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Note 30: Comparison Of Cooking Oils By Direct Thermal Extraction and Purge and Trap GC/MS

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By Santford V. Overton and John J. Manura

INTRODUCTION

Volatile and semi-volatile organic compounds present both in the matrix and the headspace aroma are primarily responsible for the flavor/fragrance qualities of commercial cooking oils. There is a concern in the food industry over the quality as well as the level of residual solvents that may be present in cooking oils due to plant origin, plant maturity, adulteration or dilution of the final product. In determining an oil's plant origin and assessing its overall flavor quality, it would be extremely advantageous to have a reliable and efficient method for the detection, identification and quantification of the volatile organic compounds responsible for the unique flavors and aromas in these oils. Because oils possess highly characteristic aromas due to specific volatile organic components, it can be anticipated that the chemical analysis of the aroma and flavor components of a given oil could give a fingerprint which could be dependent on the fruit or floral source. In previous studies of flavors and aromas in oils and food products, headspace GC techniques have been the methods of choice. However, headspace techniques are limited in their level of detection and identification of many organic volatiles, especially the semi-volatile organics.

More sensitive 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 at lower concentrations. The purge and trap (P&T) technique permits the analysis of a wider range of both volatile and semi-volatile organic compounds and is more sensitive by a factor of at least 100 as compared to the static headspace technique. In addition, a new technique entitled Direct Thermal Extraction using a thermal desorption apparatus attached to the injection port of a GC/MS system permits the direct thermal extraction of volatile and semi-volatile organics directly from small sample sizes (mg) without the need for solvent extraction or other sample preparation. The samples are ballistically heated and together with the carrier gas flow through the samples the volatiles are outgassed into the injection port and onto the front of the GC column for subsequent analysis via the GC and/or GC/MS. For this study, cooking oils were analyzed by both the Direct Thermal Extraction and P&T techniques to determine the best suitable technique for possible development of a quality control method for the food industry. The volatile organics present in the oils were quantified using matrix spiked deuterated internal standards.

Figure 1

Figure 1 - Purge & Trap Apparatus

Instrumentation

Samples to be analyzed were collected using a Scientific Instrument Services Purge and Trap System. This apparatus (Fig. 1) consists of a sparge gas inlet connected to a stainless steel purging needle that is inserted through an adapter fitting into a 10 ml test tube. A dry purge gas inlet is located at a right angle to the sparge gas inlet at the top of the apparatus. This can be left in the closed or open position. The purpose of the dry purge is to reduce the water vapor condensation on the adsorbent trap. Opposite the dry purge inlet is the connector for the glass-lined stainless steel (GLT) desorption tube containing the adsorbent resin. The Purge and Trap System also contains two ball rotameters with adjustable needle valves mounted on a stationary base and permits the visual indication and independent adjustment of the carrier gas flow to each of the gas inlets.

A new technique called Direct Thermal Extraction which utilizes a thermal desorption apparatus attached to the injection port of a GC/MS permits the direct thermal extraction of volatile and semi-volatile organic compounds directly from small sample sizes (mg) without the need for solvent extraction or other sample preparation. The sample is either placed inside a glass-lined stainless steel desorption tube between two glass wool plugs which simply hold the sample in place or injected directly into a desorption tube containing either a glass wool plug or an adsorbent resin, as Tenax®. The desorption tube containing the sample is attached to the Short Path Thermal Desorption System and a syringe needle attached. The sample is then ballistically heated and together with the carrier gas flow through the sample the volatiles are outgassed into the injection port and onto the front of the GC column for subsequent analysis.

All experiments were conducted using a Scientific Instrument Services model TD2 Short Path Thermal Desorption System accessory connected to the injection port of an HP 5890 Series II GC interfaced to an HP 5971 Mass Selective Detector. The mass spectrometer was operated in the electron impact mode (EI) at 70eV and scanned from 35 to 400 daltons during the GC run for the total ion chromatogram.

A short 0.5 meter by 0.53 mm diameter fused silica precolumn was attached to the injection port end of a 30 meter x 0.25 mm i.d. DB-5MS capillary column containing a 0.25 µm film thickness. The GC injection port was set to 250 degrees C and 5:1 split was used. The head of the column was maintained at -70 degrees C using a GC Cryotrap model 951 (Scientific Instrument Services, Ringoes, NJ) during the desorption and extraction process and then ballistically heated to 200 degrees C after which the GC oven was temperature programmed from 35 degrees C (hold for 5 minutes) to 80 degrees C at 10 degrees C/min, then to 200 degrees C at 4 degrees /min and finally to 260 degrees C at a rate of 10 degrees /min.

Experimental

Six brands of olive oil were analyzed by both Direct Thermal Extraction and P&T techniques to determine the most sensitive technique for possible development of a quality control method for the food industry. For quantification, an internal standard was spiked into the adsorbent traps after the sample had been isolated. No correction for extraction efficiency of recovery is achieved using this technique; however, it serves as a useful means of quantifying the levels of components present on the adsorbent traps.

Purge & Trap

Sample sizes of 1 ml of the various oils were pipetted into a 10 ml test tube and heated to 80 degrees C for 30 minutes. Samples were purged with high purity helium at 20 ml/min with an additional 25 ml/min dry purge using the S.I.S. Purge and Trap System (Fig. 1). Volatile analytes were purged from the liquid matrix and carried to a preconditioned 4.0 mm i.d. glass-lined stainless steel desorption tube packed with 100 mg of Tenax TA.

Direct Thermal Extraction

Approximately 10 ul of each of the samples to be analyzed by Direct Thermal Extraction were injected into a 4.0 mm i.d. glass-lined stainless steel desorption tube containing 25 mg of Tenax TA (Fig. 2). The samples were then purged for 5 minutes at 80 ml/min to remove any moisture from the traps.

Figure 2

Figure 2 - Adding An Internal Standard To Desorption For Quantitation Of Analytes

Once the samples were collected in the desorption tubes, they were spiked with a mixture of 100 ng of d-8 toluene, 200 ng of d-14 cymene and 50 ng of d-8 naphthalene internal standard by injecting 1 ul of the stock solution in methanol by syringe injection into the Tenax matrix. An additional purging of 120 ml of purge gas was required to remove the methanol from the Tenax trap.

The desorption tube with sample and internal standard were then attached to the Short Path Thermal Desorption System and a syringe needle attached. The desorption tube was injected into the GC injection port at desorption block temperatures of 220 degrees C for 5 minutes and a flow rate of 5 ml/min for samples collected via purge and trap. For samples to be analyzed by Direct Thermal Extraction, the desorption block temperatures were set at 200 degrees C.

Results and Discussion

Six different manufacturers of olive oil were analyzed by Direct Thermal Extraction and P&T techniques to identify, compare and quantify the volatile organics present, and from the data, determine the most sensitive method of analysis. Table I shows the VOC's detected by each technique and the relative amounts of each of these compounds in each cooking oil analyzed. Over 200 volatile organics were identified in the various oils studied. Most of the cooking oils studied produced 50 or more volatile organics which were identified in addition to many more that were either too weak to identify or in which a good NBS library match was not achievable (Figs. 3 & 4). The cooking oils were found to contain numerous straight and branched chain hydrocarbons, aldehydes, alcohols, ketones, esters and furans in addition to many benzene derivatives. Significant differences in the volatiles present from the different manufacturers occurred in these olive oils. Although many of the same volatiles are present in all of these oils using either Direct Thermal Extraction or a Purge and Trap technique, there is significant variation in the quantities as well as the variety of volatile organics present. This variation could be due to the origin and maturity of the harvested olive fruits as well as differences in the manufacturing processes.


Table I. Relative Amounts of Volatile Organics in Cooking Oils
                       A    AX    B     BX    C     CX    D     DX    E     EX     F    FX 

1. Pentane             x   2.3k  38.2   15k  49.9   20k  45.0   22k  37.2   26k    x    23k

2. Butanal            3.3    x     x     x    3.9   569  15.5    x     x     x    4.1    x

3. Hexane            18.4   42k    x   4.9k    x   2.1k    x   3.4k  95.3  4.7k    x   3.3k

4. Ethylacetate       165    x   15.9    x    1.3    x   82.6    x    133    x   76.2    x

5. 3-methyl-butanal  11.8    x     x     x     x     x   21.7    x   18.6    x   27.9    x

6. 2-methyl-butanal  11.2    x     x     x     x     x   22.7    x   20.0    x   17.6    x

7. 3-pentanone       62.5    x     x     x     x     x     x     x     x     x    134    x

8. Heptane             x    30k    x    36k    x    22k    x    40k   390   58k    x    33k

9. Pentanal          48.7    x   50.8  68.1    x    295    x     x     x     x     x     x

10. d-8-toluene                 internal stardard       

11. Toluene          28.3    x   19.1    x    9.1    x   22.7    x   60.7    x   28.6    x

12. 1-octene           x     x    3.8  1.9k  14.5   777  24.8  1.8k  63.2  5.0k  24.9  1.3k

13. 2-octene         17.8    x     x     x     x     x     x     x     x     x     x     x

14. Octane             x    57k   228   53k    x    33k    x    59k  3.1k   93k   245   52k

15. Hexanal           516    x     x     x    319    x   1.9k    x     x     x    366    x

16. (E)-2-octene       x     x     x     x     x     x     x     x    147    x     x     x 

17. (Z)-2-octene       x     x     x     x     x     x     x     x   79.3    x     x     x     

18. E-2-hexenal        x   1.7k   120  1.5k   8.8  1.1k   689  2.0k    x   4.3k  1.8k  3.7k

19. Z-3-hexenal       869    x     x     x     x     x     x     x   1.3k    x     x     x

20 (Z)-3-hexen-l-ol  88.2    x     x     x     x     x     x     x     x     x     x     x

21. 4-hexen-l-ol       x     x     x     x     x     x    117    x   26.0    x     x     x

22. (E)-2-hexen-l-ol  436    x    8.9    x     x     x    183    x    249    x    286    x

23. 2-heptanone      48.6   556  17.5  1.6k  17.6   569    x   1.0k  62.0  1.8k  51.3  1.0k

24. Styrene            x     x     x     x     x     x    175    x     x     x     x     x

25. Heptanal         46.1  3.7k  15.9   12k  58.4  4.5k   227  8.6k   128  1.4k  52.1  4.2k

26. (E)-2-heptenal   23.1  8.6k  20.9   13k    x    26k    x     x    200    x   24.6    x

27. (Z)-2-heptenal     x     x     x     x     x   6.5k   124   10k    x   2.3k    x   1.9k

28. Benzaldehyde       x   3.3k    x    779    x    354    x   1.3k    x   2.3k    x   1.9k

29. 2-pentyl-furan     x   1.5k    x    12k  35.4  1.6k   149  2.1k    x   2.9k    x   1.6k

30. Octanal            x   5.4k    x   2.9k  43.5  4.9k   174  9.3k    x    16k    x   6.5k

31. (Z)-3-hexen-l-ol   x     x     x     x     x     x     x     x    227    x     x     x

32. (Z)-3-hexen-l-ol, 247    x     x     x     x     x     x     x     x     x    193    x

    acetate

33. (Z)-4-hexen-1 -ol  x     x   18.0    x     x     x     x     x     x     x     x     x    

34. Acetic acid,     8.07    x     x     x     x     x   14.9    x   64.3    x   78.2    x

    hexyl ester

35. d-14 cymene             internal stardard       

36. Limonene         11.8    x   10.7    x    2.0    x     x     x    286  1.1k   6.9    x

37. (E)-2-octenal      x   4.4k    x   5.6k   9.2  5.1k   138  5.9k  49.8  9.2k    x   5.5k      

38. Nonanal          84.7   14k  18.9   23k  68.6   11k   334   21k   307   28k  63.5 16.3k       

39. trans-2-dodecenal  x    573    x     x     x    413    x     x     x    858    x     x     

40. (E)-2-nonenal      x     x     x     x     x     x     x     x     x     x     x     x       

41. (Z)-2-nonenal      x     x     x     x     x     x     x   1.3k    x     x     x     x    

42. d-8-naphthalene        internal stardard

43. 1 -octanol         x     x     x     x     x     x     x   4.1k    x     x     x      x       

44. cis-undec-4-enal   x   2.2k    x     x     x     x     x     x     x     x     x      x

45. methyl-cyclo-    38.8    x     x     x     x     x   17.8    x     x     x     x      x

    heptane

46. (2)-2-decanal     7.6    x     x     x     x     x     x     x     x     x    2.8     x   

47. 2-cyclohexen-1 -ol       x     x     x   19.9    x   28.8    x   34.8    x     x      x   

48. (E)-2-decenal      x    10k    x   6.9k    x   4.1k    x    20k    x    11k    x     6.5k

49. Decadienal         x   4.1k    x   1.9k    x   1.7k    x   6.2k    x   2.9k    x     2.6k

50. trans,trans-nona-  x   6.1k    x   2.5k    x   3.1k    x   8.9k    x   4.6k    x     3.9k 

    -2,4-dienal

51. 2-undecenal       8.2  7.3k    x   6.9k  16.8  4.5k  11.4   19k    x    10k   1.2    5.9k 

52. Farnesene          x   1.1k    x     x     x     x     x   2.1k    x    1.0k   x      638

The aliphatic C6 compounds hexanal, (E)-2-hexenal and (Z)-3-hexenal, which contribute greatly to the green notes of the aroma, were detected by Purge & Trap GC/MS in each of the cooking oils (Fig. 3). In addition to trace amounts of hexanal and (Z)-3-hexenal, higher concentrations of (E)-2-hexenal as compared to the P&T technique were found in the cooking oils using the Direct Thermal Extraction technique (Table I). These compounds and corresponding hexyl esters have previously been reported in great quantities in the volatile components of olive oils. It has been assumed for a long time that unsaturated fatty acids are the precursors of these volatile compounds.

Figure 3

Figure 3 - Volatile Organics as determined by Purge & Trap

The aliphatic compounds heptanal, octanal and nonanal were also identified by Purge & Trap GC/MS in each of the cooking oils (Fig. 3). However, much higher concentrations of these compounds were detected using the Direct Thermal Extraction technique (Table I). Previous reports indicate that several aldehydes and ketones may be of importance for the development of rancid flavor. The branched aldehydes 2-methyl-butyraldehyde and 3-methyl-butyraldehyde, which contribute to the fruity flavor notes, were found in Brands A, D, E and F using the Purge & Trap technique (Fig. 3). However, these compounds were not detected by Direct Thermal Extraction. Linear alkenes such as 1- and 2-octene from lipid oxidation decomposition products were also detected in the cooking oils using both techniques (Figs. 3 & 4), with appreciably higher concentrations detected by Direct Thermal Extraction (Table I).

Pentane, hexane, heptane and octane were found in significantly higher concentrations in the olive oils when using the Direct Thermal Extraction technique (Table I). When compared to the Purge & Trap technique for the presence of these hydrocarbons, Direct Thermal Extraction was more sensitive by a factor of at least 1000. It has been previously reported that octane concentrations generally increased with time during storage. In addition to the increased sensitivity found when using the Direct Thermal Extraction technique as compared to the Purge & Trap technique, very high concentrations of higher molecular weight compounds such as (E)-2-decenal, decadienal, trans, trans-nona-2,4-dienal and 2-undecenal were identified via Direct Thermal Extraction in the olive oils (Fig. 4). These compounds contribute to the flavor and aroma of the cooking oils and are the result of fatty acid decomposition.

Figure 4

Figure 4 - Volatile Organics as determined by Direct Thermal Extraction

Conclusion

Direct Thermal Extraction has been utilized for the identification and quantification of volatile organics, aromas and flavor components in olive oils. This analytical technique can be utilized for the quality control during the production of the cooking oils as well as a technique for the detection of adulteration or dilution of these oils. The chromatograms produced can provide for a chromatographic fingerprint for the comparison of the cooking oils to determine origin, compare different manufacturers, or for quality control. Many kinds of flavors are used in the food industry, and there is a demand for new and improved ones, especially natural ones. Such a source are oils which can produce a variety of flavors. The Direct Thermal Extraction technique offers several unique advantages over the Purge and Trap technique including greater sensitivity and the detection of a wider range of volatile organics including higher molecular weight compounds and is more sensitive by a factor of at least 1000 as compared to the Purge and Trap technique. This technique can easily be incorporated into troubleshooting techniques to detect problems in a wide variety of commercial food products, to compare various competing manufacturers products as well as the implementation of a quality control program for the food industry.

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