Food Chemistry 124 (2011) 501–513
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Volatile profiles of aromatic and non-aromatic rice cultivars using SPME/GC–MS q R.J. Bryant *, A.M. McClung USDA-ARS, Dale Bumpers National Rice Research Center, 2890 Hwy 130 E., Stuttgart, AR 72160, USA
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Article history: Received 2 September 2009 Received in revised form 30 April 2010 Accepted 16 June 2010
Keywords: Aroma Flavour Oryza sativa L. Rice Scented rice SPME GC/MS Volatiles
a b s t r a c t Rice (Oryza sativa L.) is enjoyed by many people as a staple food because of its flavour and texture. Some cultivars, like scented rice, are preferred over others due to their distinctive aroma and flavour. The volatile profile of rice has been explored by other investigators, some of whom have also determined a corresponding aroma using GC/olfactometry. However, little research has been done to determine if different aromatic rice cultivars produce different flavour volatiles that would make them more desirable than others when cooked. In this study, seven aromatic and two non-aromatic cultivars were examined for their volatile profiles both before and after storage using solid phase microextraction (SPME) fibres in conjunction with gas chromatography/mass spectrometer (GC–MS). Ninety-three volatile compounds were identified, 64 of which had not been previously reported in rice. Differences were found in the volatile compounds of aromatic and non-aromatic rice besides 2-acetyl-1-pyrroline (2-AP). Most of the volatile compounds were present in freshly harvested rice and rice following storage, with very few new compounds being identified only after storage. Dellrose, an aromatic cultivar, and Cocodrie, a non-aromatic cultivar, had the most complex volatile profiles (over 64 volatiles). Sixteen compounds were found only in the aromatic cultivars, and some volatiles were found to be unique to specific aromatic cultivars. However, no distinctive pattern was observed that would identify a cultivar as being derived from Basmati, Khao Dawk Mali 105 (i.e. jasmine), or other sources of aroma. This study showed that there is a great diversity of volatiles in both aromatic and non-aromatic rice cultivars and, with further research, this may lead to a better understanding of the combination of compounds that gives a cultivar a unique flavour. Published by Elsevier Ltd.
1. Introduction Rice (Oryza sativa L.) is a staple food for many countries. Unlike other grains, e.g. wheat, corn, or oats, rice is cooked and consumed as a whole grain. Aromatic rice has a natural nutty, popcorn flavour and accounts for 86% of imports into the United States (USDA Economic Research Service, 2010). Jasmine rice from Thailand and basmati rice from Pakistan and India are the main sources of aromatic imports. Because this is an expanding market and imported jasmine and basmati rice cultivars are not adapted for domestic production, breeders have endeavoured to develop aromatic rice cultivars that can be grown in the USA and favourably compete with imports. Jasmine, Basmati and ‘‘Della’’ type of aromatic cultivars are distinguished by their grain shape, cooked rice texture, and flavour (Bergman, Bhattacharya, & Ohtsubo, 2004), and breeders use these traits in the selection of cultivars for different market segments. Studies have demonstrated that consumers can discern
q Mention of a trademark or proprietary product in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the United States Department of Agriculture. * Corresponding author. Tel.: +1 870 672 9300x227; fax: +1 870 673 7581. E-mail address:
[email protected] (R.J. Bryant).
0308-8146/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.foodchem.2010.06.061
sensory attributes that distinguish these types of aromatic rice cultivars in preference tests (Fitzgerald & Hall, 2008; Fitzgerald, McCouch, & Hall, 2009). Currently, 2-acetyl-1-pyrroline (2-AP), which is a primary determinant of flavour in aromatic rice (Buttery, Ling, & Juliano, 1982), is the only flavour component that breeders have the ability to select for. Genetics, growing conditions, and post-harvest handling are factors which have been shown to affect the aroma and flavour of rice (Champagne, 2008). There are several studies that have evaluated a number of rice cultivars from different genetic backgrounds (Bergman et al., 2000; Laguerre, Mestres, Davrieux, Ringuet, & Boulanger, 2007; Yang, Lee, Jeong, Kim, & Kays, 2008a; Zeng, Zhang, Chen, Zhang, & Matsunaga, 2008) and rice which has been stored under different conditions (Suzuki et al., 1999; Tulyathan, Srisupattarawanich, & Suwanagul, 2008; Widjaja, Craske, & Wootton, 1996a; Wongpornchai, Dumri, Jongkaewwattana, & Siri, 2004; Zhou, Robards, Helliwell, & Blanchard, 2002). A major focus has been on 2-AP, the primary volatile compound in aromatic rice, however, lipid oxidation products have also been identified which are known to have a negative impact on acceptability (Champagne, 2008). The latter are compounds that may become more prevalent with the length of storage time or due to poor post-harvest handling. The rice that is imported to the
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R.J. Bryant, A.M. McClung / Food Chemistry 124 (2011) 501–513
United States is subjected to months of storage and variable transport conditions. Moreover, the basmati rice is frequently aged for at least a year before it is marketed (Fitzgerald et al., 2009). Thus, the imported aromatic rice that is in the USA marketplace, may have much different flavour profiles as compared to freshly harvested rice. Champagne (2008) noted that studies were needed using larger sets of rice cultivars that were handled under identical pre- and post-harvest conditions to address flavour attributes. The volatile profile of rice has been explored by several investigators, some of whom have also determined a corresponding aroma using gas chromatography (GC)/mass spectroscopy (MS) olfactometry for most of the compounds detected (Jezussek, Juliano, & Schieberle, 2002; Widjaja, Craske, & Wootton, 1996b; Yang, Shewfelt, Lee, & Keys, 2008b; Zeng et al., 2008). As a result, more than 200 volatile compounds have been identified in rice (Bergman et al., 2000; Champagne, 2008; Widjaja et al., 1996a, 1996b; Yajima, Yani, Nakamura, Sakakibara, & Hayashi, 1979; Zeng et al., 2008), some of which, e.g. 2-AP, 2-acetyl-pyrrole, a-pyrrolidone, and pyridine, have been identified as enhancing the consumer acceptability of rice, while other compounds, e.g. lipid oxidation products, such as hexanal, acetic acid, and pentanoic acid, can have a negative affect on acceptability. The analysis of volatiles using solid phase microextraction (SPME) fibres in conjunction with GC/MS has proven to be an effective method since its development in the 1990s (Arthur & Pawliszyn, 1990). Several different combinations of stationary phases and film thickness on fibres are available to choose from (Supelco, Bellefonte, PA), depending on the molecular weight of the compounds of interest. Smaller molecules are better retained using fibres containing Carboxen (CAR) or divinylbenzene (DVB) as a stationary phase, whereas, higher molecular weight compounds desorb better from fibres which contain polydimethylsiloxane (PDMS) as a stationary phase. Fibres containing a combination of stationary phases have been tested and used by several investigators. Marsili (1999, 2000) tested various fibres but found that the CAR/PDMS combination extracted more different types of volatiles when evaluating off-flavours in milk than other fibres. The DVB/ CAR/PCMS combination fibre has been used to analyse volatiles in rice (Grimm, Bergman, Delgado, & Bryant, 2001; Laguerre et al., 2007; Zeng et al., 2008). This combination has been found to trap a greater range of volatile compounds with different polarities, e.g. ketones, alcohols, aldehydes, esters and terpenic hydrocarbons, than other fibres, which is important when analysing complex components, such as rice volatiles (Ceva-Antunes, Bizzo, Silva, Carvalho, & Antunes, 2006; Mondello et al., 2005). Due to the sensitivity of SPME, contaminants are generally observed. The most predominant peaks detected when fibres containing PDMS are used containing mass m/z 73 and m/z 147, and have been identified as silyl derivatives (Grimm, Champagne, & Oktsuba, 2002; Laguerre et al., 2007; Marsili, 1999). Laguerre et al. (2007) labelled six of them (Si-1 through Si-6), identifying the first three as hexamethyl-cyclotrisiloxane, octamethylcyclotetrasiloxane, and decamethylcyclo-pentasiloxane, respectively. Grimm et al. (2002) listed other contaminants identified in the analysis of rice volatiles and their possible sources. SPME has been reported as a successful tool for screening but not for the quantification of 2-AP in fragrant rice (Grimm et al., 2001). In the Grimm et al. (2001) study, SPME gave
