Challenges in Flavor Chemistry: An Overview - American Chemical

Chapter 1

Challenges in Flavor Chemistry: An Overview

Downloaded by SUNY DOWNSTATE MEDICAL CTR on April 15, 2015 | http://pubs.acs.org Publication Date: September 10, 1998 | doi: 10.1021/bk-1998-0705.ch001

Roy Teranishi 89 Kingston Road, Kensington, C A 94707

This chapter gives an overview of some of the challenges facing flavor chemists today and how some of these challenges are met by internationally known scientists who participated in this symposium. Although the fundamentals of modern organic chemical methods used in flavor chemistry have been established, details of modifications and extensions of existing methods must be worked out in order to solve specific problems. Isolation methods such as headspace trapping, cold trapping, extraction, etc. are described, as well as special applications of gas and liquid chromatographic separations. Some applications of mass spectrometry and sensory characterizations are discussed. Flavor chemistry is simply a specialized application of organic chemistry. Organic chemistry started in the 19th century when it was shown that chemical transformations took place without supernatural influences. The first era consisting of the theory of vitalism died, and the second period of organic chemistry began with the birth of structural theory, to explain how atoms were organized to form molecules, the network of carbon to carbon bonds. In the third era of organic chemistry, the present modern era, there are three major developments (1): 1. Electronic theories of valences and bonds. 2. Understanding of mechanisms of organic reactions. 3. Development of instruments for separating, analyzing, and identifying organic compounds. Chemical industries utilizing these developments have developed enormously to produce dyestuffs, explosives, medicines, plastics, elastomers, fibers, films, perfumes, flavors, etc. Studies of natural products, volatiles from food materials, have been the backbone of flavor chemistry. In the 1950's, only about 500 compounds had been characterized for their flavor properties (2). With the advent of liquid and gas chromatography, infrared, nuclear magnetic resonance, and mass spectrometry, after a latent period of a few

©1998 American Chemical Society In Flavor Analysis; Mussinan, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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2 years in which application techniques were worked out, the number of compounds which have been characterized has rapidly reached about 15,000 (3). It must not be forgotten that micromanipulation methods which are routinely used today were developed by many pioneers, beginning with Michael Faraday, the father of microorganic chemistry methods (4). Concurrently with the advent of modern instruments, there has been a development of sensory analyses utilizing odor thresholds (5). Although there are still many isolation and identification problems to be solved, the shift of emphasis is towards the correlation of chemical structures to sensory properties. The fundamental concepts of isolation methods, such as distillation, liquid and gas chromatography, have been established, but there are still many variations, extensions, and developments necessary to meet the challenges of isolation and characterization of compounds contributing to various flavors. There are still many problems to be solved. In the early stages of flavor research, most of the emphasis was on development of methods to establish chemical identity of constituents found only in trace quantities. Because flavor chemistry is a special application of organic chemistry, the methods worked out in flavor chemistry can be applied in other fields in which small amounts of organic compounds can have profound biological effects, such as in nutrition, air or water pollution, plant and animal hormones, insect attractants, etc. Now that many of the constituents have been identified, the task remains to determine biological activities; that is, which constituent, or constituents, is/are contributing to the characteristic sensory properties of the food product being investigated. This symposium covers some of these applications and challenges which the flavor chemist, and other natural product chemists working with trace amounts, faces today. Although the fundamental chemistry used in flavor chemistry is based on information gained by fundamental organic chemical research, some of the analytical methods now used by scientists in other disciplines were developed first in flavor research. Two examples are: 1. the use of programmed temperature capillary gas chromatography (6), and 2. the use of gas chromatography combined with mass spectrometry (7). Both methods are widely used in many disciplines and fields. Similarly, various methods developed in flavor chemistry research can be utilized in studies of water and air pollution, insect attraction, toxicology, etc. ISOLATION METHODS The first area covered by the symposium and in this book is the fundamental and broad topic of isolation. No analytical method is valid unless the isolates represent the material being studied. No logical sensory conclusions can be made if the isolates do not have the characteristic sensory properties of the food product being studied. The importance of the isolation step cannot be overemphasized. Sample size requirements of analytical instruments have become smaller and smaller with new developements, especially with the application of computer methods. Nevertheless, great care must be taken in concentration and isolation so that isolates have sensory properties of the food being studied. Care must be taken so that heat labile compounds are not destroyed by harsh conditions, highly volatile compounds not lost in concentration by distillation, or low solubility compounds lost in extractions.

In Flavor Analysis; Mussinan, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.


3 Most isolations use some form of distillation or extraction, or a combination of both. These methods utilize differences in vapor pressures (distillation) and solubilities in different solvents (extraction and chromatography). Specific appli cations have special names. Headspace and vacuum headspace trapping are forms of distillation. A combination of distillation and extraction can be used in a simultaneous distillation of aqueous material and extraction with an organic solvent. Polymeric particles or a probe coated with adsorbant material can be used to trap volatiles. Gas chomatography utilizes both vapor pressure and solubility. Countercurrent chromatography utilizes solubilities in different immiscible organic solvents. So flavor chemists have many techniques and various combinations of such methods at their disposal. Headspace analysis in the early stages was done simply by sampling vapors from various products with a syringe and injecting the vapors in a gas chromatograph (8). Now there are many sophisticated applications and variations of this method: purge-and-trap, cold trap, trap with polymeric solid-phase, etc. This method permits analyses with minimum delay from sampling to analysis, with the minimum of intro duction of artifacts developed or introduced during sampling. Unless one resorts to vacuum distillation, distillation usually means that the sample has been exposed to temperatures above ambient. At high temperatures, some of the thermally unstable compounds are destroyed and some artifacts are produced by combination of frag ments formed. Thus, headspace analyses permit analyses of volatiles emitted by plants or food products to provide data representing fresh flowers, fresh fruits and vegetables, of products as they are, not data containing artifacts formed during distillation and/or extraction. Some of the analyses discussed in this symposium utilizing headspace methods are: stored and fresh citrus, strawberry, pear, peach, cherry, pineapple, an Australian truffle-like fungus, dry pet foods, milk, meat products, volatile Maillard reaction products, etc. Solubilities and reactivities on microscale samples can be used to reduce some of the complexity of volatiles isolated from natural products. Odor assessment of each stage of extraction is made to help indicate compounds of critical importance. CHROMATOGRAPHY Gas chromatography, first proposed by James and Martin (9), is a very powerful separation method utilizing very small samples. Since the early days of flavor research, more stable columns with greater separation resolution are now commer cially available. Previously, each research group had to rely on some member of the team to fabricate its own columns. Gas chromatography became even more powerful and useful with high resolution capillary columns used in combination with mass spectrometers (7). Separation of enantiomers by gas chromatography and isotope studies with nuclear magnetic resonance and mass spectrometry are very effective and powerful methods for detecting adulteration. Reducing adulteration of our foods has farreaching implications in keeping our foods pure and safe. Novel chiral sulfur compounds have been isolated by capillary chromatography utilizing chiral stationary phases. Such columns also permit the study of the efficiency of enantioselective syntheses of important optically active lactones.



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Liquid partition chromatography, especially countercurrent liquid chromato graphy (CCC), is needed in studies of flavor precursors, which are usually watersoluble low volatility compounds containing sugar moieties. The principles of this method of chromatography were reported as early as 1941 (10). Because this separa tion method is done at ambient temperatures, it is a very powerful tool for isolating labile compounds. Current capabilities and applications of countercurrent chroma tography to flavor analyses are discussed. Progress and data accumulated in studies of flavor precursors (11) have been possible by the use of liquid chromatography, especially countercurrent chromatography. This is very important because such data are necessary in biotechnological development of more flavorful foods. If genetic engineering is to be used to produce foods with more flavor, it is necessary to know which precursors must be increased in concentration in plants. Also, knowledge of precursors will permit processing parameters to be adjusted for maximum yield of flavor. MASS SPECTROMETRY Mass spectrometry was used by petroleum chemists long before flavor chemists used it. Possible usefulness and applications of mass spectrometry in flavor research (12) were recognized before the advent of gas chromatography (9). Problems of handling microsamples and of purification of samples held back progress until the combination of programmed temperature capillary gas chromatography and mass spectrometry (GC-MS) was introduced (7). Direct introduction of samples separated by tempera ture programmed gas chromatography capillary columns into fast-scan mass spectro meters solved the problems of purification and manipulation of microsamples. The early work showed some general possibilities but are crude by today's standards. The challenges today are those encountered in fine-tuning parameters to obtain precise and maximum information in specific application flavor problems. Precise and reproducible control of instruments is now possible with application of computer technology. Storage and retrieval of data are immensely facilitated by use of computers. Some of the challenges which face the flavor chemists today are in the improvement in computer handling of data for more rapid and concise use of the enormous amounts of data accumulated. In this book, compounds purified by gas and liquid chromatography which have been analyzed by mass spectrometry range from acetals, furans and furanones and other compounds from the Maillard browning reaction, to nonvolatile bio-active and flavor-active compounds such as phenols, flavonoids, glycosides, saponins, etc. Capillary gas chromatographs coupled to on-line Isotope Ratio Mass Spectrometers (GC/IRMS) are used to determine the origin of various flavors, especially citrus, peppermint, vanilla, etc., to determine whether the flavors were synthesized by plants or by man. Samples separated by high performance liquid chromatography (HPLC) analyzed with either atmospheric pressure chemical ionization-tandem mass spectrometry (APCI-MS-MS) or inductively coupled plasma-mass spectrometry (ICPMS) can be rapidly analyzed for un-derivatized organosulfur or organoselium compounds found in Allium plants.


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SENSORY ANALYSES Ever since gas chromatographs equipped with non-destructive detectors were built, scientists have been sniffing the effluents from gas chromatographs. Simple and crude sniffing methods have been developed into sophisticated sensory methods. Sniffing effluents from capillary gas chromatographs and water solution odor thres hold determinations were used to indicate the most important compounds contributing to tortilla flavor. Gas chromatographic and mass spectral data were used for chemi cal identification, and olfactometric techniques were used to characterize which compounds are the important contributors to a specific variety of apples. This method of using capillary columns to separate materials and sniffing effluents to establish olfactory properties (13-15) is important because sensory data, qualitative and quantitative, can now be obtained on very small samples. There should be many applications of these methods in the future to provide information heretofore very difficult to obtain. Chemical and sensory analyses were utilized to determine what compounds contribute to citrus off-flavors. Selective odorant measurement by a multisensor array (SOMMSA) technique has been used to sort out chemosensors for the selective detection of key odorants. The ratio of concentration to odor threshold was used to characterize the key odorants formed when cysteine and carbohydrates were heated. Variations of odor thresholds of various unsaturated branched esters are reported. Umami, a "delicious" and savory taste, is a topic of much interest and impor tance. This taste component is very important in the production of soups, stews, soup stock, gravies, and many commercial food products. A small linear peptide in an extract of papain hydrolyzed beef has been reported as a flavor enhancer. The terms flavor potentiator, flavor modulator, and flavor enhancer are discussed. This symposium covered how some of the challenges, problems facing flavor chemists in basic and applied research, have been met. Information gained in these investigations will be useful in many applications for industrial flavor chemists. Experimental methods described herein will be useful in other fields in which minor organic chemicals have important biological functions. Literature Cited 1. 2. 3. 4. 5.

Cram, D.J.; Hammond, G.S. Organic Chemistry, McGraw-Hill, New York, 1964, pp. 4-5. Weurman, C. Lists of Volatile Compounds in Foods, First Edition, Division of Nutrition and Food Research TNO, Zeist, The Netherlands, 1963. Basset, F . Informations Chime n° 300, Givaudan-Roure, Dübendorf, Switzerland, Décembre 1988, 207-209. Michael Faraday, Tube chemistry, In Chemical Manipulation, John Murray, Albemarle Street, London, 1830, pp 386-419. Teranishi, R. In Proceedings of the 5th Wartburg Aroma Symposium, Rothe, M.; Kruse, H.-P. (Editors), Eigenverlag University of Potsdam, Potsdam, 1997, in print.


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6. 7.


8.

9. 10. 11.

12. 13. 14. 15.

Teranishi, R.; Nimmo, C . C . ; Corse, J. Anal. Chem., 1960, 32 (11), 13841386. McFadden, W . H . ; Teranishi, R.; Black, D.R.; Day, J.C. J. Food Sci., 1963, 28 (3), 316-319. Teranishi, R.; Buttery, R . G . In Proceedings of the International Federation of Fruit Juice Producers Symposium, Juris-Verlag, Zurich, 1962, pp. 257266. James, A . T . ; Martin, A.J.P. Biochem. J., 1952, 50, 679-690. Martin, A.J.P.; Synge, R.L.M.; Biochem. J., 1941, 35, 1358-1368. Flavor Precursors: Thermal and Enzymatic Conversions, A C S Symposium Series 490, Teranishi, R.; Takeoka, G.R.; Guentert, M. (Editors), American Chemical Society, Washington, D C , 1992, 269 pp. Turk, A.; Smock, R.M.; Taylor, T.I. Food Technol., 1951, 5 (2), 58-63. Acree, T . E . ; Barnard, J.; Cunningham, D . G . Food Chem. 1984, 14, 273286. Fischer, K.-H.; Grosch, W. Lebensmittel-Wissenschaft und Technologie, 1987, 20, 233-236. Ullrich, F . ; Grosch, W. Zeitschrift für Lebensmittel-Untersuchung und Forschung, 1987, 184, 266-287.


Challenges in Flavor Chemistry: An Overview - American Chemical

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