RICE QUALITY AND PROCESSING. Texture Profile and Volatile Compound Analyses of Koshihikari and Basmati Rice Prepared in Different Rice Cookers

Transcription

1 RICE QUALITY AND PROCESSING Texture Profile and Volatile Compound Analyses of Koshihikari and Basmati Rice Prepared in Different Rice Cookers R.J. Bryant, G. Jones, and C. Grimm ABSTRACT Koshihikari and Basmati, two premium rices from Japan and Pakistan, respectively, were evaluated for volatile compounds and textural characteristics using three different cooking methods. Samples were analyzed for hardness, adhesiveness, and cohesiveness using the Texture Analyzer and for volatiles using the GC-MS. A trained sensory panel evaluated the samples for six textural attributes: stickiness-to-lips, hardness, cohesiveness, tooth packing/tooth stickiness, cohesiveness of mass, and roughness. Of the volatiles identified by SPME/GC/MS, dodecanal and hexanal were present in greater amounts in the samples prepared in the Hitachi cooker, whereas, acetone and naphthalene were present in greater amounts in the samples prepared in the National cooker. The Texture Analyzer showed that both rices prepared in the National cooker were the hardest with the stove top preparation being the softest. The sensory panel was unable to detect any significant difference (P>0.05) in rice texture due to preparation methods. INTRODUCTION Rice is consumed with little processing (dehulled, milled, and cooked). However, studies have shown that different cooking methods, such as oven cooking; small, medium, and large amounts of water; or steaming, can affect the texture and flavor of rice (Juliano, 1985). To date, little research has been conducted to show what effect different cookers have on the texture and flavor of rice. Consumers generally adjust their cooking methods to obtain the desired cooked rice texture and researchers generally standardize 370

2 B.R. Wells Rice Research Studies 2006 their methods and compare their results with others. Juliano and Perez (1983) looked at factors affecting cooked rice hardness using an excess-water method and a rice cooker method. In the rice cooker method, they cooked the rice in the amount of water that would be absorbed by the rice, however, they did not use a rice cooker. Juliano and Sakurai (1985) and Webb (1985) published reviews on the preparation of cooked rice. They reported on the effect of water-rice ratio, type of energy used (electric, gas, microwave), type of heating cycles (one- or two-stage, or microcomputer-controlled) and cooking times as they affect hardness. However, no one has reported the effect various cooker types would have on the volatile compounds and texture of cooked rice. Therefore, we tested an aromatic Basmati, and a non-aromatic Koshihikari, rice using two different types of cookers and a stove top method to determine if there were differences in volatile compounds and physical characteristics of the cooked rice. PROCEDURES Koshihikari rice was grown at the University of Arkansas Rice Research and Extension Center, Stuttgart, Ark. Rice was dehulled using a Satake Testing Husker and milled using a McGill No. 2 mill. Rice was stored in a ziplock bag at 4 C until time of evaluation. Basmati rice from Pakistan was purchased as milled rice at a speciality market and stored at 4 C until time of evaluation. The rice was cooked using two types of rice cookers: a National rice cooker (Model SR-1HZC-18N, Matsushita Electric Industrial Co. Ltd, Osaka, Japan); a Hitachi rice cooker (Model RD-4053, Thailand); and a 2.8 L saucepan with lid, which was used on an electric stove to simulate conventional stove-top cooking. For the rice cookers, the amount of time the red (cooking) light was on was consider as the cook time. The temperature at the bottom of each rice cooker was tested at four different locations around the edge (facing the controls they were: back, front, right, and left) and in the center using a dual channel digital thermometer (Model , Fisher Scientific, Houston, Texas) containing 2 ultra-fast response type-k thermocouple naked bead probes with teflon insulated 4 leads. The thermometer was calibrated against National Institute of Standards and Technology Traceable Instrumentation. The temperature of the saucepan was not taken since it used excess boiling water and, therefore, the temperature would remain constant. Instrumental Each rice was prepared using methods culturally practiced. Koshihikari rice (300 g) was washed with an equal amount of filtered deionized water until the water was clear. After washing, the rice was drained and weighed, then filtered water was added until the rice-to-water ratio was 1:1.5. Rice was soaked for one hour and cooked using the same water. Basmati rice was cooked using a rice:water ratio of 1:2. No rinsing or soaking was done with the Basmati rice. Rices were cooked using the conventional stove top (2.8 L saucepan with lid), National rice cooker and Hitachi rice cooker. When rice was cooked using conventional stove top method, the amount of water 371

3 AAES Research Series 550 used was kept at the same ratio as above. The rice was added after the water came to a boil and then cooked for 20 min. Each rice cooker was allowed to cook at its factoryset program. At the completion of the cooking period, the rice was allowed to rest for 15 minutes before testing. Texture Profile and Sensory Analysis The texture profile analysis was conducted using the Texture Analyzer (Model TA-XT2i, Texture Technologies, Inc., Scarsdale, N.Y.). Five grains were placed in a single layer and compressed using a 2-in.-diameter stainless steel cylinder. Pre-test speed was 2.0 mm/sec, test- and post-test speeds were 1.0 mm/sec. Samples were compressed 95%, held for 1 second, released and compressed again to complete the two-cycle compression test (Bourne, 1982). Samples were analyzed for hardness, adhesiveness, and cohesiveness. Five panelists were trained in descriptive analysis techniques according to the Spectrum method (Sensory Spectrum, Chatham, N.J.). Eleven two-hour training sessions were necessary to train panelists in the testing procedures for cooked rice. Six attributes were used to describe the textural properties of rice: stickiness-to-lips, roughness, hardness, and cohesiveness, tooth packing/tooth stickiness, and cohesiveness of mass. Attributes were defined and evaluated according to Munoz (1986) and Meilgaard et al. (1991). When necessary, standards were added to the scale to fit the needs of a rice texture profile panel. The panel used a 15-cm line scale, anchored at both ends, to evaluate attribute intensities. Samples were coded with a 3-digit random number and presented in a warmed custard cup covered with a watch glass. Six samples were evaluated at each session. GC-MS Analysis Cooked rice grains were analyzed by placing 3g of rice directly into a 10 ml vial. 2,4,6-Trimethylpyridine (TMP; Sigma-Aldrich, St. Louis, Mo.) was employed as the internal standard by adding 2 μl aliquots of a 1-ppm solution to each sample, thus effectively placing 2 ng of TMP in each vial. The standard was placed on the inside of the glass vial just below the neck. Following preparation, samples were placed in an autosampler tray and maintained at room temperature until analyzed. Samples were preheated for 25 min at 80 C prior to sampling. Collection of volatile compounds was accomplished using a 15-min adsorption period at 80 C while shaking the sample. The Solid Phase Microextraction (SPME) fiber employed was a 1-cm 50/30 divinylbenzene/carboxen/polydimethylsiloxane stableflex fiber (Supelco, Bellefonte, Pa.). A CTC SPME autosampler equipped with a heated sample shaker and a needle heater for thermal cleaning of the SPME fiber was employed (Leap Technologies, Carrboro, N.C.). Samples were desorbed for 5 minutes on an HP 5973 GC/MS system (Agilent Technologies, Palo Alto, Calif.), The injector temperature was held constant at 270 C. The GC oven temperature was held for 1 min at 50 C, then increased to 250 C at 10 C/ 372

4 B.R. Wells Rice Research Studies 2006 min. A 30-m, 0.25-mm, DB-5 capillary column was used with helium as the carrier gas under a constant flow of 40 cm/s. The total GC cycle time consisted of a 30-min run and a five-minute cool-down period. Statistical analysis was preformed using SAS System for Mixed Models (Littell et al, 1996). RESULTS AND DISCUSSION The temperature profile for each cooker is shown in figures 1 and 2. The Hitachi cooker (Fig. 1), which had a one-step heating cycle, reached 100 C between 6 to 10 min. depending on the position. It held 100 C until 17 min. and then quickly rose to 140 C at which time the warm cycle was initiated. After 15 min. on the warm cycle the temperature was still above 100 C. The National cooker (Fig. 2), which had a two-step heating cycle, went to 60 C in one minute and held it for 9 min. It then went to 65 C for 7 min. and then to 100 C in 2 min. After 5 min. the temperature began to quickly rise to 140 C; however, it quickly returned to 105 C (within 2 min.) and held that temperature for 5 min. then dropped to 100 C for 4 min. The cooker then began a slow cool-down and went to warm at 40 min. into the run. The temperature at that time was 97 C. The National cooker held the temperature to within 5 degrees and each position was close in time and temperature and the heating appeared to be cycling. In the Hitachi cooker, due to continuous heating, each position had a different temperature at a given time except when they all reached 100 C. It must be pointed out that in our study the amount of time the red (cooking) light was on after the start button was depressed was consider as the total cook time. However, a rise in temperature over 100 C would indicate total water absorption which relates to the rice being fully cooked. Therefore, when the temperature began to rise, the Hitachi cooker, which was made out of aluminum and had a high very temperature (>100 C) for a longer period of time, tended to scorch the rice near the heating element, whereas, the National cooker, which had a coating on it and remained cooler, did not scorch. The scorched rice in the Hitachi cooker could become a problem if the scorching became severe or if the rice was allowed to sit longer and/or if it was stirred. Using cookers with a coating on them would alleviate this problem; however, the high temperature may still be of concern. Koshihikari rice was analyzed for volatile compounds using SPME/GC/MS. Of the 130+ volatiles identified using this method (Grimm et al., 2002), only dodecanal and hexanal were present in greater amounts in the samples prepared in the Hitachi, whereas, acetone and naphthalene were present in greater amounts in the samples prepared in the National (Table 2). Dodecanal and hexanal are representative of lipid oxidation products and can be enhanced by high temperatures, which was found to be the case with the Hitachi cooker (Fig. 1). Acetone and naphthalene have been found in rice not cooked in rice cookers (Bullard and Holguin, 1977; Grimm et al, 2002) and the high levels found in the National cooker could be due to the low volatility of these compounds and longer cooking time (Fig. 2). There was a significant difference (P<0.05) due to the method of cooking for all attributes evaluated with the Texture Analyzer, except for cohesiveness of Koshihikari 373

5 AAES Research Series 550 and adhesiveness of Basmati (Table 1). The National cooker produced harder rices and the stove top gave the softest rices. There was no significant difference (P>0.05) in hardness between the National cooker and the Hitachi cooker for both rice varieties. Koshihikari prepared on the stove top was significantly (P<0.05) less adhesive than that prepared in Hitachi cookers, whereas, there was no significant difference (P>0.05) in adhesiveness for any of the Basmati samples (Table 1). There was no significant difference (P>0.05) in cohesiveness for Koshihikari. There was a significant difference (P<0.05) between the National cooker and the stove top for Basmati, with the National cooker being most cohesive and the stove top being the least. The difference in texture could be due to the time required to cook the rice, 19 min. for the Hitachi cooker vs 40 min for the National cooker. However, it would be expected that the rice cooked in the National cooker would be softer not harder as the data showed. The sensory panel evaluated each sample in triplicate for six attributes: stickiness-to-lips, roughness, hardness, and cohesiveness, tooth packing/tooth stickiness, and cohesiveness of mass. They were unable to detect any significant difference (P>0.05) in the attributes based on the preparation methods. SIGNIFICANCE OF FINDINGS This study showed that the heating profile of rice cookers are different and can have an effect on texture and volatiles. Therefore, if comparisons are made, it is important that the method and type of cookers used be taken into consideration. Since there are many different models of cookers available, the cooking profile and the material they are made of may be different, thus, each cooker should be tested before its use in a study. LITERATURE CITED Bourne, M.C Food Texture and Viscosity: Concept and Measurement. Academic Press. New York, N.Y. Bullard, R.W. and G. Holguin Volatile components of unprocessed rice (Oryza sativa L.). J. Agric. Food Chem. 25: Grimm, C.C., E.T. Champagne, and K. Ohtsubo Analysis of volatile compounds in the headspace of rice using SPME/GC/MS. pp In: R. Marsili (ed.). Flavor, Fragrance and Odor Analysis. Marcel Dekker, N.Y. Juliano, B.O Criteria and tests for rice grain qualities. pp In: B.O. Juliano (ed.). Rice Chemistry and Technology. Am. Assoc. Cereal Chem., St. Paul, Minn. Juliano, B.O. and C.M. Perez Major factors affecting cooked milled rice hardness and cooking time. J. Texture Studies. 14: Kohyama, K., K. Oktsubo, and H. Toyoshima Electromyographic study on cooked rice with different amylose contents. J. Texture Studies. 29: Meilgaard, M., G.V. Civille, and B.T. Carr Sensory Evaluation Techniques. CRC Press, Inc. Boca Raton, Fla. 374

6 B.R. Wells Rice Research Studies 2006 Muñoz, A.M Development and application of texture reference scales. J. Sensory Studies. 1: SAS Institute, Inc., SAS User s Guide, Version Statistical Analysis System Institute, Cary, NC (1999). Webb, B.D Criteria for rice quality in the United States. pp In: B.O. Juliano (ed.). Rice Chemistry and Technology. Am. Assoc. Cereal Chem., St. Paul, Minn. Table 1. Textural profile analyses of Koshihikari and Basmati rices. z Sample Hardness Adhesiveness Cohesiveness (N) (N.s) Koshihikari Hitachi cooker 37.1 ab 4.0 a 5.5 a National cooker 40.2 a 3.9 ab 5.6 a Stove top y 33.8 b 3.3 b 5.4 a Basmati Hitachi cooker 45.6 a 0.46 a 6.0 ab National cooker 48.2 a 0.51 a 6.4 a Stove top 41.8 b 0.63 a 5.5 b z Values represent mean of triplicate analyses; mean comparisons followed by the same letters in the same column in the same sub-heading are not significantly different (P<0.05). y Stove top = 2.8 L covered saucepan. Table 2. Comparison of selected volatile compounds observed in Koshihikari rice cooked in two different rice cookers. Hitachi cooker National cooker Retention Area counts Area counts Compounds times avg. z RSD Avg. RSD (min.) (N=3) (%) (N=3) (%) Acetone , , Hexanal , , (E)-2-heptanal , , Benzaldehyde , , Hexanoic acid , , pentylfuran , , Dichlorobenzene , , Undecane , , Nonanal , , Napthalene ,448, ,666, butyl-2-octenal , , Tetradecane , , Dodecanal , , z Due to the reproducibility of SPME, the RSD can be high for some compounds under the conditions analyzed. 375

7 AAES Research Series 550 Fig. 1. Temperature profile of the Hitachi Cooker. (Each position is an average of two analyses). Facing the controls: Position 1 = back; Position 2 = front; Position 3 = right side; Position 4 = left side; Position 5 = center. Fig. 2. Temperature profile of the National Cooker. (Each position is an average of two analyses). Facing the controls: Position 1 = back; Position 2 = front; Position 3 = right side; Position 4 = left side; Position 5 = center. 376