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Note 71: Flavor Profile Determination of Rice Samples Using Shor tPath Thermal Desorption GC Methods

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By Eric D. Butrym and John J. Manura

Presented at ASMS meeting, Orlando, FL - June 1998

INTRODUCTION

Rice is an important commodity especially in the emerging markets of the far east. It is the main staple food of more than half of the world's population. There are many different varieties of rice available, which contribute distinct characteristics to regional cooking. Flavor characteristics of different rice varieties can be attributed to the volatile compounds present in the grains as well as those generated when the rice is heated during cooking. In this work, four varieties of rice that are commonly available in the eastern U.S. were examined. Long grain White and Brown , Basmati, and Arborio varieties were subjected to a variety of purge and trap procedures as well as direct thermal extraction. Long grain white rice tends to be the most abundant variety found on supermarket shelves, although more flavorful types of eastern origin, as Basmati, are now more widely available. Arborio rice is the traditional variety used in Italian risotto and is favored for its starchy texture rather than a strong flavor of its own. As chromatograms from the different varieties and analysis methods are compared; marked differences can be seen in the array of flavor volatiles present. These methods may be expanded and used for quality analysis of individual lots of rice or other grain commodities. The qualitative information obtained can be used to identify products that have been damaged or infested, and quantification of individual flavor components may yield distinct profiles to be used in overall quality assessment.

Materials and Methods

Rice samples were purchased at retail food stores in the central New Jersey area. All samples were ground for one minute in a Micro-Mill (Bel-Art Products) just prior to analysis.

Purge and Trap

Figure 1 - Purge and Trap System

2.5 g ground rice was added to 50 ml of HPLC grade water ( EM Science Inc.) in a 500 ml round-bottomed flask. The flask was immersed in a water bath at 100 C to the level of the contents. A purge head with a 250 mm sparging needle (S.I.S. Inc. Ringoes, NJ) was fitted to the flask, and nitrogen purge flows were adjusted to 60 ml/min (sparging) and 40 ml/min (dry). A glass-lined stainless steel sample tube (GLT) filled with 100 mg Tenax® TA adsorbent resin trapped the volatile components as they exited the flask. The samples were taken for 30 minutes starting immediately after combining the rice and water in the flask. After sampling was complete, the samples were dried with 200 ml dry nitrogen before GC/MS analysis. Figure 1 shows a schematic representation of the Purge & Trap apparatus.

Solid Sampling

Figure 2 - Solid Sampling System

2.5 g ground rice was deposited between conditioned glass wool plugs in a 14" long x 1/2" glass tube. The tube was inserted into the S.I.S. solids sampling oven (S.I.S. Ringoes, NJ) which was held at 100 C (Fig. 2). Nitrogen was purged through the sample at a rate of 40 ml/min for 30 min. Volatiles were concentrated on a Tenax TA - filled GLT. Samples were dried with 200 ml dry nitrogen before GC/MS analysis.

Direct Thermal Extraction

For Direct Thermal Extraction, 5 mg of the ground rice was placed inside a preconditioned 4.0 mm i.d. glass-lined stainless steel desorption tube between two silanized glass wool plugs. The desorption tubes containing the samples were fitted with syringe adaptors and attached to the TD-3 Thermal Desorber. Samples were purged for 2 minutes to remove all traces of oxygen and some of water vapor from the sample tubes and then thermally desorbed for 2.5 minutes at a temperature of 200 degrees C at a gas flow of 1.0 ml/min into the GC.

All experiments were conducted using a S.I.S. model TD-3 Short Path Thermal Desorber accessory connected to an HP 5890 GC interfaced to an HP 5989 MS Engine. The GC injection port was maintained at 250 degrees C. Direct splitless analysis was used. The GC column was a 60 meter x .25 mm i.d. BPX-35 capillary column (SGE Co.) containing a 0.5 um film thickness with a flow rate of 1.0 ml/min (He). The column was temperature programmed from 30 degrees C to 260 degrees C at a rate of 8 degrees per minute. A Micro-Cryotrap (S.I.S. Ringoes, NJ) was employed to cryofocus analytes prior to starting the GC run. The cryotrap was held at -70 degrees C for the duration of the desorption interval, then heated ballistically to 260 C as the chromatographic run was started.

Results and Discussion

Figure 3

Figure # 3

The four different types of rice were analyzed both by P&T TD/GC/MS and Solid Sampling to compare their flavor profiles. Direct thermal extraction was not as effective as the other two methods for analyzing the volatiles in rice (Fig. 3) One possible reason for this is that residual water in the rice may form an ice plug in the GC column during the cryofocussing stage, lowering sensitivity dramatically. Although steps may be taken to avoid this problem, as using smaller samples and pre-purging with dry gas, factors, as thermal decomposition of flavors during desorption and variation in water activity between samples, make Direct Thermal Extraction less suitable for analysis of rice.

Figure 4

Figure # 4

Figure 5

Figure # 5

The rice samples were found to contain numerous flavor compounds including aldehydes, esters, and other fatty acid derivatives (Figs.4-7) with both Purge & Trap and Solid Sampling techniques. Furans and their derivatives which exhibit a 'brown', sweetish aroma were also present in the rice samples. The aromatic compound benzaldehyde was detected in both of the white rice samples. This compound also exhibits a sweet fragrance and is typical of almond flavors. The semi-volatile sesquiterpene Caryophyllene was found in white rice using the Solid Sampling technique. Purge & Trap TD/GC/MS was very good at extracting and resolving lower boiling compounds but not as effective in extracting the higher boiling analytes . Not only were semi-volatile compounds not extracted as efficiently, but labile ones, as esters, were not present or were underrepresented, probably due to the combination of heat and excess water used in the purge and trap procedure. This procedure, however, adheres more closely to the cooking conditions usually used for rice. Solid sampling was much more efficient at extracting higher boiling flavor compounds and yielded chromatograms which were more representative of the aroma of uncooked rice. Together with ease of sample preparation, these characteristics make solid sampling a good candidate for quality monitoring of raw commodities.

Figure 6

Figure # 6

Figure 7

Figure # 7

Conclusion

The S.I.S. Short Path Thermal Desorbtion system, used in combination with GC-MS is an ideal instrument for comparing flavor profiles in commodity products, as rice. Combined with the Purge and Trap sampling technique or using the Solid Sampling oven, "Short Path Thermal Desorption" can be used to monitor quality during storage or at the point of sale of bulk commodities, as well as for the characterization of flavors and fragrances associated with different cultivars or agricultural practices. The choice of technique depends on the analytes of interest. Different species of volatile compounds with different boiling points (including semi-volatiles) may be selectively analyzed by desorbing the samples at the appropriate temperatures. In addition, by optimizing experimental parameters such as purging and desorption times and temperatures, the extraction efficiency of VOC's can be improved. It is therefore possible to detect and identify various flavors, fragrances, off-flavors, off-odors, and manufacturing by-products in a wide selection of samples.

References

1. Hartman, T.G., S.V. Overton, J.J. Manura, C.W. Baker and J.N. Manos. 1991. Short Path Thermal Desorption: Food Science Applications. Food Technology. Vol. 45 (7): 104-105.

2. Overton, S.V. and J.J. Manura. 1992. Detection of Volatile Organic Compounds in Liquids Utilizing the Short Path Thermal Desorption System. The Mass Spec Source. Vol. XV (1): 26-31.

Index to Analytes in Rice Chromatographic Charts

1
Pentanal
2
Pentanol
3
Octane
4
Heptane
5
Hexanal
6
Hexanol
7
2-pentyl-Furan
8
2-Heptenal
9
Octanal
10
2-Octenal
11
Nonanal
12
2-Nonenal
13
Decanal
14
2-Decenal
15
Tetradecane
16
2,4-Nonadienal
17
2,4-Decadienal
18
Isobutyl butanoate
19
Butyl butanoate
20
Hexadecane
21
Unk. Butanoic acid ester
22
Benzaldehyde
23
Caryophyllene
24
Unknown Nitrogen heterocycle
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