2016 Volume 22 Issue 6 Pages 771-778
Milled common Japonica rice was cooked using a newly developed superheated steam rice cooking machine and the volatile odor compounds of the steamed rice and of the steam generated during cooking were analyzed. At least 22 odorous compounds were found in the steam by GC-olfactometry, of which 13 were identified by GC-MS. Hexanal and (E,E)-2,4-decadienal, which are known odor compounds characteristic of cooked rice, were identified, and four compounds, including longifolene and 2-methoxyphenol, were newly identified in Japonica rice. The amounts of hexanal and (E,E)-2,4-decadienal in steamed rice, extracted with methyl tert-butyl ether, were essentially the same as in ordinary cooked rice. On the other hand, headspace analysis showed that the levels of odor compounds in steamed rice were less than 10% that of ordinary cooked rice. Only small amounts of odorants appear to be released from the surfaces of steamed rice grains.
Rice is generally eaten without seasoning and thus the sensory properties of cooked rice are important. One important property is aroma, and small variations can make rice highly desirable or unacceptable to consumers. A large number of compounds contribute to the aroma and flavor of cooked rice and determining which compound plays the dominant role is difficult. With the exception of 2-acetyl-1-pyrroline, a characteristic aroma compound in scented rice, any other compounds can be said to determine the aroma. Some aldehyde compounds, such as (E)-2-nonenal and (E,E)-2,4-decadienal, are believed to contribute to the aroma of rice (Buttery et al., 1988; Champagne, 2008). The process by which a stale odor develops during rice storage has also been investigated. The stale odor is thought to be mainly the result of carbonyl compounds, i.e., hexanal and octanal, and 2-pentylfuran, which are produced by the oxidization of unsaturated fatty acids (Fujio et al., 1991; Fujita et al., 2005; Monsoor and Proctor, 2004; Yasumatsu et al., 1966). Kobayashi et al. investigated storage methods to prevent flavor deterioration (Kobayashi et al., 2010), and Yasunobu et al. focused on designing an electric rice cooker to eliminate these unpleasant odors (Yasunobu et al., 1995). However, there are few reports that focus on how the volatile odor components change depending on the cooking method.
In previous research, we developed a new superheated steam rice cooking machine (Fig. 1-A) and investigated the physicochemical and sensory properties of rice cooked using this machine (Sako et al., 2013; Takemitsu et al., 2013). This machine can prepare cooked rice on a large scale and is now used in some food factories. The machine comprises a conveyor net belt in a steaming chamber. Washed and soaked raw rice is fed into the chamber on a running conveyer net belt and is simultaneously heated with steam. Hot water is added regularly, and cooked rice exits the machine, ready to eat. We reported that superheated steam-cooked rice (steamed rice) is superior to rice cooked using an ordinary electric rice cooker with regard to preserving grain shape and cell wall structure for the most part, and maintaining its texture even after storage. We also confirmed that steamed rice retrogrades more slowly than ordinary cooked rice. Sensory evaluation tests showed that the odor of cooked rice prepared with the superheated steam rice cooking machine was weaker than that of ordinary cooked rice, although the preference for one odor over the other was divided between the panelists.
Schematic views of superheated steam rice cooking machine with a distillation column.
A. Over view.
The size of the whole machine: W 2500 × H 1650 × D 1100 (mm)
The size of steaming chamber: W 2100 × H 415 × D 544 (mm)
B. Distillation column.
Material: SUS304 Stainless steel
In this study, milled Japonica rice was cooked using the superheated steam rice cooking machine and volatiles carried away with the steam during cooking were analyzed by gas chromatography-olfactometry/mass spectrometry (GC-O/MS) to investigate the odor compounds in rice. We also compared the odor components of steamed rice with those of ordinary cooked rice.
Materials and cooking process Milled common Japonica rice (cv. Koshihikari) harvested in 2013 in Fukui (Japan) was stored at 10°C for over one year and used for the experiment. The rice was cooked in two ways.
Steamed rice The superheated steam rice cooking machine (SRM-20, Acesystem, Osaka, Japan) shown in Fig. 1-A was used to prepare steamed rice (Takemitsu et al., 2013). The size of the machine was W 2500 × H 1650 × D 1100 (mm) and the steaming chamber was W 2100 × H 415 × D 544 (mm). The steaming chamber of the machine was preheated with steam to over 95°C. Two kg of rice were soaked in tap water for an hour, drained, and put into the preheated machine. Rice on the conveyer net belt moved at a constant speed and was heated with steam for 21 min. Saturated steam (100°C) was used for the first half of the cooking process and superheated steam (125°C) was used for the latter half. Hot water (80 – 85°C) was added from above at regular intervals for a total amount of 3 L water per kg raw rice.
Ordinary cooked rice The traditional Japanese rice cooking method was used, in which 250 g of rice and 375 mL of tap water were placed in an automatic electric rice cooker (SR-SU105, Panasonic, Osaka, Japan) and cooked.
Chemicals The analytical standards were: pentanal, hexanal, octanal, 1-hexanol, nonanal, 1-octanol, acetic acid, benzaldehyde, (E)-2-nonenal, γ-caprolactone, hexanoic acid and benzyl alcohol from Sigma-Aldrich, Tokyo, Japan; heptanal, 2-pentylfuran, (E,E)-2,4-nonadienal, (E,E)-2,4-decadienal, γ-octalactone and γ-nonalactone from Tokyo Chemical Industry, Tokyo, Japan; (E)-2-octenal, from Santa Cruz Biotechnology, Dallas, TX, USA; pentanoic acid from Wako Pure Chemical Industries, Osaka, Japan; isovaleric acid and 2-methoxyphenol from Nakalai Tesque, Kyoto, Japan; and longifolene from Extrasynthese, Genay, France. Methyl tert-butyl ether (MTBE) (Nakalai Tesque) was used as an extraction solvent. 2-Acetyl pyrrole (Sigma-Aldrich) was used as an internal standard.
Collecting steam during the rice steaming process and condensing it using a distillation column In order to identify the volatiles escaping during the rice steaming process, we designed a distillation column and attached it to the outlet of the machine (Fig. 1-B). The attached column was 1450 mm high and had an external diameter of 320 mm. A spiral coil for flowing coolant ran the length of the distillation column and this coolant coil was jacketed by the vapor/condensate. Steam escaping from a duct at the end of the cooking machine was piped to the distillation column. Steam traveled through the jacket from top to bottom and was collected as condensation water. Steam was collected throughout the cooking process for GC-O analysis. The steam temperatures were monitored at both the inlet and outlet of the column. The amount of condensation water for the entire period was 9560 mL. The volatile compounds were extracted twice from a portion of condensation water (800 mL), each time using 200 mL of MTBE and a separatory funnel. The upper layer of each extract was collected and dried with anhydrous Na2SO4. The solvent was removed at 52°C using a Vigreux column (30 cm × 1.5 cm, Kiriyama Glass Works, Tokyo, Japan) and further concentrated using a stream of nitrogen gas. The concentrates were used as samples for GC-O and GC-MS analysis. The steam from the last 2 min of cooking was collected in order to quantify the volatile components. This fraction had a volume of 525 mL and the volatiles were extracted twice using 100 mL of MTBE each time, concentrated, and analyzed as described above.
Extraction of the volatile compounds from cooked rice Cooked rice was placed in a plastic container and kept for an hour to reach room temperature, then 20 g was homogenized with 60 mL of distilled water at room temperature and MTBE (60 mL) was added. This mixture was homogenized, centrifuged at 1610 × g for 5 min, and the upper layer was collected. The extraction was repeated two more times. Volatile materials were collected using solvent-assisted flavor evaporation (SAFE) under reduced pressure (6.7 × 10−2 Pa) at 30°C according to Engel et al. (1999).
Extraction of the volatile compounds from raw rice Raw rice (10 g) was ground with a Wonder Crusher WC-3 (Hsiangtai Machinery Industry, Taipei, Taiwan) and suspended in 15 mL of distilled water, extracted with MTBE (60 mL) twice, and the volatile compounds were collected using SAFE.
Water content Raw and cooked rice were dried at 105°C at atmospheric pressure for 48 h. Samples were weighed before and after drying to determine their moisture content.
Run-off during the steaming process Run-off was collected at the bottom of the machine during the last 2 min of cooking. The total amount was 2400 mL; volatiles were extracted from 400 mL using 100 mL of MTBE and were purified by SAFE.
Gas Chromatography-Olfactometry/Mass Spectrometry Extracts of collected steam were analyzed by GC-O on a chromatograph equipped with a mass spectrometer (2010 plus, Shimadzu, Kyoto, Japan) and a sniffing port (OP275, GL Sciences, Tokyo, Japan). Helium was used as the carrier gas at flow rate of 3.1 mL/min. The injector temperature was 230°C. A fused silica capillary column (DB-WAX, 60 m length, 0.25 mm i.d., 0.25 µm film thickness, Agilent J&W, Santa Clara, CA, USA) was used. An injection volume of 2 µL of sample was applied in splitless mode. The column temperature was initially maintained at 40°C for 2 min, then increased to 250°C at a rate of 5°C/min and then kept constant for 20 min. The odorants were determined by sniffing the GC effluent (GC-O). The odor descriptions and their intensities were confirmed by three trained panelists. The odor intensities were evaluated according to the six grade odor intensity measurement method. The mass spectrometer was used with an ionization voltage of 70 eV (EI) and ion source temperature of 240°C.
Isolation of volatiles using a Tenax trap The headspace volatiles from cooked rice were collected according to Yang et al. (2008) with some modifications. Each 1 kg of cooked rice was placed in a 5 L Smart Bag PA (GL Sciences). The bag was then placed in a thermostated chamber (70°C), purified nitrogen gas was passed through the bag (100 mL/min for 30 min), and the volatiles were collected in a 60/80 mesh Tenax TA trap (GL Sciences).
Short pass thermal desorption/GC-MS analysis The volatiles adsorbed to the Tenax were desorbed at 280°C for 3 min with helium gas at a flow rate of 4 mL/min using an automated short-path thermal desorption system (OPTIC-4, ATAS GL International BV, Eindhoven, the Netherlands). Analytes were then cooled to −90°C, kept for 3 min at this temperature, then rapidly heated to 240°C, and introduced into a GC column. A GC/MS equipped with a 30 m length, 0.25 mm i.d., 0.25 µm film thickness, fused silica capillary column (Inert Cap Pure Wax, GL Sciences) was used in split mode (1:1). The column temperature was held at 40°C for 1 min and then increased at 12.5°C/min to 240°C. MS was conducted as described above.
Identification of the volatile components The volatile compounds were identified according to Maraval et al. (2008) using mass spectra libraries (NIST 08 library). Some compounds were confirmed by comparing their mass spectra and retention indices (RI) with those of standards. The RIs were calculated using a series of n-alkanes injected under the same chromatographic conditions as described above.
Quantification of volatile components An appropriate amount of internal standard (2-acetyl pyrrole) was added to each sample before extraction. The individual volatile components were quantified based on an internal standard method (Monsoor and Proctor, 2004). The concentration of the volatile components was calculated according to the response factor determined for each volatile compound. The specific m/z for each compound was used for area measurement and an m/z of 94 was used for the area measurement of the internal standard.
Odor compounds in steam collected during the rice steaming process Milled common Japonica rice was cooked using a newly developed superheated steam rice cooking machine. Extracts from steam collected as condensation water during the entire cooking process (21 min) were analyzed in duplicate by GC-O with three trained panelists and by GC-MS to determine the odorous compounds. A typical chromatogram is shown in Fig. 2. Retention index values (RI), associated compounds, odor descriptions and odor intensity are presented in Table 1. At least 22 odor components were perceived by GC-O, of which 13 were identified by GC-MS: six aldehydes, two phenols, two lactones, one alcohol, one furan and one terpene. Some well-known odor compounds from rice, such as hexanal and 2-pentylfuran, were detected. (E,E)-2,4-Nonadienal, (E,Z)-2,4-decadienal and (E,E)-2,4-decadienal, which contribute to the characteristic aroma of cooked rice, were also found. In addition, we detected other components (longifolene, 2-methoxyphenol, γ-octalactone and γ-nonalactone) which have previously only been reported from black rice bran (Sukhonthara et al., 2009), scented Indica rice (Jezussek et al., 2002; Maraval et al., 2008) and brown rice (Jezussek et al., 2002). Furthermore, we found 15 compounds by GC-MS that were previously reported as odorous components in rice, including brown rice and scented rice (Maraval et al., 2008; Yang et al., 2008; Zeng et al., 2012; Zeng et al., 2009), although they could not be detected by GC-O in this study. These latter odorous compounds include alcohols and short-chain fatty acids such as 1-pentanol, 1-hexanol, pentanoic acid and hexanoic acid. We also detected γ-caprolactone, previously only reported as a component of Chinese rice wine (Chen et al., 2013). Thus, by collecting steam and condensing it with a distillation column during cooking and using our extraction method, we could identify a wide range of odor compounds. The superheated steam is over 100°C, and hence these little-known odor components might be carried away from rice together in the steam. In addition, we note that we detected nine unknown odor components and are in the process of identifying them by two-dimensional GC.
Gas chromatogram of odor compounds in steam collected during steaming process.
Numbers correspond to compounds numbers in Table 1.
I.S. : Internal Standard (2-Acetylpyrrole)
−: undetectable, +: barely detectable, ++: weak but recognizable, +++: easily detectable, ++++: strong, +++++: intense
Quantification of the volatile odor compounds extracted from raw and cooked rice The amounts of 14 odor compounds recovered in the organic extracts of superheated steam-cooked rice (steamed rice) and previously examined as odorous compounds in rice (Jezussek et al., 2002; Maraval et al., 2008; Zeng et al., 2012) were determined by GC-MS and the results were compared with raw rice and ordinary cooked rice (Table 2).
All compounds were identified by mass spectrum and retention time with those of authentic compounds.
Mass spectrum was also agreed with mass libraries (NIST08) and the retention indexes (RI) was agreed with the data in the literatures (Zeng at el., 2009; Chen at el., 2013). Quantification data are mean ± SEM (n=3).
Water content raw rice: 15.0%, ordinary cooked rice: 61.2%, steamed rice: 63.4%
m/z 94 was used for internal standard (2-Acetylpyrrole) area measurement.
Odor thresholds from
The quantities of most of these compounds were reduced after cooking, and especially of hexanal, 1-hexanol, 1-octanol, isovaleric acid, pentanoic acid and hexanoic acid. The ratios of the compounds in steamed rice compared to ordinary cooked rice are also shown in Table 2. A ratio under 0.5 means the component was substantially lower in steamed rice compared to in ordinary cooked rice. Such components include 1-hexanol, isovaleric acid, γ-caprolactone, pentanoic acid, hexanoic acid and benzyl alcohol. On the other hand, almost the same amounts of hexanal, 2-pentylfuran and nonanal were contained in both types of cooked rice; these compounds are known as the key odorants of old rice and their odor thresholds are low (Buttery et al., 1988; Monsoor and Proctor, 2004). Furthermore, (E,E)-2,4-decadienal, which has a typical rice odor and an extremely low odor threshold, was present at a comparable level.
Comparing the volatile compounds in collected steam and run-off During the rice steaming process, hot water is added at regular intervals (Fig. 1-A). The rice absorbs as much water as it needs, and the excess is drained off. At the same time, some water soluble components in rice, such as acids, appear to leach out into the water. To confirm this, we collected the run-off during the last 2 min of cooking and analyzed it by GC-MS. The steam was also collected during the last 2 min and compared to the run-off. As shown in Table 3, a large amount of acetic acid was found in the run-off. Leached acetic acid might influence the taste of steamed rice because acetic acid cannot be drained out of ordinary cooked rice. Furthermore, some acids and aldehydes, such as hexanoic acid and hexanal, were detected in the run-off and in the steam. In contrast, 1-hexanol, 1-octanol, γ-caprolactone and (E,E)-2,4-decadienal appeared only in the steam. Judging from their logP values, these compounds (except for γ-caprolactone) are less soluble in water and would therefore be carried away entirely by steam distillation.
The steam and run-of were collected during the last 2 min of the cooking.
Data presented are a total amount of each compound that was contained in the collected steam (525 mL) or run-off (2400 mL).
Specific m/z was used same as Table 2.
Volatile odor components in the headspace vapor of cooked rice To confirm the release of odorants from cooked rice, we compared the amounts of major volatile components in the headspace vapor of each cooked rice sample (Table 4) and found that all were much smaller in steamed rice than in ordinary cooked rice. Hexanal, heptanal, 2-pentylfuran and 1-hexanol were present at less than 10% of that found in ordinary cooked rice, and pentanal, 1-pentanol and benzyl alcohol were undetectable. (E,E)-2,4-Decadienal, which is a characteristic odor of rice, was not detected in either steamed rice or ordinary cooked rice. Aldehyde compounds such as hexanal and octanal, and 2-pentylfuran have been reported in rice stored for a long time (Kobayashi et al., 2010) and may contribute to the stale odor of cooked rice. These compounds were released in much smaller quantities from steamed rice, so steamed rice should have less odor characteristic of old rice. This confirms the results of sensory evaluation in our previous research that steamed rice has less odor than ordinary cooked rice (Takemitsu et al., 2013).
Values are mean ± SEM (n=3).
All compounds were identified same as Table 2.
The difference in the headspace vapor components between steamed and ordinary cooked rice was much larger than that in concentrates obtained by the extraction method, indicating that the odor components are less easily released from steamed rice. It is well-known that a white, sticky liquid called ‘oneba’ escapes during rice cooking. Oneba contains several water soluble rice solids which are mainly starch, and grains of cooked rice are covered with oneba (Maruyama and Satou, 2002). The ordinary cooked rice we prepared had an amorphous oneba layer on its surface. The odor components in that layer seem to be easily released and may contribute to the smell of rice. Steamed rice preserves its internal cell wall structure and little soluble solid leaches out; that which does is largely washed away by hot water during cooking. Consequently, steamed rice seems to have less oneba on its surface, and little odor is released. In cooked rice, in addition to the quality and quantity of odorants, whether odor components can be easily released from the surface of the rice would have an important influence on sensory evaluation. Our finding suggests that the superheated steam rice cooking machine we designed may be useful for cooking older rice so that the cooked rice does not have a stale odor.
Acknowledgement We express our thanks to Dr. Hironari Miyazato and Dr. Keiichiro Sugimoto (Nagaoka Perfumery Co., Ltd., Osaka, Japan) for their advice. We also thank Dr. Joseph Rodrigue (Osaka Prefecture University) for reviewing the English of this manuscript. This work was supported in part by a Grant-in-Aid for Trial and Development from the Small and Medium Enterprise Agency of Japan (27120394).