Flavor Release: A Rationale for Its Study - ACS Publications

Chapter 1

Flavor Release: A Rationale for Its Study 1

Deborah D. Roberts and Andrew J. Taylor

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Downloaded by FREIE UNIV BERLIN on February 26, 2017 | http://pubs.acs.org Publication Date: September 7, 2000 | doi: 10.1021/bk-2000-0763.ch001

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Nestlé Research Center, P.O. Box 44, Vers-Chez-les-Blanc, 1000 Lausanne 26, Switzerland Division of Food Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire L E 12 5RD, United Kingdom

Release of flavor compounds from foods is an essential prerequisite for flavor perception. Although measurement of release at equilibrium has been practiced for many years, measuring the dynamics of release was a more difficult task, due to the speed of change and the low concentrations of flavor compounds. However, sensory techniques like time-intensity analysis showed there were perceptible changes in flavor during, and after, eating. More recently, instrumental methods have been developed to follow the change in concentrations of flavor compounds above foods, or in the mouth and nose, during consumption of food. The availability of these techniques has led to renewed interest in the fields of modeling of flavor release, in the link between flavor compounds and flavor perception, and in the interactions of flavor compounds with food matrices.

Flavor release is the study of the mechanisms that influence the volatilization of aroma compounds (or the release of tastants) from food during specific situations, especially release in the mouth during eating This covers a wide range of situations that are mainly investigated by chemists but also by food scientists, sensory scientists, and theoretical modelers. At the one extreme, it can describe the partitioning between an aqueous solution of volatiles and the air above it (the headspace). Such systems are usually allowed to come to equilibrium where the ratio of the concentrations of volatile compound in the gas and water phases is the air-water partition coefficient. Such systems are often referred to as "static equilibrium". At the other extreme, the release of flavor compounds during eating can be monitored in the mouth and/or nose where the system represents real time flavor release in vivo. In between, there are various systems that have been designed to measure the effect of specific parameters (e.g. binding of volatiles to proteins, effect of saliva on rate of volatile release) or under specific release situations (e.g. during processing). Historically, equilibrium systems were studied because of their simplicity but the emphasis now is on the dynamic measurement of flavor release. Most published studies have investigated the release of flavor compounds that are volatile as they are considered to impart the

© 2000 American Chemical Society

Roberts and Taylor; Flavor Release ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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flavor character to foods (non-volatiles provide the base flavor) and because they are easier to isolate from foods. There is growing interest in the interaction of volatile and non-volatile flavor (sapid) compounds at the molecular and perceptual levels and, thus, the dynamics of sapid flavor release have been investigated recently. Verification of analytical results with sensory perception is one of the final and needed steps, especially for food development.


Historical Development of Flavor Release Research Flavor release has been reviewed by several authors over the last 10 years (1-9) In this book, the chapters by Escher, de Roos and Taylor review, respectively, progress in starch-flavor compound interactions, modeling of flavor release, and methodology for the real time analysis of flavor release to give a current view of these important fields. The physicochemical study of volatile compounds was a subject for early scientific research and Henry's Law is an excellent example from that era and still widely used to express the partial pressure of a compound above an aqueous solution (10). In the late 1960s and early 1970s, partition coefficients for many flavor volatiles were published from the U S D A Western Regional Laboratory, an institution that has contributed so much to flavor research in the last thirty years (77). A sensory approach to studying hydrocolloid-aroma interactions was demonstrated by Pangborn (12) and later further expanded by also correlating with headspace results from diverse systems (13). The 1970s also saw seminal work on the interactions of starch with aroma compounds through the work of Solms' group (14, 15). In 1977, Voilley published results to explain why the addition of solutes to an aqueous solution of volatiles could affect the concentration of volatiles in the headspace (16) and has since contributed much to our knowledge of the physical chemistry of flavor. In the 1980's, Kinsella's group reported data on protein-flavor binding (5) and an A C S Symposium considered progress in flavor encapsulation (77). This process was originally intended to protect flavors from deterioration during storage but is now often used to deliver flavor compounds at the appropriate point during food preparation and consumption. Lee demonstrated flavor release from a simple device, which mimicked some of the processes that occur in-mouth during eating (18) and was followed by more complex mouth simulators (19-21). McNulty approached flavor release from a theoretical modeling angle and predicted flavor release from emulsion systems of varying oil/water composition (22). In the late 80's, a series of papers describing flavor release in mouth, was published by Unilever researchers. These publications contained many interesting ideas and hypotheses. For example, an Overbosch paper in 1986 (23-25), redefined Steven's Law to include the effect of adaptation on the intensity of the chemical stimulus. Although intuitively correct, no data have yet been published to support this hypothesis. The 1990's saw progress in several areas. Models for flavor release were proposed by de Roos (26, 27). These were based on a mixture of mechanistic and empirical principles but, most importantly, they were validated with data from several different food systems. Subsequently, Harrison and Hills published a series of



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mechanistic models for flavor release, which tackled difficult release mechanisms like in-mouth solubilisation and melting of thermo-reversible gels. The widespread use of GC-olfactometry to evaluate the contribution of individual flavor compounds to overall flavor and to determine odor thresholds was a result of pioneering work at Cornell by Acree (28) and in Munich by Grosch and coworkers (29). In fact, the advances in related flavor chemistry fields (e.g. determination of key odor-active compounds, reaction mechanisms, flavor generation, analytical methodology) have generated the technical platform for flavor release research. Methods for measuring flavor release in vivo were also developed. Initial methods, based on trapping (30-32) to increase analyte concentration prior to analysis, were superceded by direct on-line methods (33-36). In the area of flavor matrix binding, a concerted research program in France saw inter-disciplinary teams studying binding of volatiles to β-lactoglobulin. Another French-led activity was the European Union COST96 action, a program that funded a series of meetings between 1995 and 1999 to discuss the interactions between small molecules (mainly flavor compounds) and matrices. There were four separate strands covering instrumental methods, dynamic and sensory methods, theoretical modeling, and kinetic and thermodynamic constants. These meetings brought together practitioners from all over Europe, mainly from academia but also, as word spread, from the food industry and further afield so that a world-wide picture of flavor research could be obtained. The C O S T action was both a forum for learning from other colleagues and a platform for the dissemination of new ideas; it was a huge success.

Current Key Areas of Published Research Looking at the scientific literature and the contributions to this book, it is obvious that flavor-matrix interaction is a major research area. Within this category, the work ranges from understanding interactions of flavor compounds with solutes in aqueous media, to interactions with macromolecules (proteins and polysaccharides) and with food structure (e.g. micelles, gels). In vivo and dynamic release, modeling, and relating analytical results to human perception are the other major categories. There is no doubt that the 1990's saw a small but discernible step forward in terms of method development and these are now being applied assiduously to try to understand flavor release.

Future Areas for Research Instrumental - Sensory Correlation Our ability to relate sensory properties to flavor compounds is very limited. There are some mathematical relationships that link the amount of a single compound with its perceived intensity but they have only been tested under very simple conditions. Now that dynamic flavor release can be measured routinely, the validity of these relationships should be tested more rigorously. The importance of time in determining flavor intensity and quality also needs attention. However, these



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Psychophysical Laws apply only to single compounds and there are few hypotheses to explain why a particular mixture of flavor chemicals is perceived as "good" whereas a slightly different formulation is not so well-received. In this area, we should draw from the experience already available from other sensor systems like vision and hearing and examine innovative ideas such as those proposed by Booth (37) and the receptor model proposed by Ennis (38). These types of studies will need groups of people working together with different specialities. Sensory scientists and flavor chemists are seeing that the relationship between headspace G C - M S and quantitative descriptive analysis is not always evident and perhaps more basic studies will be necessary to link the two disciplines. In this aspect, statisticians and sensory psychologists may help to connect chemistry with perception.

In-Vivo Analysis As the number of labs participating in experiments measuring flavor release from humans increases, the goal to better link chemical and sensorial data will hopefully be met. Although requiring specialized equipment, this area has high promise for resolving many questions regarding the importance of timing of volatile release and specific enzymatic or temperature-related changes that can occur in the mouth during eating. A review of this area shows which types of techniques are suitable for in-vivo analysis (see chapter by Taylor and Linforth in this book).

Molecular Level of Food-Flavor Interactions Global methods for measuring interactions such as sensory evaluation or headspace analysis have demonstrated the degree of aroma compound retention by various food ingredients. More specific methods such as Hummel and Dreyer using H P L C (see chapters by Semenova et al.) can allow binding parameters to be measured, which give information about the number of binding sites and strengths of the bonds. Spectroscopic techniques such as N M R (see Jung et al chapter), Electronic Paramagnetic Spectroscopy (Goubet et al. chapter), and FT-IR (Lubke et al chapter) can be used to determine the region or specific binding site on the macromolecule. Information on interactions at the molecular level can help in the design of encapsulation agents and also in studies of how changing the structure of the macromolecule or functional groups of the compound, may change the binding parameters.

Texture-Structure Influences on Flavor Release Liquid systems are relative simple to study and are necessary for forming a basic understanding of physical partitioning and release. When moving to viscous or solid systems, more parameters are involved such as diffusion and breakdown into


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\ articles, as seen in the complex models for solid systems developed by Harrison and Hills (see chapter in this book). Real foods are often even more complex, composed of several different solid phases, and whose characteristics can be regarded more generally as texture or microscopically as structure. Two papers in this book looked specifically at the influence of gel texture and emulsion structure (see chapters by Gwartney and Charles). Benefits can begin to be reaped in this field when theoretical models predict the experimental and sensory release profiles.


Non-Volatile Release Analytical research in flavor release has mainly concentrated on the volatile fraction, while only through sensory analysis is the effect of non-volatiles on flavor perception studied. Those who have chewed chewing gum past the point of sugar release where the gum seems flavorless (or liked the aroma but not the consumed flavor of bitter coffee) have well noted that the sensations of taste have strong influences on perceived flavor. The role of taste-active substances such as organic acids, bitter peptides, astringent polyphenols, salty ions, or meaty glutamic acid in modifying flavor release, more specifically on the perceptual processing level, may be a relevant area for future investigations.

Conclusions The past, present, and future developments in flavor release all have the overall final goal of giving a value-added flavor perception to the consumer. This entails using a flavoring that has the correct amount, timing, and quality of flavor. Flavor release studies can provide this information from static or dynamic release data that emanate from specific release situations, such as in-mouth release. Optimizing flavor for products in the 2000s may rely heavily on this fundamental knowledge.

References 1. 2. 3. 4. 5. 6.

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Flavor Release: A Rationale for Its Study - ACS Publications

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