RICE QUALITY AND PROCESSING. Effect of Surface Lipids on Cooked Long-Grain Rice Textural Properties

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1 RICE QUALITY AND PROCESSING Effect of Surface Lipids on Cooked Long-Grain Rice Textural Properties M.I. Saleh and J.-F. Meullenet ABSTRACT The effects of milled rice degree of milling [based on surface lipids content (SLC)] on cooked rice textural properties were investigated. Milling to lower SLC (i.e., higher degree of milling) significantly decreased (p<0.05) cooked rice firmness and increased cooked rice stickiness across cultivars, harvest locations, and harvest moisture contents (MC). INTRODUCTION Rice is usually used as milled whole grain after removing the outer hull and bran layers of rough rice. However, the amount of bran removed varies depending on the milling processes employed. Therefore, variations in the degree to which rice is milled, degree of milling (DOM), result in changes in the rice kernel gross composition. Disproportionate losses of lipids, proteins, reducing sugars, and minor components as well as an increase in the total amount of starch are reported with increasing milling (Azhakanandam et al., 2000; Singh et al., 1998). These variations in rice chemical composition are considered as key factors that affect rice functional properties (Eliasson and Krog, 1985; Vandeputte et al., 2003). Various methods have been used to mill rice samples to different DOMs. For instance, Perdon et al. (2001) used milling durations of 15, 30, 45, and 60 sec using a McGill No. 2-laboratory mill to produce samples with varying DOM while other authors (Kim et al., 2001) have used the percentage of bran removed after milling as an indication of DOM. However, other factors such as rice cultivar, MC, or temperature can yield rice of varying DOM when milled for a set duration (Bennett et al., 1993; 387

2 AAES Research Series 540 Archer and Siebenmorgen, 1995). Practices for target DOM seems to vary quite extensively throughout the world. A higher DOM tends to make rice whiter, which is often associated with higher quality rice but may also negatively impact the sensory quality of cooked rice (Kwon and Jeon, 1991). A thorough investigation of the effect of rice milling on rice quality is of importance to understand the relationship between DOM, rice water uptake during cooking, and cooked rice textural properties. Therefore the objective of this study was to investigate the effect of DOM (based on SLC) of rice harvested at various MC on the instrumental textural properties of cooked rice. PROCEDURES Sample Collection and Preparation Francis and Wells cultivars harvested in fall 2004 from Stuttgart, Ark., and Essex, Mo., were used in this study. Low (12.8 to 16.3%) and high (21.0 to 22.0%) harvest MC samples were collected from each cultivar and location. Rice samples were air-dried to approximately 12% MC and size-fractionated using a precision sizer (Carter- Day Co., Minneapolis, Minn.) into large, medium, and thin fractions. Only milled rice kernels of medium thickness (1.69 to 1.72 mm) were used in this study to provide as uniform a sample in size as possible for instrumental texture testing. Rice Milling A preliminary milling experiment was conducted to determine the milling durations necessary to achieve target DOMs for each sample. Initially, 150 g of rough samples were de-hulled using a de-husker (THU-35, Satake, Hiroshima, Japan), and milled for 10, 20, 30, 40, or 50 sec using a McGill No. 2 mill (RAPSCO, Brookshire, Texas) with a 1.5 kg weight placed on the lever arm, 15 cm from the centerline of the mill chamber. A double-tray-sizing device (GrainMan Machinery Mfg. Corp., Miami, Fla.) was used to separate head rice from the broken kernels. Milled rice SLC was determined using a Soxtec system. Depending on the SLC and milling duration relationship for each sample, a set of milling durations was determined to achieve our target SLCs. Rice samples were milled, as described earlier, to achieve target SLCs of 0.2, 0.3, 0.4, 0.5, and 0.6%. Head rice yield (HRY) was determined for each sample in duplicates. Milled Rice Chemical Composition Milled rice samples protein content was determined using AACC method 46-11A (AACC, 1996). SLC was determined using a Soxtec system (Avanti 2055, Foss, Eden Prairie, Minn.) according to AACC method (AACC, 1997) with modifications. 388

3 B.R. Wells Rice Research Studies 2005 Rice Cooking and Instrumental Texture Measurements A water-to-rice ratio of 2:1 was used for rice cooking. Rice was cooked using a small-scale precision rice cooker, featuring a heating mantle controlled by a temperature controller ( , Eutech Instruments Pte Ltd, Singapore), for 20 minutes at a maximum cooking temperature of C. The cooking conditions were identical for all rice samples to eliminate differences in cooked rice textural properties due to the cooking method. Cooked rice textural attributes were determined by a uniaxial single compression method using a TA-XT2 plus texture analyzer (Texture Technologies, Scarsdale, N.Y.). Textural attributes were obtained using the texture exponent software [Stable Microsystems, version 1,0,0,92 (2000) Surrey, England]. The maximum compression force (N) was used as an indicator of cooked rice firmness while the adhesion energy measured during the upward travel of the compression plate was used as an indicator for cooked rice stickiness. Each rice sample was cooked in duplicate and six measurements were conducted for each cook. A drying oven method was used to determine cooked rice MC. RESULTS AND DISCUSSION Milling Quality Table 1 shows the milling properties for the various treatments (i.e., cultivars, harvest MC, and harvest location). Milling durations ranged from 15 to 48 sec to achieve milled rice with SLC of 0.2 and 0.6%, respectively. A significant decrease in proteins and SLC was found with increasing milling duration across cultivars, harvest MC, and harvest location (Table 2). Results also indicated an interactive effect of cultivar and harvest location on rice millability, defined as the amount of milling time necessary to achieve a particular SLC. HRY results indicated that cultivars harvested at low MC (with the exception of Wells harvested from Essex, Mo.) had lower HRY than those harvested at higher MC. Variation of HRY in our samples is potentially due to differences in drying conditions. Samples dried in the field (low harvest MC) could have been subjected to a wide range of temperatures and humidity (i.e. in contrast to samples dried in our laboratories). Wide swings in temperature and relative humidity increase chances of kernel fissuring, which is known to decrease HRY. Cooked Rice Textural Properties Cooked rice firmness ranged from to N for Francis and Wells cultivars (Table 3). Samples milled to the lowest (0.2%) SLC were significantly (P<0.05) softer than those milled to the highest (0.6%) SLC. However, the difference was not significant (P>0.05) for Francis and Wells samples harvested from Essex, Mo. Changes in the surface chemical composition of a rice kernel appear to be an important factor determining rice water uptake during cooking and consequently cooked rice textural properties. This is in agreement with Kim et al. (2001) who suggested that changes in 389

4 AAES Research Series 540 the sensory textural properties of cooked rice and pasting properties of rice flour are mainly due to differences in proximate composition. The increase in cooked rice MC after milling to lower SLC supports textural analysis results (Table 3). Lipids play a nonnegligible role in determining cooking properties through the formation of complexes with amylose (LAM) and amylopectin (LAP) (Biliaderis and Tonogai, 1991). These complexes are believed to restrict starch granule swelling during heating and prevent leaching of amylose during gelatinization (Eliasson and Krog, 1985; Eliasson et al., 1988), thereby increasing cooked rice firmness and decreasing stickiness. The increase in cooked rice MC with increasing DOM or decreasing SLC (Table 3) supports this hypothesis and agrees with Eliasson and Krog (1985) and SeneViratne and Biliaderis (1991) who indicated that amylose lipid complexes tend to decrease water uptake. Stickiness values ranged from 5.96 to N. sec for Francis and from 5.39 to N. sec for Wells (Table 3). Cooked rice stickiness increased with milling degree across cultivars, harvest MC, and harvest location. Greater stickiness positively correlated with lower lipid and protein contents and with higher water uptake during cooking. The significant decrease in protein and SLC with increasing DOM probably contributed to decreasing starch-protein interactions, weakening the starch-protein matrix formed during cooking. These conditions provided more water for starch granules to uptake, greater starch granule swelling and an increase in soluble starch leaching out of the granules. SIGNIFICANCE OF FINDINGS Both lipid and protein contents in milled rice decreased with increasing DOM. Milling to lower SLC decreased cooked rice firmness and increased cooked rice stickiness across cultivar, harvest MC, and harvest location. Proteins are believed to interact with starch during cooking, forming a protein-starch network that is believed to restrict water absorption during cooking by preventing rice kernels inner constituents to fully interact with water. Moreover, lipids tend to form complexes with starch during cooking that prevent starch from absorbing moisture. The abundance of lipids and proteins results in lowering rice moisture absorption during cooking, which results in harder cooked rice and in lowering starch leaching, which results in less sticky cooked rice ACKNOWLEDGMENTS The authors wish to thank the Arkansas Rice Research Promotion Board and the corporate sponsors of the University of Arkansas rice processing program for financial support of this project. LITERATURE CITED American Association of Cereal Chemists Crude protein-improved Kieldahl method, copper catalyst modification. Method 46-11A. St. Paul, Minn. 390

5 B.R. Wells Rice Research Studies 2005 American Association of Cereal Chemists Crude fat in grain and stock feeds. Method St. Paul, Minn. Archer, T.R. and T.J. Siebenmorgen Milling quality as affected by brown rice temperature. Cereal Chem. 72(3): Azhakanandam, K., J.B. Power, and K.C. Lowe Qualitative assessment of aromatic Indica rice Oryza sativa L. Proteins, lipids and starch in grain from somatic embryo- and seed-derived plants. J. Plant Physiol. 156: Bennett, K.E., T.J. Siebenmorgen, and A. Mauromoustakos Effects of Mc- Gill No. 2 miller settings on surface fat concentration of head rice. Cereal Chem. 70(6): Biliaderis, G. C. and J.R. Tonogai Influence of lipids on the thermal and mechanical properties of concentrated starch gels. J. Agric. Food Chem. 39: Eliasson, A.C. and N. Krog Physical properties of amylose monoglyceride complexes. J. Cereal Sci. 3: Eliasson, A.C., H. Finstad, and G. Ljunger Study of starch lipid interactions for some native and modified maize starches. Starch 40: Kim, K.O., J.K. Park, and S.S. Kim Effect of milling ratio on sensory properties of cooked rice and on physicochemical properties of milled and cooked rice. Cereal Chem. 78(2): Kwon, Y.W. and W.B. Jeon Effect of period and storehouse-grade in grain storage and degree of milling on the sensory taste of cooked rice. Korean J. Crop Sci. 36(3): Perdon, A.A., T.J. Siebenmorgen, A. Mauromoustakos, V.K. Griffin, and E.R. Johnson Degree of milling effects on rice pasting properties. Cereal Chem. 78(2): SeneViratne, H. D. and C.G. Biliaderis Action of -amylase on amylose lipid complex superstructures. J. Cereal. Sci. 13 (2): Singh, S., Y.S. Dhaliwal, H.P.S. Nagi, and M. Kalia Quality characteristics of six rice varieties of Himachal Pradesh. J. Food Sci. Technol. Mysore, 35: Vandeputte, G.E., V. Derycke, J. Geeroms, and J.A. Delcour Rice starches. II. Structural aspects provide insight into swelling and pasting properties. J. Cereal Sci. 38:

6 AAES Research Series 540 Table 1. Milling quality of Francis and Wells rice cultivars harvested at various MC from Stuttgart, Ark., and Essex, Mo. Stuttgart, Ark. Essex, Mo. Harvest Harvest Cultivar MC DOM z Milling % Bran z HRY z MC DOM Milling % Bran HRY (%) (SLC) (sec.) (%) (%) (SLC) (sec.) (%) Francis a y 62.5 a c 36.7 a b 62.4 a b 35.7 a ab 62.1 ab ab 34.7 a ab 30.4 ab ab 34.1 a a 59.1 b a 34.0 a a 71.2 a b 67.9 a a 69.8 a b 67.5 a a 69.1 a b 65.5 a a 67.9 a ab 65.3 a a 67.4 a a 63.5 a Wells c 68.8 a c 67.3 a bc 66.8 ab bc 65.6 a ab 66.5 b abc 64.1 a ab 66.0 bc ab 63.7 a a 64.1 c a 63.0 a d 70.9 a a 61.9 a cd 69.5 a a 61.6 a bc 68.4 a a 61.2 a ab 67.1 a a 59.8 a a 66.5 a a 59.3 a z DOM (SLC), HRY, and % Bran represent target surface lipids after milling, head rice yield, and percentage of rice bran removed during the milling process, respectively. y Means of HRY, % bran for each cultivar harvested at given harvest MC and milled to various DOMs with different letters are significantly (P<0.05) different according to LSD. 392

7 B.R. Wells Rice Research Studies 2005 Table 2. Protein and SLC of Francis and Wells rice cultivars harvested at various MCs and milled to various degrees. Stuttgart, Ark. Essex, Mo. Harvest Protein Measured Harvest Protein Measured Cultivar DOM z MC content SLC MC content SLC (SLC) (%) (%) Francis a y 0.59 a a 0.64 a a 0.49 b 7.15 a 0.51 b a 0.41 c 7.12 a 0.41 c b 0.29 d 7.14 a 0.32 d b 0.20 c 7.08 a 0.23 c a 0.62 a a 0.65 a b 0.52 b 6.44 a 0.53 b b 0.43 c 6.44 a 0.40 c c 0.31 d 6.49 a 0.29 d c 0.20 e 5.86 b 0.20 e Wells a 0.59 a a 0.60 a a 0.52 b 7.42 a 0.51 b a 0.41 c 7.39 a 0.41 c b 0.33 d 7.54 a 0.33 d ab 0.18 e 6.77 b 0.25 e ab 0.59 a a 0.61 a a 0.50 b 7.63 bc 0.50 b a 0.42 c 7.57 c 0.38 c b 0.31 d 7.69 ab 0.27 d c 0.20 e 7.26 d 0.18 e z DOM (SLC) represents the target surface lipid content (SLC) after milling. y Means of protein content and SLC for each cultivar and harvest MC milled to 0.2 to 0.6^ SLC with different letters are significantly (P<0.05) different according to LSD. 393

8 AAES Research Series 540 Table 3. Means of hardness, stickiness, and cooked rice MC of long-grain rice cultivars Francis and Wells milled to various degrees. Hardness Stickiness Cooked rice MC HMC HMC HMC HMC HMC HMC Cultivar DOM z 14.7% 21.8% 14.7% 21.8% 14.7% 21.8% (SLC) Stuttgart, Ark. Francis a y A x a B 8.84 b A 6.55 a B c A c A ab A a B 9.80 b A 7.45 a B c A bc A bc A ab B 8.54 b A 6.74 a A bc A b A c A ab B 9.37 b A 8.16 a A ab A a A d A b A a A 6.59 a B a A a A HMC HMC HMC HMC HMC HMC 13.7% 21.4% 13.7% 21.4% 13.7% 21.4% Wells a A a B 5.39 c A 6.14 c A a A a A a A ab B 7.25 bc A 7.78 bc A a A a A a A bc B 7.39 b A 7.26 c A a A a A a A cd B 9.33 a A a A a A a A b A d B 9.49 a A 9.60 ab A a A a A continued 394

9 B.R. Wells Rice Research Studies 2005 Table 3. Continued. Hardness Stickiness Cooked rice MC HMC HMC HMC HMC HMC HMC 12.8% 21.0% 12.8% 21.0% 12.8% 21.0% Essex, Mo. Francis a A a B 5.96 c A 7.01 a A b A b A a A a B 7.74 b A 7.69 a A b B b A a A a B 8.39 ab A 7.22 a A ab A ab A a A a B 9.83 a A 8.98 a A a B a A a a B a A 8.36 a B a A ab A HMC HMC HMC HMC HMC HMC 16.3% A 22.0% 16.3% 22.0% 16.3% 22.0% Wells a A a B 7.29 b A 6.69 a A b A b A c A ab B 8.03 b A 8.05 a A b A a A c A b B 7.32 b A 7.81 a A a A a A bc A b B 9.19 b A 7.65 a A a A a A ab A b B a A 7.94 a B a A a A z DOM (SLC) and HMC represent target surface lipids after milling and MC of rough rice at harvest, respectively. y Means of hardness, stickiness, and cooked rice MC for a given cultivar and MC at harvest milled to various DOMs with different lowercase letters are significantly (P<0.05) different according to LSD. x Means of hardness, stickiness, and cooked rice MC for a given cultivar milled to a similar DOM but harvested at different HMCs with different uppercase letters are significantly (P<0.05) different according to LSD. 395