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Understanding flavour quality: relating sensory to chemical and physical data
Conference Report
Understanding flavour Food flavour cannot be measured directly by instruments; it is an interaction of product and consumerl. Rapid progress in the development of ever more sensitive analytical instrumentation has allowed the measurement of more compounds in foods at lower and lower concentrations, and the number of compounds shown in the Volatile Compounds in Food Iistings 2 has recently shown a near-linear increase. However, the rest of the equation, the interaction with the consumer, has been very much less well understood, and the increase in data has not been matched by a corresponding increase in understanding. This conference was jointly organized by Elsevier, Oliemans Punter & Partners, and Sensory Research Laboratories Ltd to review current progress in understanding the interaction. Seven technical sessions and a poster display were arranged over three days to discuss different aspects of the problem of understanding flavour. The sessions were intended to illuminate general aspects of the problem, rather than examples of the methodology applied to specific technical questions. In this respect the organizers were largely successful, though some of the contributed papers and posters did deal with particular products or processes. The first session presented an overview of the problems, the kinds of data that might be available, and a view of some of the methods that might be used to improve understanding (A.A. Williams, Sensory Research Laboratories Ltd, UK). This paper served well as an introduction to the conference as a whole, and told 'the story so far' by picking out significant areas of past progress and outlining the questions to be tackled in later presentations. J-M. Sieffermann (ENSIA - Ecole Nationale Superieure des Industries Agricoles et Alimentaires, France) discussed some of the methodological considerations of sensory profiling, dealing with what could be described as 'data optimization'. The final paper of the session (Terry Peppard, Givaudan-Roure, USA) would have been better placed in the later session on mathematical modelling. It discussed the use of chemometric techniques such as principal components analysis and partial least squares regression analysis for exploratory data analysis, prediction and classification. A marker for the future appeared here, in a brief introduction to the use of neural network simulations to model complex relationships.
Biological and psychological approaches The second session, 'The biological system', was opened by Doron Lancet (Weizmann Institute of Science, Israel), who reviewed some recent progress in understanding the interactions at the heart of the system: those between odorant and receptor. The mechanism of
*Held in Bristol, UK, 20-23 September 1992 '.R. Piggott is at the Department of Bioscience & Biotechnology, University of Strathclyde, l31 Albion Street, Glasgow, UK G1 ISO.
Trends in Food Science & Technology December 1992 [Vol.
31
quality: relating sensory to chemical and physical data* J.R. Piggott
action of olfaction has been the subject of intense speculation and hypothesis over the years, and it appears that the techniques of molecular biology are finally beginning to yield an understanding of the structure and function of the system. We are still far from being able to predict the odour of a single compound, however, let alone that of a natural flavour mixture. For the chemist or food scientist trying to understand a flavour, one of the most far-reaching consequences of this work is that the complexity of the system and the number of possible individual variations suggests that everyone might smell things differently. It is possible that a complete understanding of flavour might be impossible, since every observer might need to be modelled individually. Joseph Brand (Monell Chemical Senses Center, USA) concluded this session with a review of receptor mechanisms for taste, olfaction and chemesthesis. The next session was devoted to psychological and physiological factors. David Booth (University of Birmingham, UK) provided a useful antidote to the chemical-biological approach of most of the other speakers, and stressed that the processes that relate the physicochemical stimulation of receptors to sensory data operate within the mind of the assessor. Hence, experiments should be designed and analysed taking account of the principles of experimental psychology. Florence Heil (INRA - Institut National de la Recherche Agronomique, France) then gave an account of an experiment designed to assess the impact of the level of information provided on the acceptance of novel food items in an institution menu. This neatly illustrated one of the many factors other than sensory character that affect acceptability of food, and also showed that French women were less tolerant than French men of the experimental products. It might be entertaining to repeat this kind of experiment in other cultures. The next paper, presented by Ulrich Fischer (University of Hannover, Gennany), addressed physiological factors, and showed that differences in salivary flow while tasting wines could be partially responsible for individual variations in perception.
©l g92, Elsevier S(
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Puhlishers l to, (UK)
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The fourth session, on 'Interactive approaches to understanding relationships', contained a mix of papers on psychological aspects and on data optimization. Jan Frijters (Landbouw University, The Netherlands) and David Laing (University of Western Sydney, Australia) discussed different aspects of the perception of mixtures, and demonstrated that the perception of even simple mixtures of two taste or odour chemicals is not straightforward. In particular, Laing's results suggested that most observers can pick out only about three of the components of mixtures of only a few odorants. It seems that chefs and perfumers must have a different strategy for assessing a typical mixture of odours or flavours in a food or perfume. The complexity of real mixtures of possibly hundreds of volatile flavour compounds seems to render a genuine understanding impossible. Stephen Ellis (Arthur D. Little, Inc., USA) presented two papers on the many factors that can affect threshold measurements, and on sensorydirected chemical analysis. This paper complemented Terry Acree's (Cornell University, USA) on gas chromatography - olfactometry, and both suggested methods to guide the analysis of complex flavour mixtures.
Structure-activity relationships and flavour release The fifth session, on 'Structure-activity relationships', contained two papers dealing with smell and taste. John Dearden (Liverpool John Moores University, UK) described the background of research in structure-activity relationships, largely in pharmacology, and presented some examples of successful applications to the study of olfaction. Recent work has been moderately successful in predicting odour thresholds from molecular data for homogeneous groups of compounds, but we are still a long way from being able to predict the smell of a new molecule. Gordon Birch (Reading University, UK) described some recent work in understanding sweet taste chemoreception. It is striking how much progress has been made in understanding the chemical senses, and particularly taste. J.P.M. Plug (Unilever Research, The Netherlands) opened the following session with a paper on flavour interactions and the release dynamics of flavour materials from foods in the mouth. This paper and a subsequent one by Johanna Bakker (AFRC Institute of Food Research, UK) clearly demonstrated the importance of considering the rate of release of flavouring materials from foods, not just their concentrations in foods. The use of headspace analysis, where it is possible, is a step in the right direction, but the mouth is a very different environment from a sample vial, and in some cases it is necessary to sample from within the mouth or nose to determine which stimuli are available at the receptors. Pieter Punter (Oliemans Punter & Partners, The Netherlands) described the use of timeintensity measurements to study the dynamics of flavour release and perception in the mouth, and Hal MacFie (AFRC Institute of Food Research, UK) picked up the topic of individual variability shown by time-intensity
330
data, discussing methods of averaging such data to reduce its quantity without losing the essential features.
Mathematical modelling The final technical session covered mathematical modelling of sensory-physicochemical relationships. Hal MacFie discussed the principles of multivariate modelling of such relationships, and then generalized Procrustes analysis was discussed in more detail as a potential approach (Garmt Dijksterhuis, Oliemans Punter & Partners, The Netherlands). The next paper (R. Virgili, Universita di Parma, Italy) described an application, using generalized Procrustes analysis and partial least squares regression analysis, to a study of relationships between analytical and sensory data in Italian ham. Finally, two papers (Peter Smith, Quantum Chemical, USA; Laurent Bochereau, ENSIA, France) described the principles and some applications of neural networks to modelling relationships between sets of data.
Future directions The posters covered a range of topics, from data acquisition and optimization to mathematical modelling, and provided many examples of applications of the techniques covered in the oral sessions to real problems. As in many conferences, the posters provided a good indication of what is actually being done in laboratories worldwide, including some surprising results. The most unexpected may have been those presented by Ikuo Ohara (Kobe Womens University, Japan) who showed that, in rats, a diet containing bitter-tasting substances (chamomile and quinine) was associated with elevated serum ~-endorphin levels. The authors concluded that some bitter tastes mediate neuroendocrine reflexes, but did not evaluate the biological significance of the elevated levels found. In summary, three topics seemed to be the most important, and the areas most likely to repay new work. The first is flavour release - what happens in the mouth, the effect of food structure, and what happens when a real food is eaten. Secondly, there appear to be good opportunities for progress in looking at chemoreceptor mechanisms. Thirdly, neural networks seem to offer the hope of solving some of the more intractable mathematical modelling problems. In conclusion, the organizers managed the task they set themselves very successfully, and the conference matched expectations well. The proceedings should provide a valuable future reference to the state of the art, rather than simply a snapshot of the present situation.
References 2
von Sydow. E. (1971) Food Technol. 25(11. 40-45 Maarse. H. and Visscher. CA. (1989) Volatile Compounds in FoodQualitative and Quantitative Data. TNO-CiVO Food Analysis Institute. Zeist. The Netherlands
Trends in Food Science & Technology December 1992 [Vol. 31
Understanding flavour Food flavour cannot be measured directly by instruments; it is an interaction of product and consumerl. Rapid progress in the development of ever more sensitive analytical instrumentation has allowed the measurement of more compounds in foods at lower and lower concentrations, and the number of compounds shown in the Volatile Compounds in Food Iistings 2 has recently shown a near-linear increase. However, the rest of the equation, the interaction with the consumer, has been very much less well understood, and the increase in data has not been matched by a corresponding increase in understanding. This conference was jointly organized by Elsevier, Oliemans Punter & Partners, and Sensory Research Laboratories Ltd to review current progress in understanding the interaction. Seven technical sessions and a poster display were arranged over three days to discuss different aspects of the problem of understanding flavour. The sessions were intended to illuminate general aspects of the problem, rather than examples of the methodology applied to specific technical questions. In this respect the organizers were largely successful, though some of the contributed papers and posters did deal with particular products or processes. The first session presented an overview of the problems, the kinds of data that might be available, and a view of some of the methods that might be used to improve understanding (A.A. Williams, Sensory Research Laboratories Ltd, UK). This paper served well as an introduction to the conference as a whole, and told 'the story so far' by picking out significant areas of past progress and outlining the questions to be tackled in later presentations. J-M. Sieffermann (ENSIA - Ecole Nationale Superieure des Industries Agricoles et Alimentaires, France) discussed some of the methodological considerations of sensory profiling, dealing with what could be described as 'data optimization'. The final paper of the session (Terry Peppard, Givaudan-Roure, USA) would have been better placed in the later session on mathematical modelling. It discussed the use of chemometric techniques such as principal components analysis and partial least squares regression analysis for exploratory data analysis, prediction and classification. A marker for the future appeared here, in a brief introduction to the use of neural network simulations to model complex relationships.
Biological and psychological approaches The second session, 'The biological system', was opened by Doron Lancet (Weizmann Institute of Science, Israel), who reviewed some recent progress in understanding the interactions at the heart of the system: those between odorant and receptor. The mechanism of
*Held in Bristol, UK, 20-23 September 1992 '.R. Piggott is at the Department of Bioscience & Biotechnology, University of Strathclyde, l31 Albion Street, Glasgow, UK G1 ISO.
Trends in Food Science & Technology December 1992 [Vol.
31
quality: relating sensory to chemical and physical data* J.R. Piggott
action of olfaction has been the subject of intense speculation and hypothesis over the years, and it appears that the techniques of molecular biology are finally beginning to yield an understanding of the structure and function of the system. We are still far from being able to predict the odour of a single compound, however, let alone that of a natural flavour mixture. For the chemist or food scientist trying to understand a flavour, one of the most far-reaching consequences of this work is that the complexity of the system and the number of possible individual variations suggests that everyone might smell things differently. It is possible that a complete understanding of flavour might be impossible, since every observer might need to be modelled individually. Joseph Brand (Monell Chemical Senses Center, USA) concluded this session with a review of receptor mechanisms for taste, olfaction and chemesthesis. The next session was devoted to psychological and physiological factors. David Booth (University of Birmingham, UK) provided a useful antidote to the chemical-biological approach of most of the other speakers, and stressed that the processes that relate the physicochemical stimulation of receptors to sensory data operate within the mind of the assessor. Hence, experiments should be designed and analysed taking account of the principles of experimental psychology. Florence Heil (INRA - Institut National de la Recherche Agronomique, France) then gave an account of an experiment designed to assess the impact of the level of information provided on the acceptance of novel food items in an institution menu. This neatly illustrated one of the many factors other than sensory character that affect acceptability of food, and also showed that French women were less tolerant than French men of the experimental products. It might be entertaining to repeat this kind of experiment in other cultures. The next paper, presented by Ulrich Fischer (University of Hannover, Gennany), addressed physiological factors, and showed that differences in salivary flow while tasting wines could be partially responsible for individual variations in perception.
©l g92, Elsevier S(
j{-,rI( t'
Puhlishers l to, (UK)
329
The fourth session, on 'Interactive approaches to understanding relationships', contained a mix of papers on psychological aspects and on data optimization. Jan Frijters (Landbouw University, The Netherlands) and David Laing (University of Western Sydney, Australia) discussed different aspects of the perception of mixtures, and demonstrated that the perception of even simple mixtures of two taste or odour chemicals is not straightforward. In particular, Laing's results suggested that most observers can pick out only about three of the components of mixtures of only a few odorants. It seems that chefs and perfumers must have a different strategy for assessing a typical mixture of odours or flavours in a food or perfume. The complexity of real mixtures of possibly hundreds of volatile flavour compounds seems to render a genuine understanding impossible. Stephen Ellis (Arthur D. Little, Inc., USA) presented two papers on the many factors that can affect threshold measurements, and on sensorydirected chemical analysis. This paper complemented Terry Acree's (Cornell University, USA) on gas chromatography - olfactometry, and both suggested methods to guide the analysis of complex flavour mixtures.
Structure-activity relationships and flavour release The fifth session, on 'Structure-activity relationships', contained two papers dealing with smell and taste. John Dearden (Liverpool John Moores University, UK) described the background of research in structure-activity relationships, largely in pharmacology, and presented some examples of successful applications to the study of olfaction. Recent work has been moderately successful in predicting odour thresholds from molecular data for homogeneous groups of compounds, but we are still a long way from being able to predict the smell of a new molecule. Gordon Birch (Reading University, UK) described some recent work in understanding sweet taste chemoreception. It is striking how much progress has been made in understanding the chemical senses, and particularly taste. J.P.M. Plug (Unilever Research, The Netherlands) opened the following session with a paper on flavour interactions and the release dynamics of flavour materials from foods in the mouth. This paper and a subsequent one by Johanna Bakker (AFRC Institute of Food Research, UK) clearly demonstrated the importance of considering the rate of release of flavouring materials from foods, not just their concentrations in foods. The use of headspace analysis, where it is possible, is a step in the right direction, but the mouth is a very different environment from a sample vial, and in some cases it is necessary to sample from within the mouth or nose to determine which stimuli are available at the receptors. Pieter Punter (Oliemans Punter & Partners, The Netherlands) described the use of timeintensity measurements to study the dynamics of flavour release and perception in the mouth, and Hal MacFie (AFRC Institute of Food Research, UK) picked up the topic of individual variability shown by time-intensity
330
data, discussing methods of averaging such data to reduce its quantity without losing the essential features.
Mathematical modelling The final technical session covered mathematical modelling of sensory-physicochemical relationships. Hal MacFie discussed the principles of multivariate modelling of such relationships, and then generalized Procrustes analysis was discussed in more detail as a potential approach (Garmt Dijksterhuis, Oliemans Punter & Partners, The Netherlands). The next paper (R. Virgili, Universita di Parma, Italy) described an application, using generalized Procrustes analysis and partial least squares regression analysis, to a study of relationships between analytical and sensory data in Italian ham. Finally, two papers (Peter Smith, Quantum Chemical, USA; Laurent Bochereau, ENSIA, France) described the principles and some applications of neural networks to modelling relationships between sets of data.
Future directions The posters covered a range of topics, from data acquisition and optimization to mathematical modelling, and provided many examples of applications of the techniques covered in the oral sessions to real problems. As in many conferences, the posters provided a good indication of what is actually being done in laboratories worldwide, including some surprising results. The most unexpected may have been those presented by Ikuo Ohara (Kobe Womens University, Japan) who showed that, in rats, a diet containing bitter-tasting substances (chamomile and quinine) was associated with elevated serum ~-endorphin levels. The authors concluded that some bitter tastes mediate neuroendocrine reflexes, but did not evaluate the biological significance of the elevated levels found. In summary, three topics seemed to be the most important, and the areas most likely to repay new work. The first is flavour release - what happens in the mouth, the effect of food structure, and what happens when a real food is eaten. Secondly, there appear to be good opportunities for progress in looking at chemoreceptor mechanisms. Thirdly, neural networks seem to offer the hope of solving some of the more intractable mathematical modelling problems. In conclusion, the organizers managed the task they set themselves very successfully, and the conference matched expectations well. The proceedings should provide a valuable future reference to the state of the art, rather than simply a snapshot of the present situation.
References 2
von Sydow. E. (1971) Food Technol. 25(11. 40-45 Maarse. H. and Visscher. CA. (1989) Volatile Compounds in FoodQualitative and Quantitative Data. TNO-CiVO Food Analysis Institute. Zeist. The Netherlands
Trends in Food Science & Technology December 1992 [Vol. 31