Volatile Compound Release during Consumption: A Proposed Aroma

Chapter 33

Volatile Compound Release during Consumption: A Proposed Aroma Stimulus Index 1

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ConorM.Delahunty and Brendan Guilfoyle

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 29, 2018 | https://pubs.acs.org Publication Date: September 7, 2000 | doi: 10.1021/bk-2000-0763.ch033

1

Department of Food Science and Technology, Division of Nutrition, University College Cork, Cork, Ireland Mathematics Department, IT Tralee, Tralee, Kerry, Ireland 2

The flavor of a food can be described as the generally expressed response by a number of consumers to the food's stimulus of their senses of smell, taste, and trigeminal mouthfeel that occurs during and immediately after consumption. To fully understand flavor it is necessary to understand the nature of the flavor stimulus, the mechanisms of sensory perception, and the psychology of the expressed response. Studies that had an objective to understand the nature of the aroma stimulus have resulted in quantitative data of volatile compounds released in the mouth of individual consumers during consumption of different food types. Differences in the release of total volatile compounds from food to food and differences in the release of total volatile compounds between individuals have been observed. However, in many cases, the total quantity of all volatile compounds released during consumption was similar in proportion from one consumer to another. When volatile release data were related in this regard it was found that an index, termed here the Aroma Stimulus Index (ASI), could be used to quantify this relationship. In this paper, we also postulate that this index is constant in time during consumption and we consider its potential shortfalls and applications.

The study of flavor release from foods has received much attention in recent years, recognized most recently by a European Union Concerted Action (1) and this dedicated Symposium (2). Volatile compounds released from food contribute much to perceived flavor quality, and therefore knowledge of the relative contribution to flavor of those found in a food, or produced during consumption of a food, is sought. In recent years apparatus and methodology to measure volatile compound release in the mouth have been developed to study food systems (3-6), and now there are data of the

© 2000 American Chemical Society

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

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406 total, and the dynamics, of volatile compounds released by different consumers eating the same food (3-17).


Aroma Stimulus Index Theory The quantity and balance of volatile compounds, which are released from a food during consumption, and are available for perception, represent an aroma stimulus. A n individual aroma stimulus is influenced by the composition and structure of the food and by the physiology of the consumer. In studies of volatile compounds released from cheese during consumption, O'Riordan et al. (10) and Delahunty et al. (11) found that although the total quantity of a compound released from any single cheese varied considerably from one consumer to another, there was a relationship between consumers of that cheese. This relationship was that the difference between any two consumers in the total quantity of any individual compound released during consumption of the cheese was a constant across all compounds and by scaling out this difference between the consumers, differences in volatile compounds released between cheeses could be determined. Ingham et al. (14) found a similar relationship

Figure 1. Representation of the relationship between three consumers (A, Β and C) of a hypothetical food which released six compounds during consumption (including compound X and compound Y). between volatile compounds released by consumers of mint sweets, regardless of whether they sucked or chewed the sweet. Van Ruth et al. (16) found this relationship between consumers of French beans, bell peppers, and leeks, and Linforth et al. (17) found this relationship between consumers of tomatoes. This finding can be illustrated as in Figure 1.


407 This common finding is interesting if one considers the relationship between the volatile compounds illustrated in Figure 1 (X and Y ) released by two consumers (A and B) in the following terms:


Concentration of X released by A Concentration of X released by Β

= Concentration of Y released by A = Concentration of Y released by Β

λ

In addition, when you consider more than two consumers, and the many more volatile compounds which are released during consumption, it could be said that any individual consumer has a certain capacity to release volatile compounds from a food (for those which were tested). Then the ability of all consumers to release volatile compounds from a given food can be represented by an Aroma Stimulus Index (ASI), and the difference between any two consumers on the index can be denoted by λ. The fact that λ is different from unity may arise from differences between consumers in physiological and habitual factors of consumption e.g. variations in saliva production (10,18), mastication behavior (10, 18, 19), time of swallow (13), and breathing volume and behavior (18). However, while these factors are extremely complex and inter-related, it is remarkable that the single constant λ appears to describe the final concentration released during consumption across different compounds.

The Aroma Stimulus Index and Dynamic Flavor Release Up to now, this paper has not considered volatile release over time. However, release from the food begins as soon as the food is placed into the mouth and continues until just after the food is swallowed. In parallel, perception of aroma begins very soon after the food is placed in the mouth and continues until all released volatile compounds (the aroma stimulus) have disappeared from the buccal cavity some time after the food is swallowed (due to adsorption onto the buccal cavity tissues and regurgitation from the stomach). The quantity and the nature of the aroma stimulus will change during the time of consumption, as will the intensity and the quality of the aroma perception. Therefore, the temporal aspects of volatile compound release and subsequent perceptions by consumers are an important dimension of perceived flavor quality (13, 18) Given the evidence presented to determine relationships between the total aroma stimulus of different consumers, we postulate that the A S I is also a constant in time during consumption (at least for the food types referenced above). This hypothesis was formed by giving consideration to the potential of any volatile compound to re establish aroma stimulus proportionality (λ) with any other compound at the time of measured total release, if it had erred from this proportionality at any time during consumption. We believe that this could not be possible. Therefore, one could consider the dynamic relationship between any two consumers (A and B) as they release any volatile compound (X) during consumption as follows:


408 Concentration for A of X at any time t = Total concentration for A of X = Concentration for Β of X at any time t Total concentration for Β of X

λ


To illustrate this relationship in an easy to understand way, assume initially that the accumulation of a volatile compound released over time of consumption, and trapped, is linear. Then, for compound X , Figure 2 would describe the release over time for consumers A and Β and the ASI postulate would hold as:

Figure 2. Linear cumulative release of a volatile compound (X) during consumption of a hypothetical food by two consumers (A and B). A measure of the quantity ofX released at two time points during consumption is illustrated at the vertical crosssections. However, it is known that the accumulation of a trapped volatile compound released from a food over time of consumption will not be linear as the rate of its release will not be constant. This is because a volatile compound's affinity for a food's compositional components (e.g. fat, protein, carbohydrate, pH) differs according to the physical and chemical properties of the volatile compound. In addition, there are also well documented mechanisms of volatile release, namely partition between bulk phases (18) and mass transfer (20), either of which can have the greatest role in release depending on the food system. Yet, it is still possible for the reasons already provided, that regardless of the shape of a cumulative release curve of any individual compound, the A S I is constant in time. Figure 3 illustrates how this can be so, where again: λ = 70/50 = a/ b



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Figure 3. Non-linear cumulative release of a volatile compound (X) during consumption of a hypothetical food by two consumers (A and B). A measure of the quantity of X released at two time points during consumption is illustrated at the vertical cross-sections.

Determining the Validity of the ASI Theory

Is the ASI Constant Across a Wide Range of Compounds? Data of many different volatile compounds released during consumption exists (although from a limited number of food types)(5,10,14,16,17). These compounds have different physical and chemical properties. However, it will be necessary to examine these data and data generated in the future more carefully to determine whether specific compound properties (e.g. degree of hydrophobicity) influence the validity of the ASI.

Is the ASI Constant for Compounds Generated During Consumption (e.g. by Enzymes) and for Compounds Already Present that are Released During Consumption? Van Ruth et al. (16) found relationships, which could be described by the ASI, between twelve consumers of French beans, bell peppers and leeks. Volatile compounds will have been generated as these plant tissues were disrupted by


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mastication. Linforth et al. (17) found that for tomatoes, when compounds were generated enzymatically during consumption, that three different consumers had closely related volatile release ratios. Data that supports the A S I has also been described by O'Riordan et al. (10) for cheese and by Ingham et al. (14) for mint sweets where compounds are not generated during consumption.


Is the ASI Constant Across Different Food Types? In the study of Van Ruth et al. (16), the same twelve consumers consumed all the three food types. Where similar compounds were released from the foods, we have further analyzed the data by Friedman two-factor analysis of variance to observe ratios of release between consumers. No significant differences were found between consumers for the three foods, which would support the ASI theory. However, much more work with foods having greater differences should be carried out.

Is the ASI Constant During Time of Consumption as Postulated? Ingham et al. (15) found that for mint sweets consumed over 300s, the relative release of compounds measured was similar for four consumers. We analyzed timeconcentration data, generated by Legger and Roozen (5), to further test the time independence theory of the ASI. Although the difference in the aroma stimulus between two of the six consumers was a consistent proportion at each of the nine time intervals measured, variable differences were found between other pairs of consumers. However, as the coefficient of variation in measurement of total quantity released is often as great as 50% between samples for the same consumer (17), this finding is perhaps not surprising when the A S I is tested at numerous time intervals during consumption. More data, and more precise data, is required for analysis before any conclusion can be made.

What are the Physiological Determinants of the Value of the ASI for any Consumer? This question may be ahead of its time. However, if the A S I is shown to have some validity then it will be a reasonable objective to categorize consumers by their ASI. One means of doing so may be to determine physiological determinants such as time of swallow, mastication rate, breathing behavior or saliva composition and production volume. Could there be differences between consumers by gender? Does a consumer's A S I change as they grow older? Are there differences between ethnic groups in the population, for example between Asian and Caucasian populations?


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Why Might the ASI be Important and What are the Potential Applications of it? If consistent differences exist between consumers in their aroma stimulus, and i f such differences can be generalized to population groups by an ASI, then there may be a need to provide foods that have tailored differences in how they release their flavor. Also, for fundamental understanding of flavor, both instrumental and sensory methodology could be optimized using this knowledge. For example, a model mouth would have a much greater role in volatile release analyses, i f it were calibrated to a particular point on the A S I . In sensory time-intensity methodology, where unaccountable differences in the data generated by assessors causes problems for data interpretation, pre-training and establishing individualized protocol for assessors using A S I information and knowledge of physiological determinants of the A S I would be advantageous and could improve the determination of instrumental / sensory relationships.

Conclusions A n Aroma Stimulus Index (ASI) that can be used to define a consumer's ability to release volatile compounds from a food has been postulated. Data we have analyzed supports this hypothesis. It is also postulated that the A S I is independent of time during consumption. However, the validity of this argument needs further investigation for general acceptance. If the A S I is shown to be a constant for consumers, it will be a very useful tool in both instrumental and sensory studies to understand flavor.

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Volatile Compound Release during Consumption: A Proposed Aroma

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