Flavor Volatiles in Three Rice Cultivars With Low Levels of Digestible Protein During Cooking
By Zeng, Zhi Zhang, Han; Zhang, Tao; Chen, Jie Yu
ABSTRACT A modified headspace solid-phase microextraction (SPME) method in conjunction with gas chromatography-mass spectrometry (GC- MS) has been used for the analysis of the flavor volatiles in three rice cultivars with low levels of digestible protein during cooking. Altogether, 77 volatile compounds were identified, of which 13 components were not previously reported in rice. A total of 61, 71, and 74 peaks, respectively, were assigned to Shunyou, LGC-katsu, and LGC-soft. Compounds that have been highlighted previously as flavor molecular markers in rice, including indole, vanillin, (E,E)-2,4- decadienal, (E)-2-nonenal, 2-pentylfuran, and 2-methoxy-4- vinylphenol, etc., were on the list of those identified components. Furthermore, similarities and differences of the flavor volatiles among the three rice cultivars were observed. Shunyou was characterized by a relatively higher amount of indole and LGC-katsu had a very high amount of 4-vinylphenol while both rice cultivars displayed an absence of vanillin, pentyl hexanoate, and hexyl hexanoate. In contrast, LGC-soft contained vanillin and had an abundance of fatty acid esters such as pentyl hexanoate and hexyl hexanoate, together with a higher amount of gamma-nonalactone.
Cereal Chem. 85(5):689-695
Flavor volatiles or aroma, and texture are the principal sensory qualities of rice (Oryza sativa L.). Some of the volatiles contribute to consumer acceptance of certain types of rice, and other components contribute to consumer rejection. The issues currently of greatest importance to rice agriculture are the flavor volatiles of rice and off flavors due to staleness. Neither spices nor sauces are added to the rice, and cooked rice is generally eaten without any seasoning when the traditional Japanese cooking method is used. When rice is cooked by this traditional Japanese cooking method, the properties of the rice itself are the most important, and the flavor plays one of the key roles in consumer acceptance. Furthermore, the flavor volatiles are derived from an array of nutrients including amino acids, fatty acids, and carotenoids, etc., according to a recent review (Goff and Klee 2006) and these flavor volatiles provide important information about the nutritional make up of the food items.
The number of elderly people is increasing in Japan, and some of them are in poor health with kidney disease. They require foods with low levels of digestible protein, which are specially processed. Rice grains contain two major proteins: an easily digested glutelin and a hard to digest prolamine (Furukawa et al 2003). Because of the difficulty in digesting prolamine, it may not be considered as a source of digestible protein for the elderly people in poor health with kidney disease. Therefore, the lowglutelin content cultivars of rice could be considered as one of the foods with low levels of digestible protein. Thus, efforts have been ongoing by several rice- breeding programs in Japan which recently developed low-glutelin content cultivars of rice, such as Shunyou, LGC-katsu, and LGC- soft, which are part of a movement to grow new crop cultivars with functional ingredients to improve elderly people’s health (Uehara et al 2002; Iida et al 2004; Nishimura et al 2005). The glutelin content in Shunyou, LGC-katsu, and LGC-soft is lower compared with that in ordinary rice cultivars such as Koshihikari and Akitakomachi. In lowglutelin rice cultivars, the LGC 7 gene confers the low glutelin content phenotype by RNA silencing (Kusaba et al 2003). The new rice cultivars could be used as the natural food with low levels of digestible protein in the diet of the patients with chronic renal failure.
In breeding new rice cultivars, however, there is the danger of changing other subtle desirable features such as the flavor characteristics. Some rice breeding programs incorporate flavor volatile analysis and sensory evaluation of breeding lines into their evaluation protocols. For example, selected line B5-3, cultivated in Italy from American cv. A301, was analyzed for its total aroma composition, which was collected by steam distillation- solvent extraction (Tava and Bocchi 1999). However, the conventional sample preparing methods, like steam distillation-solvent extraction, have various drawbacks such as complicated and timeconsuming procedures, the requirement for a large amount of samples and organic solvents, and loss of volatile compounds during solvent removal.
Simple and rapid methods that focus on rice flavor volatiles could be useful for the evaluation of the flavor of new rice cultivars. To clarify the flavor volatiles of the new rice cultivars that contribute to the flavor of rice, an important problem is how to collect the volatiles. The effective collection of these volatile compounds can now be accomplished using solid-phase microextraction (SPME) without qualitative changes during the collection procedure, which has been considered as a sensitive, reproducible, cost- efficient, and solvent-free technique that incorporates extraction, concentration, and sample introduction in a single step (Pawliszyn 1997). This technique was introduced in a study on the flavor indicators of rice by Grimm et al (2001). They reported the screening of 2-acetyl-l-pyrroline and indole in the headspace of rice using SPME (Grimm et al 2004). Wongpornchai et al (2004) studied the effects of drying methods and storage time on the aroma and milling quality of rice and examined the concentration of 2- acetyl-l-pyrroline, hexanal, and 2-pentylfuran in rice using the SPME method. Champagne et al (2004, 2005) investigated the volatile microbial metabolites in rice based on the SPME technique. Laguerre et al (2007) reported the rapid evaluation of rice aroma by analysis of the volatile fraction using SPME directly coupled with mass spectrometry (MS) recently.
Even some rice-breeding programs now incorporate flavor volatile analysis and sensory evaluation of breeding lines into their evaluation protocols. However, no data have been published concerning the use of SPME for the analysis of the total flavor volatiles of different rice cultivars. The objectives of this research were to identify the volatile components in three rice cultivars with low-glutelin content (Shunyou, LGC-katsu and LGC- soft) during cooking to compare these rice cultivars with the profiles of those volatile components based on the SPME technique, and to gain an understanding of the flavor similarities and differences among the three rice cultivars. The variations in the composition and amounts of the volatile compounds in the three rice cultivars during the course of the cooking process were analyzed using a modified headspace SPME method in conjunction with gas chromatography-mass spectrometry (GC-MS).
MATERIALS AND METHODS
Samples of three rice cultivars with low-glutelin content (Shunyou, LGC-katsu, and LGC-soft) harvested in Ogata Village, Akita Prefecture, Japan, were obtained. They were dehulled at the growing area and were transported to the laboratory in the form of brown rice, which was then stored at 4[degrees]C until experiments. These samples were milled by an RSD-AlOO machine (TIGER, Japan) to a 90% milling yield (brown rice basis) in the form of white rice, which was used for the experiments.
Authentic compounds hexanal, 2-pentylfuran, (E)-2-heptenal, 1- hexanol, nonanal, l-octen-3-ol, pentyl hexanoate, (E)-2nonenal, (E,E)-2,4-decadienal (with (E,Z)-2,4-decadienal as an isomer in it), gamma-nonalactone, 2-methoxy-4-vinylphenol, and vanillin were purchased from Wako Pure Chemical Industries, Japan; indole was purchased from Kanto Chemical, Japan.
The traditional Japanese rice cooking method was used, where a mixture of 150 g of white rice and 250 mL of distilled water were placed in an automatic electric rice cooker (SR-A18H, National, Japan). The mixture was heated for 48 min and kept warm for another 30 min to cook completely according to the traditional Japanese method. The whole process was divided into four stages: I) 25 min from the start of heating to the start of steam coming out of the rice cooker; II) 13 min from the start of steam coming out of the rice cooker to the end of steam coming out of the rice cooker; III) 10 min from the end of steam coming out of the rice cooker to automatic stop of heating; and IV) keeping the rice warm for another 30 min from automatic stop of heating. A thermo-recorder (TR-72S, T & D, Japan) was applied for the measurement of the variations in temperatures at the sampling dots during the rice cooking process as reported previously.
Modified Headspace Solid-Phase Microextraction Sampling
The extraction and concentration of the flavor volatiles of rice were performed using an SPME fiber (Supelco) 1 cm long, coated with triple-phase 30/50 [mu]m divinylbenzene/carboxen/ polydimethyl siloxane (DVB/CAR/PDMS) preconditioned in an SPME fiber conditioner (GL Sciences) at 250[degrees]C for 1 hr before the first measurement. This kind of fiber has proven to be the most efficient in trapping volatile compounds with different polarities and the most useful in covering the wide range of physicochemical properties of flavor volatiles (Mondello et al 2005; Ceva-Antunes et al 2006). As described previously (Zeng et al 2007), a sampling apparatus used to collect volatile compounds of rice during cooking using SPME was designed. The volatiles of rice during cooking were coming out of the cooker and then releasing out through the second side arm with a flexible septum, while the SPME fiber has been placed in the effluent of flavor volatiles at the sampling port through the first arm of the sampling apparatus. At cooking stages I, II, and III, the fiber mounted in the manual SPME holder was in its protective sheath when it was inserted through the Teflon-coated silicone septum into the sampling port of the Pyrex glass sampling apparatus tightly connected to the open part of the automatic electric rice cooker. Inside the sampling port, the fiber was lowered to expose it to the effluent of the flavor volatiles of the rice during cooking. It was left there for adsorption from the beginning and then withdrawn at the end of cooking stages I, II, and III. But at cooking stage IV, the fiber was inserted directly into the automatic electric rice cooker through the Teflon-coated silicone septum that tightly covered the open part of the cooker after the Pyrex glass sampling apparatus had been removed. Inside the cooker, it was exposed to the headspace of the sample and left there for adsorption at this stage. The SPME fiber thermally desorbed the flavor volatiles in the injection port of the GCMS instrument for 5 min at 250[degrees]C. Then it was left in the SPME fiber conditioner at 250[degrees]C for 1 hr for reconditioning before it was exposed to the flavor volatiles of the next sample. The same kind of fiber was used in all applications. Gas Chromatography-Mass Spectrometry
Gas chromatography-mass Spectrometry (GC-MS) analyses were conducted on a TurboMass GC-MS system with AutoSystem XL and TurboMass Upgrade MS software (Perkin Elmer). Desorption time was 5 min in the injection port at 250[degrees]C. A column, DB Wax, 30 m x 0.25 mm, i.d. x 0.25 [mu]m (stationary phase thickness) (J&W Scientific) was applied. It was temperature programmed at 40[degrees]C for 2 min, then increased to 230[degrees]C at a rate of 4[degrees]C/min, and maintained at 230[degrees]C for 4.5 min. The carrier gas was helium, which was delivered at a linear velocity of 2 mL/min. The mass selective detector was operated in an electron impact ionization mode at 70 eV, in a scan range of m/z 40-400. The interface temperature was 23[degrees]C. Retention time of each volatile was converted to the Kovats retention index (RI) using n- alkanes (Supelco) as the references. The volatile compounds were positively identified by matching their mass spectra and RI values with those of authentic compounds or tentatively identified by matching their mass spectra with the spectra of reference compounds in both the Wiley mass spectral library (6th Ed) and the NIST/EPA/ NIH mass spectral library (v. 1.5a), and verified on the basis of mass spectra and RI values reported in the literature (Buttery and Ling 1998; Buttery et al 1999; Mahatheeranont et al 2001; Fan and Qian 2006; Mottram 2007). Fourteen compounds were selected for flavor volatile analysis. Because the recovery factors vary greatly among the flavor volatiles identified, no quantification was performed. The chromatograms obtained from the total ion current were integrated without correction for coelution. The results from the volatile analyses were provided in peak area counts. All experiments were performed in triplicate.
RESULTS AND DISCUSSION
Modified Headspace Solid-Phase Microextraction
Direct extraction of rice flavor volatiles during cooking using a modified headspace SPME was developed recently by our group (Zeng et al 2007, 2008). The flavor volatiles of rice during cooking can be extracted by inserting the SPME fiber in the effluent of the headspace of rice during cooking or into the headspace region above the rice samples during cooking. After extraction and concentration of the volatiles on the fiber, the syringe assembly of SPME was inserted into the injection port of the GC-MS instrument (Vas and Vekey 2004), where the volatiles were thermally desorbed from the fiber and trapped on the head of the capillary column.
The results previously reported in the literature (Mondello et al 2005) showed that lower molecular weight compounds reached what can be considered equilibrium after extraction for 10-20 min and decreased slightly afterwards. However, other high molecular weight volatiles required 40-50 min to reach equilibrium and remained mainly stable thereafter. Because full equilibration is not necessary for precise analysis by SPME (Ai 1997), adsorption times were selected from 10-30 min in most cases reported (Kataoka et al 2000), whereas less volatile compounds may take 1 hr or even more. The adsorption times at the four different cooking stages in this study were applied based on the automatic electric rice cooker as described above. The variations in temperature at the sampling dots in the course of the rice cooking process have been reported previously (Zeng et al 2007).
The direct extraction used in this study was also a headspace extraction method, but in this case, the headspace was inside the apparatus instead of a vial, which is usually used. During cooking stages I, II, and III, the cooker was heating and the volatile compounds were continually coming out of the cooker. Therefore, direct extraction during these stages was a nonequilibrium method. However, direct extraction during cooking stage IV (keeping the rice warm) may be considered as an equilibrium method.
Flavor Volatile Compounds During Rice Cooking
The three rice cultivars with low levels of digestible protein (Shunyou, LGC-katsu, and LGC-soft) were officially registered by the Japanese Ministry of Agriculture, Forestry and Fisheries, as Paddy Rice Norin 374, 396, and 381, respectively (Uehara et al 2002; Iida et al 2004; Nishimura et al 2005). Shunyou is a lowglutelin rice cultivar; LGC-katsu is a low-glutelin rice cultivar with an absence of globulin at the molecular weight (MW) of 26,000; while LGC-soft is a low-glutelin and low-amylose rice cultivar.
Flavor or aroma is an especially essential factor in cooked rice quality. Flavor volatiles generally play a role in consumer acceptability of rice (Zhou et al 2002). Several hundred volatile compounds can be observed in cooked rice, and more than 100 components have been identified. Most rice flavor components are produced in low amounts at the onset of lipid oxidation and some of them are produced by thermal decomposition of the nonvolatile constituents existing in rice. Several studies have addressed the binding of lipid oxidation products to proteins (O’Keefe et al 1991). These bound components need to be liberated before they can be extracted and then determined (Grimm et al 2001; Laguerre et al 2007). Except for the free flavor volatiles, both the bound flavor components and the compounds formed by the thermal decomposition of the nonvolatile constituents existing in rice could be liberated during rice cooking and then they could be extracted, detected, and identified by the modified headspace SPME method in conjunction with GC-MS. The rice cooking process with the steam coming out of the rice cooker was preferred to other methods for the effective liberation of both the bound flavor components and the compounds formed by the thermal decomposition of the nonvolatile constituents existing in rice.
Concerning the general pattern of the volatile components of the three rice cultivars, there were dramatic differences in the volatiles of rice during the four different cooking stages. The major compounds identified at cooking stage I were aldehydes such as nonanal and hexanal. In contrast, fatty acids were the dominating components identified at cooking stage II. The major constituents identified at cooking stages III and IV were aldehydes and fatty acids, etc. However, this does not necessarily mean that they play an important role in the characteristic rice flavor, which is dependent on the odor characteristics and the thresholds of the volatiles.
Cooked rice has a desirable characteristic flavor while uncooked rice has little or no aroma. Previous studies (Zeng et al 2007, 2008) have revealed that the volatile components at cooking stage I were representative of the flavor volatiles of uncooked rice while the volatile constituents at cooking stage IV were representative of the flavor volatiles of cooked rice. The major components identified at cooking stage I, such as nonanal and hexanal, are very common components of other foods (Buttery et al 1988).
As shown in Table I, altogether 77 volatile compounds were identified. A total of 61, 71, and 74 peaks, respectively, were assigned to Shunyou, LGC-katsu, and LGC-soft. Of them, hexanal, 2- pentylfuran, (E)-2-heptenal, 1-hexanol, nonanal, l-octen-3-ol, pentyl hexanoate, (E)-2-nonenal, (E,Z)-2,4-decadienal, (E,E)-2,4- decadienal, gamma-nonalactone, 2-methoxy-4-vinylphenol, indole, and vanillin were positively identified by comparing their mass spectra and RI values with those of authentic compounds, while others were tentatively identified by their corresponding mass spectra (Wiley & NIST/EPA/NIH mass libraries) and RI values, when their RI values on the DB Wax capillary column are available in the literature (Buttery and Ling 1998; Buttery et al 1999; Fan and Qian 2006; Mottram 2007). In Shunyou, 26, 36, 60, and 65 vola tile compounds, respectively, were identified at cooking stages I, II, III, and IV. In LGC-katsu, 26, 37, 63, and 68 volatile components, respectively, were identified at cooking stages I, II, III, and IV. In LGC-soft, 30, 39, 69, and 71 volatile constituents, respectively, were identified at cooking stages I, II, HI, and IV. Overall, the volatile compounds in the three rice cultivars used during cooking in this study belonged to the chemical classes of aldehydes, ketones, alcohols, and heterocyclic compounds, as well as fatty acids and esters, phenolic compounds, and hydrocarbons, etc. Interestingly, compounds such as (E)-4-nonenal, (E)-2-octenl-ol, and ethanol were detected only at cooking stage I. Most of the compounds in Table I were previously reported in rice. However, there were 13 components [pentyl hexanoate, hexyl hexanoate, 2-methyl-3-octanone, 2- hexylfuran, 2-heptylfuran, (E)4-nonenal, (E)-3-nonen-2-one, (Z)-2- octen-l-ol, 6-dodecanone, 6,10-dimethyl-2-undecanone, dodecanal, hexadecanal, and 5-iso-propyl-5H-furan-2-one], which were not previously reported in rice, based on research reports (Buttery et al 1988; Widjaja et al 1996; Tava and Bocchi 1999; Mahatheeranont et al 2001; Jezussek et al 2002) and reviews (Maga 1984; Tsugita 1986).
Some compounds such as 4-vinylphenol and vanillin, which had higher boiling points and could not be detected by using the conventional headspace sampling method and could only be detected in the concentrate by using the steam distillation-solvent extraction method (Tsugita et al 1983), have also been successfully extracted, detected, and identified by using this modified headspace SPME technique.
Of the identified components in the three rice cultivars during cooking, indole is a nitrogen-containing component with a floralfruity, and herbaceous scent at low concentration (Kaiser 2006). It was selected as one of the flavor indicators of rice by Grimm et al (2004) in a study on screening for sensory quality in foods using solid-phase microextraction tandem mass spectrometry. gamma-Nonalactone has a sweet, coconut-like aroma. Although its contribution to the flavor of rice has not been established yet, some lactones with low-odor thresholds (Buttery et al 1999; Slaughter 1999) and a pleasant flavor associated with fruity aromas (Kaiser 2006) pointed out an important significance.
Of the volatiles identified, (E)-2-nonenal, (E,E)-2,4- decadienal, 2-methoxy-4-vinylphenol, and 4-vinylphenol have been reported among the most important flavor volatiles of rice (Buttery et al 1988); Widjaja et al (1996) listed alk-2-enals, alka-2,4- dienals, and 2-pentylfuran among the most important compounds that contribute to the aroma profile of rice; Jezussek et al (2002) confirmed (E,E)-2,4-decadienal, 2-methoxy-4-vinylphenol, and vanillin as the most important odorants in four brown rice cultivars.
Similarities and Differences Among Three Rice Cultivars
To meet consumer needs, knowledge of the similarities and differences in the flavor of different rice cultivars is required. The similarities and differences among the three rice cultivars have been achieved through a comparison of the volatile components. The samples of the different rice cultivars were tested under the same operating conditions. The chromatograms obtained from the total ion current were integrated without correction for coelution. We made a comparison of the amount of the components among different samples using the chromatographic peak area counts directly and no further calibration method was used, as previously reported (Champagne et al 2004, 2005; Alissandrakis et al 2007; Soares et al 2007). Using chromatographic peak area counts for quantity comparison does not require extensive sample preparation, but the sampling procedure and chromatographic conditions must remain constant for all samples and there should be no sample matrix effect.
Flavor Volatile Compounds Described and Identified in Three Rice Cultivars (Shunyou, LGC-Katsu, and LGC-Soft) During Four Different Cooking Stages
Flavor Volatile Compounds Described and Identified in Three Rice Cultivars (Shunyou, LGC-Katsu, and LGC-Soft) During Four Different Cooking Stages
Therefore, those data are only intended to give some idea of the order of magnitude of the amounts. To obtain a more reliable result for an individual component, a calibration method should be used for correcting the sample matrix effect.
The typical total ion current chromatograms that were obtained for the flavor volatiles of the three rice cultivars at cooking stage IV on the DB Wax capillary column are shown in Fig. 1. Hexadecanoic acid is excluded from the figure, however, due to its extremely high peak. Authentic compounds are also shown in Fig. 1 as references. Figure 2 shows the variations in amount (peak area counts x 10^sup 6^) for the flavor molecular markers as representatives of their chemical classes based on their importance as potential impact odorants in rice for the three rice cultivars at cooking stage IV.
Some brief observations can be made on the tested samples. As shown in Figs. 1 and 2, Shunyou was characterized by a relatively high amount of indole and LGC-katsu had a very high amount of 4- vinylphenol, while both showed an absence of vanillin and hexyl hexanoate, etc. In contrast, LGC-soft contains vanillin and had an abundance of the fatty acid esters, such as pentyl hexanoate and hexyl hexanoate, together with a high amount of gammanonalactone.
Fig. 1. Total ion current chromatograms for flavor volatiles of three rice cultivars at cooking stage IV. Tetradecanoic acid is included while hexadecanoic acid is excluded due to extremely high peak. Peak numbers refer to authentic compounds as references: hexanal (1), 2-pentylfuran (2), (E)-2-heptenal (3), 1-hexanol (4), nonanal (5), 1-octen-3-ol (6), pentyl hexanoate (7), (8)-2-nonenal (8), (E,Z)-2,4-decadienal (9), (E,E)-2,4-decadienal (10), gamma-non- alactone (11), 2-methoxy-4-vinylphenol (12), indole (13) and vanillin (14).
Fig. 2. Differences in chromatographic peak area counts ( x 10^sup 6^) of flavor molecular markers selected for three rice cultivars at cooking stage IV. Results are mean +- standard deviation (n = 3). Odor thresholds (ppb) in water solution are reported in the literature (Buttery et al 1988, 1999; Goff and Klee 2006) for some flavor molecular markers: 2-pentylfuran (6); (E)-2- heptenal (13); 1-octen-3-ol (1); benzaldehyde (350); (1)-2-nonenal (0.08); (E,E)-2,4-nonadienal (0.01); (E,E)-2,4-decadienal (0.07); geranyl acetone (60); gamma-nonalactone (30); 2-methoxy-4- vinylphenol (3); indole (140); and vanillin (58). Data are unavailable for pentyl hexanoate and hexyl hexanoate.
The results presented from the flavor volatiles during cooking in three rice cultivars with low levels of digestible protein were based on the application of a modified headspace SPME in conjunction with GC-MS, in which the headspace was inside a designed apparatus instead of a vial. As extraction and concentration are combined, all of the flavor volatiles of rice extracted by the SPME fiber are directly introduced into the analytical system. This is particularly important for the volatile compounds found at trace levels. Moreover, all of the free flavor volatiles, the bound flavor components, and the compounds formed by the thermal decomposition of the nonvolatile constituents in rice could be liberated during rice cooking and then they could be extracted, detected, and identified. Therefore, a broad range of the flavor volatiles of rice during cooking could be identified in one single headspace SPME/GC-MS run. For these reasons, the modified headspace SPME is preferred to other sampling techniques for the study of the flavor volatiles of rice during cooking. In addition, the similarities and differences in the composition and amounts of the flavor volatiles among the three rice cultivars were also observed in this study. This simple and rapid method is potentially useful in assisting rice breeders to make selections from a multitude of lines and evaluate rice-breeding programs in which only a limited amount of an individual rice sample is available together with a large number of different rice samples to be analyzed.
This work was supported by research funding (No. 17-05778) from Japan Society for the Promotion of Science (JSPS).
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[Received January 3, 2008. Accepted April 22, 2008.]
Zhi Zeng,1 Man Zhang,2,3 Tao Zhang,4 and Jie Yu Chen2
1 School of Chemistry and Environment, South China Normal University, Guangzhou 510631, P.R. China. E-mail: firstname.lastname@example.org
2 Faculty of Bioresource Sciences, Akita Prefectural University, Kaidobata-Nishi 241-438, Shimoshinjo-Nakano, Akita-shi, Akita 010- 0195, Japan.
3 Corresponding author. Phone: +81-18-872-1500. Fax: +81-18-872- 1676. E-mail: email@example.com
4 Institute of Opto-electronic Materials and Technology, South China Normal University, Guanezhou 510631, P.R. China.
Copyright American Association of Cereal Chemists Sep/Oct 2008
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