the McGill #2 laboratory mill. The CM rice sub-samples contributed by the other two processors ranged from 6.3 percentage points more to 4.6 percentage points less head rice percentage than the McGill #2 laboratory-milled counterpart. Color parameters were not statistically different between the CM and LM samples within the L* and a* values, but were different for b* values (Fig. 3). The CM samples had mean L* a* b* values of 72.8, 0.25, and 16.0, respectively. The LM samples had mean L* a* b* values of 73.3, 0.24, and 15.8, respectively. The L* values linearly increased as the milling duration increased and the SLC correspondingly decreased for both milling systems (Fig. 3). Concurrently, the a* and b* values linearly decreased as milling duration increased in both systems (Fig. 3). The rates of linear decrease of L*, and increase of a* and b*, with SLC were not statistically different between the CM and LM rice samples. For both milling systems, SLC explained 77, 78, and 66 percent of the variation in L*, a*, and b*, respectively. Overall, the CM rice samples were of statistically equal or greater size, by length, width, and thickness, than those milled in the McGill #2 laboratory mill (Fig. 4), based on a means comparison of each of the 29 samples. The mean length of the 27 long-grain samples was 6.5 mm for the CM rice, and 6.4 mm for the LM samples. The mean width and thickness of the 27 long-grain samples was 2.1 mm, and 1.6 mm, respectively, for rice milled in both systems. The mean length, width, and thickness of the two medium-grain samples was 5.9 mm, 2.7 mm, and 1.8 mm, respectively, for rice milled in both systems. The CM rice samples had a statistically greater number of bran streaks than the LM counterparts in 27 of the 29 samples using a means comparison. The number of bran streaks for the CM samples averaged 12 per 100 kernels, whereas the LM samples averaged 5 per 100 kernels. The incidence of bran streaks increased as the SLC increased for both the CM and LM samples (Fig. 5). Of the 29 samples, 17 of the CM samples had statistically equal, and 11 had statistically greater peak viscosity values than the corresponding LM sample by means comparison. The difference in peak viscosity is attributed to the different bran removal methods in the two milling systems. Laboratory mills can leave disproportionate amounts of bran and germ at the kernel ends which would decrease the peak viscosity of ground rice. There was no statistical difference in the final viscosities between the CM and LM samples. The CM rice samples had a peak viscosity mean of 220 RVUs and a final 266

4 B.R. Wells Rice Research Studies 2008 viscosity mean of 257 RVUs. The LM samples had a peak viscosity mean of 211 RVUs and a final viscosity mean of 258 RVUs. There was no statistical difference in the firmness or stickiness between the CM and LM samples. The CM rice samples had a mean firmness value of 95.8 N, whereas the LM samples had a mean of 94.9 N. The mean stickiness was 2.8 N s for the CM rice samples and 2.9 N s for the LM rice samples. The end-use cooking parameters of volumetric expansion and water uptake were not statistically different between the CM and LM samples. The mean volumetric expansion was 316% and 314% for the CM and LM rice, respectively. Water uptake averaged 229% and 231% for CM and LM rice, respectively. SIGNIFICANCE OF FINDINGS The results of this study indicate a degree of similarity between rice milled in a McGill #2 laboratory mill and commercial milling processes with respect to color parameters L* and a*, kernel thickness, final viscosity, texture, and end-use cooking properties. However, there were differences in head rice percentage, the color parameter b*, kernel length and thickness, bran streaks, and peak viscosity between rice milled in the two systems. Furthermore, the similarity of the regression analysis of the L* a* b* color parameters between the CM and McGill #2 LM samples extends previous research conducted in a McGill #2 laboratory mill. The aggressive nature of the McGill #2 milling process is evident in the decrease in kernel dimensions and the incidence of bran streaks compared to the CM rice samples. LITERATURE CITED Bhattacharya, K.R. and C.M. Sowbhagya Technical note: An alkali degradation test and an alcoholic alkali bran-staining test for determining the approximate degree of milling of rice. J. Food Tech. 11: Cooper, N.T.W. and T.J. Siebenmorgen Correcting head rice yield for surface lipid content (degree of milling) variation. Cereal Chem. 84: Matsler, A.L. and T.J. Siebenmorgen Evaluation of operating conditions for surface lipid extraction from rice using a soxtec system. Cereal Chem. 82: Perdon, A.A, T.J. Siebenmorgen, A.A. Mauromoustakos, V.K. Griffin, and E.R. Johnson Degree of milling effects on rice pasting properties. Cereal Chem. 78: Saleh, M.I. and J.-F. Meullenet Effect of moisture content at harvest and degree of milling (based on surface lipid content) on the texture properties of cooked long-grain rice. Cereal Chem. 84:

5 AAES Research Series 571 Fig. 1. Sample procurement schematic showing collection and comparison of commercially-milled (CM) and McGill #2 laboratory-milled rice subsamples. Fig. 2. Graph depicting the mean values of head rice percentage for the commercially-milled (CM) and McGill #2 laboratory-milled (MG) rice samples. Each data point represents the average of two measurements. 268

6 B.R. Wells Rice Research Studies 2008 (a) (b) (c) Fig. 3. Graphs depicting L* (a), a* (b), and b* (c) by surface lipid content for commercially-milled (CM) and McGill #2 laboratory-milled (MG) samples. Each data point represents the average of two measurements. 269

7 AAES Research Series 571 (a) (b) (c) Fig. 4. Graphs depicting particle size distributions of length (a), width (b), and thickness (c) by sample number for commercially-milled (CM) and McGill #2 laboratory-milled (MG) samples. Each data point represents the average of two, 200-kernel measurements. 270

8 B.R. Wells Rice Research Studies 2008 Fig. 5. Graphs depicting the number of bran streaks by sample number (a), and surface lipid content (b) for commercially-milled (CM) and McGill #2 laboratory-milled (MG) samples. Each data point represents the average of two, 100-kernel measurements. *Samples with SLC greater than 0.8 were not used in this analysis. 271