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The aromas and flavors of coffee are responsible for their aesthetic value and consumer appeal. The flavor/fragrance qualities are greatly dependent on the volatile and semi-volatile organic compounds contained both in the liquid matrix and the headspace aroma (1). Analytical techniques are needed to profile and identify flavors, fragrances, off-flavors, off-odors and potential contaminants that may be present as flavor and fragrance additives, residual solvents from the manufacturing process or as impurities in raw materials. Results of such analyses may be used for developmental research, as well as production quality control.
Two techniques were utilized to compare flavor profiles of two different brands of instant coffee and one brand of ground coffee. The first technique involved collecting VOC's using a newly designed purge and trap system (P&T) (2), followed by trapping on an adsorbent resin and subsequent analysis by thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) using a Scientific Instrument Services (S.I.S.) Model TD-2 Short Path Thermal Desorber accessory. The second technique, Direct Thermal Desorption (DTD) (3) involving the Short Path Thermal Desorption accessory, permits the analysis of coffee samples placed directly in glass-lined stainless steel (GLT) desorption tubes and the volatiles and semi-volatiles thermally extracted into the GC injection port for subsequent analysis.
MATERIALS AND METHODS
To simulate a cup of coffee, approximately 45 mg of coffee was dissolved in 5 ml of distilled H2O in a 25 ml sample tube and incubated at 60 degrees C for 15 minutes in a heater sleeve. Heating and temperature controlling of the heater sleeve were supplied by a compact bench top temperature controller Model CT101 (Figure 1). The coffee samples were sparged with helium at 15 ml/min with an additional 15 ml/min dry purge for 60 minutes using the Purge and Trap System. The volatiles purged from the coffee samples were trapped on a preconditioned 4.0 mm i.d. glass-lined stainless steel desorption tube packed with 100 mg of Tenax® TA 60/80 mesh. The desorption tubes were then fitted with syringe adaptors and connected to the TD-2 Thermal Desorber. Desorption tubes were purged for 3 minutes to remove any excess water and then thermally desorbed for 10 minutes at a temperature of 150 degrees C and gas flow of 1.0 ml/min into the GC injection port and column.
Figure 1 - Purge & Trap System with Bench Top Temperature Controller Model CT 101
For Direct Thermal Desorption, approximately 5.0 mg of the coffee powder to be analyzed 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-2 Thermal Desorber. Samples were purged for 30 sec to remove all traces of oxygen from the sample tubes and then thermally desorbed for 10 minutes at a temperature of 150 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-2 Short Path Thermal Desorber accessory connected to an HP 5890 GC interfaced to an HP 5971 MSD. The theory and operation of the Short Path Thermal Desorption System was previously described (4). The GC injection port was maintained at 260 degrees C. Direct splitless analysis was used. The GC column was a 30 meter x .25 mm i.d. DB-5MS capillary column (J&W Scientific) containing a 0.5 um film thickness with a flow rate of 1.0 ml/min (He). The column was temperature programmed from -40 degrees C (hold for 10 minutes to cryofocus during thermal desorption interval) to 280 degrees C at a rate of 10 degrees per minute.
Figure 2 - Instant Coffee A, 45 mg In 5 ml H2O Collected at 60 degrees C for 1 h at 15 ml/min With 15 ml/min Dry Purge and Thermally Desorbed at 150 Degrees C for 10 min
Figure 3 - Instant Coffee A, 5 mg With Direct Thermal Desorption at 150 Degrees C For 10 min
Figure 4 - Instant Coffee B, 45 mg In 5 ml H2O Collected at 60 Degrees C for 1 h at 15 ml/min With 15 ml/min Dry Purge and Thermally Desorbed at 150 Degrees C for 10 min
Figure 5 - Instant Coffee B, 5 mg With Direct Thermal Desorption at 150 degrees C for 10 min
RESULTS AND DISCUSSION
Two different brands of instant coffee and one brand of ground coffee were analyzed both by P&T TD/GC/MS and DTD to compare their flavor profiles. The coffees were found to contain numerous flavor compounds including pyridine, furfural and other furfural derivatives (Figures 2 - 7) with both techniques. Trace amounts of pyrazine derivatives which exhibit a nutty, roasted aroma were present in the coffees analyzed by P&T TD/GC/MS (Figures 2, 4 & 6). The aromatic compound toluene was detected in one of the instant coffees (Figure 2), as well as in the ground coffee (Figure 6) using the P&T TD/GC/MS technique. Trace amounts of this compound may be present as residual solvent from the manufacturing process or occur naturally. Styrene which may have leached from the plastic wrapper was found in the ground coffee (Figure 6) by P&T TD/GC/MS. However, styrene was not detected in the instant coffees which were packaged in glass jar containers. Purge & Trap TD/GC/MS was very good in extracting and resolving lower boiling compounds but not very effective in extracting the higher boiling compounds. Not only were semi-volatile compounds not extracted, but lower molecular weight compounds could escape with the additional purging to remove excess water. However, extraction efficiency may be improved by optimizing parameters such as: collection, purge and desorption times and desorption temperatures.
The technique of DTD permits the analysis of coffee samples without any solvent extraction or other sample preparation required by Purge and Trap. Direct Thermal Desorption utilizes small sample sizes and injects them directly into the injection port for maximum sensitivity of analysis.
By using the DTD technique, higher boiling compounds were able to be detected in each of the coffees such as: the saturated fatty acids capric, lauric, myristic and palmitic acids and as well as caffeine (Figures 3, 5 & 7). Direct Thermal Desorption was a very good technique in extracting higher boiling compounds with minimum loss of low boilers. Loss of these low molecular weight compounds may be due to an overloaded sample resulting in the coelution of closely related species which are not sufficiently resolved. The various boiling ranges of compounds including the semi-volatiles can be selectively analyzed using DTD by desorbing the coffee samples at the appropriate temperatures.
Figure 6 - Ground Coffee, 45 mg in 5 ml H2O Collected at 60 Degrees C For 1 h at 15 ml/min With 15 ml/min Dry Purge and Thermally Desorbed at 150 Degrees C For 10 min
Figure 7 - Ground Coffee, 5 mg With Direct Thermal Desorption at 150 Degrees C For 10 min
The S.I.S. model TD-2 Short Path Thermal Desorber accessory used in combination with GC-MS is an ideal instrument for comparing flavor profiles in liquid commercial products such as coffee. Combined with the Purge and Trap sampling technique or using Direct Thermal Desorption, "Short Path Thermal Desorption" can be used during the production of liquid commercial products such as coffee, tea and other herbal products (2) as well as for the quality control of flavor/fragrance additives. The technique of choice or utilizing both techniques depends on the analytes of interest. Different species of VOC's with different boiling points including the semi-volatiles may be selectively analyzed by desorbing the samples at the appropriate temperatures. In addition, by optimizing experimental parameters such as purge and desorption times as well as desorption temperatures, the extraction efficiency of VOC's can be improved. It is therefore possible to detect and identify the various flavors, fragrances, off-flavors, off-odors, and manufacturing by-products in a wide diversity of liquid samples.
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