Heteroatomic Aroma Compounds - ACS Publications - American

Downloaded by 80.82.77.83 on May 19, 2018 | https://pubs.acs.org Publication Date: August 7, 2002 | doi: 10.1021/bk-2002-0826.pr001

Preface Although the flavor industry has been in existence for more than 150 years, flavor research in its current form is relatively new beginning with the advent of gas chromatography (early 1960s). As academic institutions have developed both teaching and research programs to service the flavor and related food industries, more information has become available in the public domain. Prior to academic involvement, very little information related to flavor science was published because this is traditionally a very secretive industry. As publications started to appear, the major venue for the publication of scientific information has been the proceedings of various international symposia. These have included symposia offered in Europe (e.g., Weurman, Wartburg and Greek symposia) as well as those offered through the flavor subdivision of the American Chemical Society (ACS) Division of Agricultural and Food Chemistry. This book is the result of a symposium sponsored by the A C S in Washington D.C. in the fall of 2000. The editors (symposium organizers) invited leading scientists from academia and industry to present lectures and ultimately to publish this book, which is a comprehensive treatment of the topic of flavor chemistry. This book is organized into sections dealing with various heteroatomic molecules (e.g., nitrogen, sulfur, oxygen and miscellaneous atoms). This was done because many of the methodologies for study are based on the detection of a specific atom. For example, one may choose an analytical method that selectively extracts or detects sulfur compounds. Also, particular atoms are often associated with certain mechanisms of formation and sensory character. Therefore, an organization based on the occurrence of a specific atom seemed reasonable. The first section of this book (six chapters) focuses on sulfur-containing volatiles. There is little question that sulfur chemicals are very often the key to a given aroma and unfortunately present the greatest problems in analysis and identification. The

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problems in analysis and identification are the result of the extremely low sensory thresholds of this group o f compounds and the inherent instability or reactivity. Thus, the first chapter of this book written by Rouseff discusses analytical techniques to determine volatile sulfur compounds in foods. The application of aroma isolation techniques such as distillation, codistillation, solid phase microextraction, liquid-liquid extraction, and sublimation are discussed in terms of efficiencies and potential artifact formation. The author has presented a thoughtful discussion of the advantages, limitations, and potential problems associated with using these aroma isolation methodologies. Sample-analyte dependent concentration techniques such as adsorption on mercury-based solid sorbents, metal foils, and impregnated filters are also presented as well as separation approaches based on H P L C and G C . The chapter ends with a discussion of the limits of detection, sensitivity, selectivity, and cost of flame photometric, chemiluminescence, atomic emission, and pulsed flame photometric G C detectors. The second chapter considers the sensory significance of sulfurcontaining aroma compounds. Blank points out that although volatile sulfur compounds account for only about 10 percent of all volatile components identified in foods, these compounds are extremely important constituents of the flavor of many foods. Blank has chosen to demonstrate the role of sulfur volatiles in foods by selecting certain sulfur-containing aroma-impact compounds and discussing in detail the sensory relevance. Having presented a discussion of the sensory importance of sulfur-containing volatiles in food and how we approach the analysis of the volatiles, the discussion then turns to how these compounds are formed in foods via natural processes (plants and microorganisms) and during thermal processing. Spinnler et al. (Chapter 3) focus their discussion on the conversion of various sulfur precursors (e.g., sulfates, sulfites, amino acids, and peptides) into sulfur-containing volatiles. They place special attention on the importance of acyl CoAs in thioester synthesis. Mottram and Mottram (Chapter 4) then present an overview of how sulfur-containing aroma compounds are formed during the thermal processing of foods. They note that both heterocyclic sulfur compounds (e.g., thiophenes, thiophenones, dithiolanes, trithiolanes, trithianes, thiazoles, and thienothiophenes) and non-heterocyclic sulfur

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compounds (e.g., thiols, sulfides, and disulfides) and typical products of thermal reactions in food. They provide a discussion of how precursors such as cysteine, methionine, and thiamin are converted to aroma compounds during thermal processing. These first four chapters are review in nature and are intended to provide the reader with an appreciation of flavor research focused on sulfur compounds. The final chapters in the sulfur section present original research on sulfur volatiles. The chapter by Mottram and Elmore focuses on the role of lipids in forming sulfur volatiles. They note that a number of 3thiazolines, thiazoles, thiapyrans, and thiophenes with 2-alkyl substituents have been found in cooked beef and lamb. These compounds derive from the interaction of lipid autoxidation products, such as saturated and unsaturated aldehydes, with simple intermediates of the Maillard reaction, such as hydrogen sulfide, ammonia, and dicarbonyls. They point out that, although the aromas of these compounds are weak, they may influence flavor by modifying the formation of other compounds in the Maillard reaction or autoxidation of lipids. Thus the effect on food flavor may be indirect as opposed to making a direct contribution to aroma. The final sulfur chapter reports on original work by L i n et al. employing GC-Olfactometry (GC-O) to identify five sulfur-containing odorants in commercial not-from-concentrate (NFC) grapefruit juice. They found these sulfur compounds imparted both characterizing and supporting aroma attributes to the juice. Two compounds, 3-mercaptohexyl acetate and 3-mercaptohexan-1 -ol were reported for the first time in grapefruit juice. They also reported on the effect of pasteurization on sulfur-containing aroma compounds. The next section of this book presents a similarly organized discussion of nitrogen-containing aroma compounds in foods. Although nitrogen-containing aroma compounds perhaps are less important to food aroma on a global basis, there is little question that nitrogen compounds sensorially characterize the aroma of some foods (discussed in Chapter 9). Rajesh and Peppard start this section (Chapter 7) with a discussion of numerous techniques to isolate, identify, and quantify nitrogen-containing aroma compounds in foods. They have included a discussion of solvent extraction, adsorption, and ion exchange chromatography, molecular and steam distillation, static and dynamic headspace sampling, and solid phase microextraction for the isolation of nitrogen-containing volatiles. Analytical techniques used to characterize

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and/or quantify nitrogen heterocycles include multidimensional chroma tography and G C - O , as well as G C with various types of nitrogenspecific detection or atomic emission detection. The most commonly employed techniques are described, and examples are cited for analyzing food and beverage flavors for nitrogen-containing aroma compounds. Rizzi presents an overview of the biosynthesis of nitrogencontaining aroma compounds beginning with the incorporation of atmospheric Ν into ammonia and then ammonia into a host of other aroma precursors (Chapter 8). He discusses how some of these aroma compounds are formed in nature through a combination of enzyme and nonenzyme catalyzed reactions. The next chapter (by Demyttenaere et al., Chapter 9) focuses on the formation of nitrogen-containing aroma compounds during the thermal processing of foods. This overview presents the chemistry and flavor characteristics of cracker-like flavors, such as 6-acetyl-l,2,3,4-tetrahydro-pyridine and 2-acetyl-l-pyrroline, to illustrate how this group of aroma chemicals are formed in foods. These authors detail the mechanisms of formation, instability, and synthesis of these compounds. The final two chapters of this section present original research involving nitrogen-containing aroma compounds. Le Quéré et al. present their work on the formation and sensory character of a homologous series of 2-alkyl-2,4,5-trimethyl-2,5-dihydro-oxazoles in blue cheese (Chapter 10). Their work illustrates the application of G C sniffing, G C / M S , GC/FTIR, N P D (nitrogen specific detector), i H - N M R and two-dimensional N M R in the characterization of these nitrogencontaining odorants. Rhlid et al. present their work on the formation and enhancement of an important nitrogen-containing volatile, 2-acetyl-2thiazoline in roasted products (Chapter 11). The final section of this book that is organized around a particular atom is the section on oxygen-containing odorants. This section again starts with a discussion of methods to determine oxygencontaining aroma compounds in foods, and then chapters are included that present overviews of their formation during thermal processing and biogenesis. The initial chapter written by Budin (Chapter 12) focuses on the methods used in the isolation and analysis of oxygen-containing aroma compounds. Like the other analytical chapters, his discussion includes some common methods of aroma isolation. However, he goes on to discuss some of the classical methods used in the analysis of this

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group of odorants. These classical methods include various derivatization methods such as reaction with 2,4-dinitrophenyl hydrazine, 1pyruvylchloride-2,6-dinitrophenyl hydrazine, and cystamine. Unlike nitrogen- or sulfur-containing molecules, there are few truly selective techniques for the analysis of oxygen-containing molecules other than these classical methods. Schieberle and Hofmann present an overview of the formation of oxygen-containing aroma compounds in foods during processing and storage (Chapter 13). They note that carbohydrates are the primary source of these odorante (lipid oxidation is a secondary source). They illustrate the mechanisms of formation of these odorants using 4-hydroxy-2,5-dimethyl3(2//)-furanone and 3-hydroxy-4,5-dimethyl-2(5//)-furanone as examples. Peterson and Reineccius discuss the biosynthesis of several key oxygencontaining aroma compounds as examples of how these compounds are formed in nature (Chapter 14). The final chapter of this section presents an example of original research focused on oxygenated aroma compounds (i.e., free fatty acids in Parmesan cheese). Qian and Reineccius note that free fatty acids appear to be extremely important in characterizing this cheese (Chapter 15) for these compounds were found to have high odor activity values. As one would expect, the short chain fatty acids (C4 to CIO) were found to be most important of the acids. Although aroma compounds containing sulfur, nitrogen, and oxygen are generally considered to largely comprise the molecules of odor significance, some literature considers the sensory significance of selenium-containing and halogenated odorants. K i m and Reineccius present a discussion of methodologies used to study halogenated aroma compounds. This chapter (Chapter 16) includes numerous examples of methods used in the isolation and quantification of these compounds. Kittle and Deibler discuss the sensory significance of halogenated aroma compounds noting that these compounds are generally associated with off odors as opposed to desirable odors (Chapter 17). The chloroanisoles and chlorophenols are associated with medicinal, chemical, or musty aromas. The primary example of a food where these chemicals may make a positive contribution is in brominated aroma compounds found in some seafood. Wei and Ho report on the formation of seleniumcontaining aroma compounds during thermal processing of foods (Chapter 18). This study investigated the Maillard reaction of

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selenomethionine and glucose. Organoselenium, organoselenium-sulfur and pyrazine compounds were identified by G C - E I / M S and G C - N H 3 CI/Ms and dimethyl diselenide was the most abundant selenium compound in the extracts. By use of a buffered model system, the authors found maximum yields of pyrazines and dimethyl diselenide at p H 7, 80 min. However, the formation of ethyl selenoacetate was favored at a lower pH, with a maximum yield at pH 3, 80 min. The addition of diallyl disulfide to the model system produced several organosulfur, organoselenium, and organoselenium-sulfur compounds of interest. The final section of this book (last three chapters) considers new methods for the analysis of aroma compounds. Although not closely related to the focus of this symposium, symposia offer the advantage of drawing large audiences where cutting edge research can be disseminated in a timely fashion (by oral presentations). Thus, these presentations (i.e., chapters) are included in this book. The first chapter in this group (by Buettner and Schieberle, Chapter 19) details how realtime magnetic resonance imaging (MRI) can be applied to understand how aroma is released from a food and perceived by the olfactory receptors. They demonstrated that no significant release of odorants occurs in the mouth during chewing and that the majority of odorants is, in fact, released to the sensory receptors in the first breath after swallowing. This information is remarkable and considerably advances our knowledge of aroma release during eating and the potential role of temporal profile in this context. Engel et al. presented their work on evaluating taste active substances in tomato and camembert cheese (Chapter 20) using omission testing. Little has been published on taste substances or methodologies for their study. This was a very thoughtful study adding a very useful methodology to the field. The final chapter in this book describes a method to monitor flavor development in coffee during roasting (Hofmann et al., Chapter 21). This method is based on a chemosensor array developed at the Technische Universitat of Munchen. Through classical flavor chemistry, they found that 2-furfuryl alcohol was a good indicator of flavor development during roasting (i.e., the formation of the key roast and ground coffee component 2-furfurylmercaptan). The authors were able to design a sensor that could specifically detect the indicator odorant (the key odorant is present at too low

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concentrations to be monitored using existing methodologies in real time) and thereby be used to monitor flavor development in coffee during roasting. In summary, this book provides detailed discussions of methods of analysis, mechanisms of formation in plants, during fermentation and during thermal processing; sensory properties of some key aroma compounds; and their role as off flavors all based on the presence of a given atom. A n effort was made to get a comprehensive treatment of these topics that will serve as a reference book in this field. Although many books are now available in this general topic area, no other single book offers such a comprehensive discussion of the analysis, formation, and sensory significance of these aroma compounds. The primary audiences for this book are those involved in flavor research in the flavor and food industriesas well as in government and academic research laboratories. Related fields that would benefit from this book include environmental (e.g., the control of off odors), cosmetics or fragrance areas. The fundamental discussions of how to analyze volatiles, how they are formed, and how they might contribute to desirable and undesirable aroma would prove most useful.

Acknowledgments We express our appreciation to the authors of each chapter, the reviewers who were so helpful in improving the chapters, and the companies who provided financial support to bring in speakers. The generosity of Robertet Flavors, Inc., Firmenich, Kraft Foods, and Quest International contributed greatly to the success of the symposium and ultimately the publication of this book.

Gary A . Reineccius Terry A. Reineccius Department of Food Science and Nutrition University of Minnesota 1334 Eckles Avenue St. Paul, MN 55108

xvii Reineccius and Reineccius; Heteroatomic Aroma Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

Heteroatomic Aroma Compounds - ACS Publications - American

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