M Source: Ref 18.

Copyright  2002 by Marcel Dekker, Inc. All R ights Reserved.

FIGURE9 TIC of a roast and ground coffee sample moistened with water and extracted with methylene chloride.

VII. HIGH VACUUM DISTILLATION OF LIPIDS

A number of the procedures described in Sec. VI will yield a material that is primarily lipid in nature. In addition, many samples available to the researcher are themselves lipids. A few materials that one may encounter are coffee oil, vegetable and nut oils, cocoa butter, lard, butter oil, lipids used for deep fat frying, and lipids used as the solvent for Maillard reaction systems. Such materials can be a relatively rich source of aromatic compounds because aroma compounds are typically lipid soluble. A number of procedures can be used to prepare a sample. In this section we will cover three useful ones.

A. Steam Distillation

The lipid material may be steam distilled at atmospheric pressure or under vac- uum, as was described in Sec. IV, and subsequently subjected to solvent extrac- tion. Alternatively, a modified Likens-Nickerson extractor has been described (19), which permits the introduction of steam into the system. Recoveries of model compounds from lipid systems were not as satisfactory as for aqueous samples.

FIGURE10 Falling film molecular still for the removal of volatiles from lipids. (From Ref. 20.)

B. High Vacuum Distillation

When large amounts of lipid materials are present, the sample may be subjected to a falling film molecular still. The apparatus utilizes the principle of vaporiza- tion of the flavor from a heated thin film of the oil under high vacuum. One such apparatus is shown in Fig. 10 (20). Several hundred milliliters of oil are placed in vessel A and slowly passed through the foaming chamber into the heated bellows chamber. The distillate is collected in a series of traps cooled with liquid nitrogen. The oil may be recycled. Another series of apparatus described by Chang et al. at Rutgers (21) has accomplished similar goals. This type of apparatus generally falls into the same category of equipment as that used to deodorize lipids.

C. Short Path Distillation

One version of the apparatus is shown inFig. 11a.The nonvolatile material is

FIGURE11 Apparatus for the removal of aromatics from lipids. (a from Ref. 22; b from

Ref. 23.)

uum is applied. The inner condenser is cooled with liquid nitrogen or dry ice- solvent (22). We have found this apparatus very useful for separating the volatile aromatics from nonvolatile residues (i.e., lipids) such as those generated in Sec- tion VI. In that case the sample size may be only a few grams or less, and a smaller version of the short path distillation apparatus is appropriate. This apparatus can be easily fabricated by a glassblower.

An example of the application of such an apparatus is shown inFig. 12.

The sample was produced by high vacuum distillation of 10 g of coffee oil ex- pelled from roast and ground coffee. The volatiles were condensed with liquid nitrogen and subsequently washed off the cold finger with methylene chloride. Figure 12 shows the total ion chromatogram of the sample. The large peak eluting at 25 minutes is caffeine.

Nawar (23) has commented that the apparatus shown in Fig. 11a may pres- ent problems if the sample contains water. He suggested the apparatus shown in Fig. 11b. Vacuum is applied at point A, and vessel L is filled with liquid nitrogen during the 1-hour distillation period. At the end of the distillation, the cold trap is disconnected and the coolant is discarded. The condenser is inverted, the ice melted, and condensed volatiles and water extracted with a solvent. He reported greater than 80% recovery of high-boiling hydrocarbons in a model system study.

VIII. CO-DISTILLATION OF SAMPLE WITH SOLVENT

A new technique has been suggested by a group of Russian workers (24). They compared three methods of isolation, namely, distillation-extraction and two methods based on co-distillation of sample from solvent-water mixtures. In the

FIGURE12 TIC of volatiles from roast and ground coffee oil, distilled in apparatus shown inFig. 11a.

co-distillation technique, a solvent such as diethyl ether, pentane, or methylene chloride is dispersed in the sample and the sample is distilled rapidly (at, e.g., 200°C) until all the solvent and a small amount of water have passed over. The sample is analyzed by gas chromatography. Their co-distillation technique (at atmospheric pressure) compared favorably with the Likens-Nickerson tech- nique. They analyzed three samples: a model system, a meat sample, and a fish sample.

The chromatogram of an R&G coffee that was dispersed in water and co- distilled with solvent in our laboratory is presented inFig. 13.The curve is the total ion chromatogram of the sample, which has large caffeine peak eluting at 25 minutes.

The advantages of co-distillation are that isolates are generated without a boiled note, the process is efficient and reproducible, and it takes only 15–20 minutes for a distillation.

IX. SUMMARY

Over the years numerous procedures have been proposed for the isolation and identification of aromatic compounds. Because of the variation of sample types encountered, no single technique will always suffice. One must always be aware that none of these techniques will produce an isolate that quantitatively represents the composition of the starting material.

FIGURE13 TIC of a roast and ground coffee sample co-distilled with methylene chloride.

This chapter reviewed techniques that involve distillation and extraction procedures. These have the advantage of being simple and rapid, and they do not require a complex apparatus. For typical food products some version of the Likens-Nickerson distillation apparatus is probably the technique of choice; for lipid materials, some high-vacuum distillation procedure is worth investigating initially.

REFERENCES

1. T. H. Parliment, ‘‘Sample Preparation Techniques for Gas-Liquid Chromatographic Analysis of Biologically Derived Aromas,’’ in Biogeneration of Aromas (T. H. Parli- ment and R. Croteau, eds.), American Chemical Society, Washington, DC, 1986, p. 34.

2. G. Wasserman, H. Stahl, W. Rehman, and P. Whitman, Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., Vol. 6, John Wiley and Sons, New York, 1993, p. 793.

3. W. Jennings and M. Filsoof, Comparison of sample preparation techniques for gas chromatographic analysis, J. Agric. Food Chem. 25:440 (1977).

4. C. Weurman, Isolation and concentration of volatiles in food odor research, J. Agric. Food Chem. 17:370 (1969).

5. H. Sugisawa, ‘‘Sample Preparation: Isolation and Concentration,’’ in Flavor Re- search, Recent Advances (R. Teranishi, R. Flath, and H. Sugisawa, eds.), Marcel Dekker, Inc., New York, 1971, p. 11.

mal Generation of Aromas (T. H. Parliment, R. J. McGorrin, and C.-T. Ho, eds.), American Chemical Society, Washington, DC, 1989, p. 42.

7. H. Maarse and R. Belz, Isolation, Separation, and Identification of Volatile Com- pounds in Aroma Research, Akademie-Verlag, Berlin, 1981.

8. C.-T. Ho and C. H. Manley, eds., Flavor Measurement, Marcel Dekker, New York, 1993.

9. P. Schreier, Chromatographic Studies of Biogenesis of Plant Volatiles, Huthig, New York, 1984.

10. Restek Advantage 4:7 (1993).

11. M. Leahy and G. Reineccius, ‘‘Comparison of Methods for the Isolation of Volatile Compounds from Aqueous Model Systems,’’ in Analysis of Volatiles. Methods. Applications (P. Schreier, ed.), de Gruyter, New York, 1984, p. 19.

12. W. Schultz and J. Randall, Liquid carbon dioxide for selective aroma extraction, Food Technol. 24:1283 (1970).

13. D. A. Moyler, Carbon dioxide extracted ingredients for fragrances, Perf. Flav. 9: 109 (1984).

14. T. H. Parliment, A new technique for GLC sample preparation using a novel extrac- tion device, Perf. Flav. 1:1 (1986).

15. T. H. Parliment and H. D. Stahl, ‘‘Generation of Furfuryl Mercaptan in Cysteine- Pentose Model Systems in Relation to Roasted Coffee,’’ in Sulfur Compounds in Foods (C. Mussinan and M. Keelan, eds.), American Chemical Society, Washington, DC, 1994, p. 160.

16. T. Parliment and H. Stahl, ‘‘Formation of Furfuryl Mercaptan in Coffee Model Sys- tems,’’ in Developments in Food Science V37A Food Flavors: Generation, Analysis and Process Influence (G. Charalambous, ed.), Elsevier, New York, 1995, p. 805. 17. G. B. Nickerson and S. T. Likens, Gas chromatographic evidence for the occurrence

of hop oil components in beer, J. Chromatog. 21:1–3 (1966).

18. T. Schultz, R. Flath, R. Mon, S. Eggling, and R. Teranishi, Isolation of volatile components from a model system, J. Agric. Food Chem. 25:446 (1977).

19. C. Aug-Yeung and A. MacLeod, A comparison of the efficiency of the Likens and Nickerson extractor for aqueous, liquid/aqueous, and lipid samples, J. Agric. Food Chem. 29:502 (1981).

20. B. Johnson, G. Waller, and A. Burlingame, Volatile components of roasted peanuts: basic fraction, J. Agric. Food Chem. 19:1020 (1971).

21. S. Chang, F. Vallese, C. Hwang, O. Hsieh, and D. Min, Apparatus for the isolation of trace volatile constituents from foods, J. Agric. Food Chem. 25:450 (1977). 22. J. de Bruyn and J. Schogt, Isolation of volatile constituents from fats and oils by

vacuum degassing, J. Am. Oil Chem. Soc. 38:40 (1961).

23. J. J. Balboni and W. W. Nawar, Apparatus for direct collection of volatiles from meat, J. Agric. Food Chem. 18:746 (1970).

24. T. Misharina, R. Golovnya, and I. Beletsky, ‘‘Comparison of the Efficiency of Isola- tion of Volatiles from Foodstuffs by Co-Distillation and Likens-Nickerson Meth- ods,’’ in Developments in Food Sci. V35. Trends in Flavor Research (H. Maarse and D. van der Heij, eds.), Elsevier, New York, 1994, p. 117.

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Analysis of Food Volatiles