Flavour release in the mouth

Interaction’ in Pbysiol. Behav. 24, 601-605 11 Cillan, D.J. (1983) ‘Taste-Taste, Odor-Odor, and Taste-Odor Mixtures: Greater Suppression Within Than Between Modalities’ in Percept. Psychophys. 33,183-l 85 12 Kuo, Y.L., Pangborn, R.M. and Noble, AC. (1992) ‘Temporal Patterns of Nasal, Oral, and Retronasal Perception of Citral and Vanillin’ in Int. J. Food Sci. Technol. 28, 127-l 37 13 Burdach, K.J., Kroeze, J.H.A. and Koster, E.P. (1984) ‘Nasal, Retronasal and Gustatory Perception: An Experimental Comparison’ in Percept. Psychophys. 36,205-208 14 Hornung, D.E. and Enns, M.P. (1994) ‘The Synergistic Action of the Taste and Smell Components of Flavour’ in Synergy(Birch, C. and Campbell-Platt, C., eds), pp. 145-l 54, Intercept, Andover, UK 15 Opet, J.M., Pangborn, R.M., Noble, AC. and Perfetti, T.A. (1990) ‘Effect of Caffeine, Ethanol and Sucrose on Temporal Perception of Menthol’ in Chem. Senses15, 611612 16 Frank, R.A. and Byram, J. (1988) ‘Taste-Smell Interactions are Tastant and Odorant Dependent’ in Chem. Senses 13,445-455 17 von Sydow, E., Moskowitz, H., Jacobs, H. and Meiselman, H. (1974) ‘Odor-Taste Interaction of Fruit Juices’ in Lebensm.-Wiss. Technol. 7, 18-24 18 Cliff, M. and Noble, A.C. (1990) ‘Time-Intensity Evaluation of Sweetness and Fruitiness and Their Interaction in a Model Solution’ in 1. Food Sci. 55, 450-454 19 Bonnans, S. and Noble, AC. (1993) ‘Effect of Sweetener Type and of Sweetener and Acid Level on Temporal Perception of Sweetness, Sourness and Fruitiness’ in Cbem. Senses 18, 273-283 20 Stampanoni, C.R. (1995) ‘Influence of Acid and Sugar Content on Sweetness, Sourness and the Flavour Profile of Beverages and Sherbets’ in Food Qua/. frei. 4, 169-l 76

21 Matysiak, N.L. and Noble, A.C. (1991) ‘Comparison of Temporal Perception of Fruitiness in Model Systems Sweetened with Aspartame, an Aspartame + Acesulfame K Blend, or Sucrose’ in 1. Food Sci. 56, 823-826 22 Clark, CC. and Lawless, H.T. (1994) ‘Limiting Response Alternatives in Time-intensity Scaling: An Examination of the Halo-dumping Effect’ in Cbem. Senses 19, 583-594 23 Frank, R.A., Wessel, N.E. and Shaffer, C. (1990) ‘Enhancement of Sweetness by Strawberry Odor is Instruction Dependent’ in Chem. Senses 15, 576 24 Frank, R.A., van der Klaauw, N.J. and Schifferstein, H.N. (1993) ‘Both Perceptual and Conceptual Factors Influence Taste-Odor and Taste-Taste Interactions’ in Percept. Psychophys. 54, 343-354 25 O’Mahoney, M. (1991) ‘Descriptive Analysis and Concept Alignment’ in Sensory Science Theory and Applications in Foods (Lawless, H.T. and Klein, B.P., eds), pp. 223-267, Marcel Dekker 26 van der Klaauw, NJ. and Frank, R.A. (1994) ‘Matching and Scaling of Taste-Smell Mixtures: Individual Differences in Sweetness Enhancement by Strawberry Odor’ in Chem. Senses 19, 567-568 27 Roberts, D.D. and Acree, T.E. (1995) ‘Simulation of Retronasal Aroma Using a Modified Headspace Technique: Investigating the Effects of Saliva, Temperature, Shearing and Oil on Flavor Release’ in 1. Agric. food Chem. 43,2179-2186 28 Schlich, P. and McEwan, J.A. (1992) ‘Preference Mapping. A Statistical Tool for the Food Industry’ in Sci. Aliment. 12, 339-355 29 Kobal, C. and Hummel, T. (1991) ‘Olfactory Evoked Potentials in Humans’ in Smell and Taste in Health and Disease (Getchell, T.V., Doty, R.L., Bartoshuk, L.M. and Snow, J.B., eds), pp. 255-276, Raven Press 30 Plattig, K-H. (1991) ‘Gustatory Evoked Brain Potentials in Humans’ in Smell and Taste in Health and Disease (Cetchell, T.V., Doty, R.L., Bartoshuk, L.M. and Snow, J.B., eds), pp. 277-286, Raven Press

Flavour release in One of the key areas of interest in flavour science has been to reconcile the flavour composition of foods with the flavour perceived by consumers. Despite substantial increases in understanding, it is still not known how the various flavour compounds combine to produce a particular flavour experience. Prediction of flavour is therefore not possible, which hinders the development of flavours in new products.

the mouth A.J. Taylor and R.S.T. Linforth Eating is the stage at which food flavour and judged by consumers. The relationships compounds

and sensory perception

is released, sensed between flavour

are still not entirely clear.

Analysing the total flavour composition of a food does not reflect the flavour profile experienced during eating; however, new methods of analysis that model flavour release in-mouth have been developed. in the air expired

Recently, direct analysis of volatiles

through

the nose and mouth

has been

achieved during eating, confirming that the volatile profile changes over time, as had been suspected from results produced by time-intensity

sensory analysis. Data from these

analyses may explain the link between perception

and food

composition as well as providing a tool for optimizing formulation of flavours in low-fat foods. A.J.

Taylor

Biochemistry Campus,

and R.S.T. Linforth are at the Department of Applied and Food Science, University of Nottingham, Sutton Bonington

Loughborough,

UK

LEl2

5RD (fax: +44-l 1S-951-61 54; e-mail:

[email protected]).

444

the

Copyright 01996, Elsevier Science Ltd. All rights resewed. 0924 -2244/96/$15.00 PII: 50924.2244(96)10046-7

What constitutes flavour? Crudely, flavour can be considered as comprising volatile components that are sensed in the nose (aroma), non-volatile components that are sensed on the tongue (taste) along with compounds and structures that are perceived in the mouth as mouthfeel and/or texture. It is believed that aroma is more important than taste in determining overall flavour, a generalization that can be easily demonstrated by observing the difficulties subjects have identifying flavoured drinks if the air flow through their noses is prevented by pinching off the nostrils. The interactions of taste and aroma are discussed in the previous article by Ann Noble. Methods for analysing flavour have tended to concentrate on the volatile components because of their importance in overall flavour and because they are more amenable to analysis by instrumental means (e.g. by gas chromatography - mass spectrometry; GC-MS). Some sensory information can also be obtained from volatile analyses by sniffing the GC eluent to identify the odoriferous compounds by gas chromatography olfactometry, a technique that has been developed by W. Grosch (for a Trends

in Food Science

& Technology December 1996 [Vol. 71 Special Issue on Flavour Perception

review, see Ref. 1) and Acree2 into a powerful analytical tool. The analysis of non-volatile flavours has focused mainly on compounds that are sweet. Other compounds associated with taste have received less attention, probably because of the difficulty of identifying those nonvolatiles that are associated with taste in a complex high-performance liquid chromatography (HPLC) chromatogram. According to Maarse and Van den Berg3, taste is perceived much more slowly than odour; in addition, there are difficulties in sensing eluting peaks that are close together, although this can be partly overcome by tasting fractions from HPLC ‘off-line’. Furthermore, the eluents used in HPLC (buffers, organic solvents) possess strong flavours themselves and may mask the compounds of interest. This review deals solely with volatile compounds and their roles in flavour perception. Volatile sampling and analysis methods Volatiles can be sampled by several different methods before analysis. Total volatile composition requires the complete extraction of all compounds from a food, which is difficult to achieve without affecting labile compounds. Classical headspace analysis measures those volatiles contained in the air above a food, usually in a sealed system at equilibrium. The composition of the headspace depends on the partitioning of volatiles among the air phase and the different phases present in the food (such as oil and water). Other factors that affect partitioning of the compounds between the air and the food, such as surface area and temperature, also influence the headspace composition. Because the partitioning of volatiles is dynamic, many workers have opted to seal the system, allowing equilibrium to be attained. This approach simplifies the analytical procedure and provides fundamental physical data on the model systems that can be applied to real food systems, although it is doubtful whether equilibrium is actually achieved when food is eaten. Headspace measurements can also be made under non-equilibrium conditions but, because of the time taken to collect a sample, the values obtained are often averages of the volatile profile over a specified period of time. However, it is clear from sensory analysis using the time-intensity technique (in which subjects are asked to eat a food and indicate how a flavour changes over time; see Cliff and Heymann4, for review) that the flavour intensity reported by panellists changes over time in the mouth. There are, hoGever, few data available on these temporal changes with respect to individual volatiles. Neither total volatile composition, nor simple headspace profiles of whole foods, represents fully the volatile profiles experienced during eating, and this may explain why attempts at correlating total volatile or headspace volatile profiles with sensory perception studies have not been totally successful. Theoretically, the measurement of volatiles released during eating would indicate the profile sensed by the olfactory epithelium and might also provide some information on how the profiles change over time. For these reasons, alternative methods of collecting volatile flavours have been developed Trends

in Food Science

& Technology

December

1996

[Vol. 71

that take into account some of the changes that foods undergo during eating. Changes during eating Foods exist in different physical states ranging from a simple aqueous liquid (a beverage) through fat-based foods to complex heterogeneous matrices (e.g. breakfast cereals). It is therefore difficult to generalize about the changes that occur during eating; some foods are little changed by passage through the mouth, whereas others are changed markedly. For instance, a simple aqueous beverage will not be diluted by saliva to any significant extent; temperature changes, however, may be more important depending on the length of time the food is held in the mouth. In the case of fat-based foods, the melting of the fat in-mouth or the inversion of emulsion phases (i.e. change from water-in-oil to oil-in-water) can cause substantial changes in volatile release. In the case of a breakfast cereal, the chewing action and mixing with saliva serve to hydrate the matrix, changing the components from the glassy state into the rubbery state with a hydration level and temperature profile that change with each chew. The combined action of the tongue and cheeks places food between the teeth ready for each chew; the tongue also acts to collect pieces of food from around the mouth, which is crucial for mixing food with saliva in an even fashion. It is almost impossible to recreate all of the processes that occur in the mouth in model mouth systems because the motion of the teeth and tongue is extremely complex. However, using model mouths to study single factors, such as hydration, is easier to achieve. Measurement of volatile release in model systems Because of the difficulties of measuring in-mouth, attempts have been made to simulate some of the mouth processes in model systems that can be closely controlled in terms of temperature, shear rate or hydration. One of the first systems described used steel balls to disrupt food and simulate chewing while released volatiles were removed by a gas strean?. More recently, the retronasal aroma simulator system has been described by Roberts and Acree6x7,in which food can be sheared (to simulate eating), hydrated with water or saliva, and those volatiles that are released removed by a gas stream. Roozen’s group8,9 has developed a plunger system that imparts shear to food by a screw action and can also add artificial saliva to the food to simulate hydration. Other workerslO*” have hydrated samples to various extents and then measured volatile release as a function of water content. These types of experiments are excellent for providing information on the effects of factors such as hydration on volatile release. They allow large samples to be used, thus increasing the levels of volatiles present so that they can be easily detected (this is in contrast with the relatively small amounts of food that can be chewed in the mouth at any one time). However, they cannot always reflect what happens in-mouth; thus, where possible, it is wise to check the results from such model systems against real in-mouth measurements. 445

time-release curves can be drawn for individual compounds. This opens up 90 new possibilities for comparing volatile 80 time-release curves with sensory time70 intensity curves and determining which 60 compounds are involved in flavour per50 ception. Roozen’s groupzo demonstrated 40 a correlation between 2-methylbutanal 30 time-release curves and the time20 intensity curves of overall flavour in 10 chocolate. Ingham et aLzl demonstrated that different volatiles were released 0 from mint-flavoured sweets at different rates, and characterized the volatile Time (min) profiles at the start of eating and in the aftertaste. In the case of Ryvita crispbreadz2, volatile release in a model Fig. 1 system (based solely on hydration) was Trace showing the change in the levels of three volatiles present in breath as tomatoes are eaten: compared with release measured inisobutylthiazole (*), hexenal (0) and hexenol (m); each point is the mean of five replicates. mouth, and possible explanations for (Reproduced from Ref. 29.) the differences were proposed. One suggestion was that the-formation of a An alternative approach is to estimate volatile release bolus during eating reduced the effective surface area using mathematical models based on physical constants for release in-mouth compared with the model system, such as partition coefficients r2-14.Such models have been in which the surface area was constant. applied to certain fabricated foods with some success. These investigations have shown the value of following volatile release in-mouth, but progress has been Volatile measurement in-mouth slow, because the trapping methods are very timeOne of the first attempts to measure volatile release consuming, and thus it can take several weeks to obtain from food ingredients followed the levels of sulphur- sufficient data to plot a single volatile-release profile. containing compounds in expired air from humans using Ideally, a method for measuring release in real time is GC in combination with a flame photometric detector required. Soeting and Heidema23 addressed this problem (FPD)r5. Onion oil or peppermint oil was fed to subjects with their membrane interface for an electron impact and their breath samples were injected onto the GC mass spectrometer, which allowed the introduction of column; the sensitivity of the FPD helped to detect the volatile molecules into the electron impact source while low levels of volatiles present. To overcome the prob- to a large degree excluding air and water, both of which lems of sensitivity, several groups have used some form drastically affect the efficiency and general operation of of volatile trapping from breath samples to concen- the source. The membrane has some disadvantages astrate them before GC-MS analysis; trapping methods sociated with it (e.g. selective permeability); moreover, involved the use of low temperatures (cryo-trappingr6J7) the method also suffered from poor sensitivity, with or adsorbing polymers (e.g. Tenax) 8,9J8J9.These studies volatiles being analysed only in the 25-250mg kg-’ have revealed several important findings. One is that range. The technique has recently been revisited, but volatiles are released at different rates during eating and produced similar levels of performance*“. Various mass spectroscopic methods have been proposed for breath analysis25,26,although some of them Table 1. T,, and /,,,,,(k SD) of ethyl hexanoate in the expired air of two have slow response times, making them unsuitable for subjects eating strawberry-flavoured sweets” the analysis of volatile release during eating, where changes take place in periods measured in seconds. Subject L, (min) I,, x 1O-4 (peak area) A recent development by our group is an interface to allow the use of atmospheric pressure ionization MS to Subject 1 0.51 * 0.11 213+41 follow real-time volatile release from subjects during eatingz7Tz8. The release of several compounds can be folSubject 2 0.42kO.07 392k104 lowed simultaneously at concentrations of -1Oppb (v/v) “Data taken from Ref. 29. Two subjects were asked to eat strawberry-flavoured sweets and the response time is sufficiently rapid to analyse (gelatin-sugar base), but no instructions were given on how they should eat them. Expired breath-by-breath release from the nose. If the peak air was sampled from the nose; ethyl hexanoate levels were monitored by the technique heights from each breath are plotted, traces like that described in Refs 27 and 28. Each value is the mean of 12 replicates shown in Fig. 1 are obtained. Figure 1 demonstrates the J,,, Time to maximum intensity release of volatiles from a tomato during eating29. Isob,, Maximum intensity butylthiazole is a flavour component that is present in SD, Standard deviation the intact tomato and its release is thought to be due 100

446

Trends

in Food Science

& Technology

December

1996 [Vol. 71

solely to physical changes such as increased surface area when tissue is macerated in-mouth. Hexenal, however, is produced only when tomato tissue is macerated30 and lipid oxidation enzymes come into contact with lipid substrates. Hexenol is formed by the action of alcohol dehydrogenase on hexenal. In Fig. 1, isobutylthiazole is the first compound to appear in breath samples, followed by hexenal and then hexenol, which is gratifying because this is the order in which hexenal and hexenol are biosynthesized. Further development of this technique may allow the kinetics of the generation of tomato flavour to be determined in uiuo rather than by extrapolating in vitro measurements. Table 1 shows the application of the technique to investigate the differences in volatile release between subjects. Two subjects chewed sweets on 12 occasions, and some parameters commonly used in sensory timeintensity analyses (maximum intensity, I,,, and the time to maximum intensity, r,,) were calculated from the time-release curves for each individua129. It can be seen that both subjects released volatiles in a fairly consistent way; the coefficient of variation for Tmaxin each case is between 17% and 21%. However, differences between the two individuals are also evident, with subject 2 showing a more rapid increase in volatile release (earlier Tm,.) than subject 1. Subject 2 also shows a more intense volatile-release pattern (greater Z,,) compared with subject 1. These differences are probably due to factors such as variability in the chewing, breathing and saliva-flow patterns of the two subjects. The ability to follow individual volatile release breath-by-breath therefore provides new opportunities for studying volatile release in different subjects with sufficient replicates for statistical analysis over reasonable periods of time (l-2 h). Conclusions The ability to measure the volatile profile in the nose during eating and then compare it with time-intensity data represents a powerful tool that still is being investigated, following the development of suitable instrumental methods, and should allow us to further our understanding of the relationships between specific compounds and the overall sensory perception of food flavour. Few attempts 3L have been made to correlate nosespace volatile profiles with the time-intensity curves reported by panellists; however, the importance of the temporal dimension has been established, and appropriate analysis of the data is now needed. Early results have revealed interesting differences between the time-release curves and the time-intensity curves. The

techniques

now

available

should

allow

further

studies on volatile release behaviour from foods during eating, which may explain some of the subtle differences we experience among foods that are very similar, for instance between the flavour of butter and that of regular and low-fat spreads. Applications might include the development of improved flavour in fabricated foods with desirable nutritional properties, as described in Box 1. Trends in Food Science & Technology December 1996 [Vol. 71

Box 1. Future trends: The development of low-fat foods Removing fat from a food has serious effects on its favour. The over-expression of some volatile components can cause an imbalante in the flavour, which leads to poor ftavour quality. Currently, flavour formulation for these foods is targely empirical, but the ability to measure the volatile release profile in-mouth has provided flavourists with an additional tool to assist in this task,

Acknowledgements Research on aroma release at Nottingham University is supported by the BBSRC, MAFF and industrial companies including Firmenich and Micromass (formerly VG Organic). References 1 Schieberle, P. (1995) ‘New Developments in Methods for Analysis of Volatile Flavor Compounds and Their Precursors’ in Characterization of Food: Emerging Methods (Gaonkar, A.C., ed.), pp. 403-431, Elsevier 2 Acree, T.E. and Barnard, J. (1994) ‘Gas Chromatography-Olfactometry and Charm Analysis’ in Trends in Flavour Research (Maarse, H. and van der Heij, D.G., eds), pp. 21 l-220, Elsevier 3 Maarse, H. and Van den Berg, F. (1989) ‘Current Issues in Flavour Research’ in Distilled Beverage Flavour (Piggott, J.R. and Paterson, A., eds), pp. l-l 5, Horwood Ellis 4 Cliff, M. and Heymann, H. (1993) ‘Development and Use of Time-Intensity Methodology for Sensory Evaluation’ in Food Jechnol. 26, 375-385 5 Lee, W.E. (1986) ‘A Suggested Instrumental Technique for Studying Dynamic Flavor Release From Food Products’ in j. Food Sci. 51, 249-250 6 Roberts, D.D. and Acree, T.E. ‘Effects of Heating and Cream Addition on Fresh Raspberry Aroma Using a Retronasal Aroma Simulator and Gas Chromatography Olfactometry’ in 1. Agric. Food Chem. (in press) 7 Roberts, D.D. and Acree, T.E. (1995) ‘Simulation of Retronasal Aroma Using a Modified Headspace Technique: Investigating the Effects of Saliva, Temperature, Shearing and Oil on Flavor Release’ in /. Agric. Food Chem. 43,2179-2186 8 Van Ruth, S.M., Roozen, J.P. and Cozijnsen, J.L. (1995) ‘Volatile Compounds of Rehydrated French Beans, Bell Peppers and Leeks. Part I. Flavor Release in the Mouth and in Three Mouth Model Systems’ in Food Chem. 53, 15-22 9 Van Ruth, S.M., Roozen, J.P. and Cozijnsen, J.L. (1995) ‘Volatile Compounds of Rehydrated French Beans, Bell Peppers and Leeks. Part II. Gas Chromatography/Sniffing Port Analy& and Sensory Evaluation’ in Food Cbem. 54,14 10 Dalla Rosa, M., Pittia, P. and Nicoli, M.C. (1994) ‘Influence of Water Activity on Headspace Concentration of Volatiles Over Food and Model Systems’ in Ital. 1. Food SC;. 4, 421-432 11 Clawson, A.R., Linforth, R.S.T., Ingham, K.E. and Taylor, A.J. (1996) ‘Effect of Hydration on Release of Volatiles From Cereal Foods’ in Lebensm.-Wiss. Technol. 29, 158-l 62 12 Overbosch, P., Afterof, W.G.M. and Haring, P. (1991) ‘Flavor Release in the Mouth’ in Food Rev. Int 7, 137-184 13 DeRoos, K.B. and Wolswinkel, K. (1994) ‘Non-equilibrium Partition Model for Predicting Flavor Release From Chewing Gum’ in Trends in F/avow Research (Maarse, H. and van der Heij, D.C., eds), pp. 15-32, Elsevier 14 Hills, B.P. and Harrison, M. (1995) ‘Two Film Theory of Flavour Release From Solids’ in Int. 1. FoodSci. Technol. 30, 425-436 15 Mackay, D.A.M. and Hussein, M.M. (1978) ‘Headspace Techniques in Mouth Odor Analvsis’ in Analysis of Foods and Beverages (Charalambous, G., ed.), pp. 283-357, Academic Press 16 Linforth, R.S.T. and Taylor, A.J. (1993) ‘Measurement of Volatile Release in the Mouth’ in Food Chem. 48,115-l 20 17 NaBI, K., Kropf, F. and Klostermeyer, H. (1995) ‘A Method to Mimic and to Stuby the Release of Flavour Compounds From Chewed Food’ in Z. lebensm.Unters. Forsch. 201,62-68 18 Taylor, A.J. and Linforth, R.S.T. (1994) ‘Methodology for Measuring Volatile Profiles in the Mouth and Nose During Eating’ in Trends in Flavour Research (Maarse, H. and van der Heij, D.G., eds), pp. 3-14, Elsevier 19 Delahunty, C.M., Piggott, J.R., Connor, J.M. and Paterson, A. (1994) ‘Low-fat Cheddar Cheese Flavor: Flavor Release in the Mouth’ in Trends in Flavour Research (Maarse, H. and van der Heij, D.C., eds), pp. 47-52, Elsevier 20 Roozen, J.P. and Legger-Huysman, A. (1995) ‘Sensory Analysis and Oral Vapour Gas Chromatography of Chocolate Flakes’ in Proceedings of the 4th

447

21

22

23

24

25

Wartburg Aroma Symposium, Aroma, Perception, formulation, Evaluation (Rothe, M. and Kruse, H-P., eds), pp. 627-632, Deutsche lnstitut fur Ernahrungsforschung, Potsdam-Rehbrucke, Germany Jngham, K.E., Linforth, R.S.T. and Taylor, A.J. (1995) ‘Effect of Eating on the Rate of Aroma Release From Mint Flavoured Sweets’ in Lebensm..Wiss. Technol. 28, 105-l 10 Ingham, K.E., Clawson, A.R., tinforth, R.S.T. and Taylor, A.]. g996) ‘Hydration Effects on Volatile Release in the Mouth’ in Lebensm;Wiss, Technol. 29,158-l 62 Soeting, WJ. and Heidema, J. (1988) ‘A Mass Spectrometric Method for Measuring Flavour Concentration/Time Profiles in Human’ in Chem. Senses 13,607-617 Reid, W.J. and Wragg, S. (1995) ‘Flavour Release Measurement by In-breath Mass Spectrometry. 1. Development of Membrane Interface’ in Leatherhead Food RA, UK Research Report No. 724, Leatherhead Food RA, Leatherhead, UK Jordan, A., Hansel, A., Holzinger, R. and Lindinger, W. (1995) ‘Acetonitrile and Benzene in the Breath of Smokers and Non-smokers Investigated by Proton Transfer Reaction Mass Spectrometry’ in Int. 1. Mass Spectrom. /on

Process. 148, Ll-L3 26 Benoit, F.M., Davidson, W.R., Lovett, A.M., Nacson, S. and Ngo, A. (1983) ‘Breath Analysis by Atmospheric Mass Spectrometry’ in Anal. Chem. 55,805~807 27 Linforth, R.S.T. and Taylor, A.]. (1996) ‘Apparatus and Methods for the Analysis of Trace Constituents in Gases’, UK Patent Application 9615303.6 28 Taylor, A.J. and tinforth, R.S.T. (1996) ‘Improvements Relating to Volatile Compound Detection’, UK Patent Application 9615304.4 29 Linforth, R.S.T., Ingham, K.E. and Taylor, A.]. ‘Time Course Profiling of Volatile Release From Foods During the Eating Process’ in flavour Science: Recent Developments (Mottram, D.S. and Taylor, A.]., eds), Royal Society of Chemistry, London, UK (in press) 30 Calliard, T., Matthew, ].A., Wright, A.J. and Fishwick, M.J. (1977) ‘The Enzymic Breakdown of Lipids to Volatile and Non-volatile Carbonyl Fragments in Disrupted Tomato Fruits’ in 1. Sci. food Agric. 28, 863-868 31 Delahunty, CM., Piggott, J.R., Conner, J.M. and Paterson, A. (1996) ‘Comparison of Dynamic Flavour Release From Hard Cheeses and Analysis of Headspace Volatiles From the Mouth with Flavour Perception During Consumption’ in 1. Sci. Food Agric. 71, 273-281

Physiological effects of flavour perception Karen 1. Teff

The flavour of food, a critical determinant tion and consumption,

of consumer selec-

also has the potential to regulate how

ingested food is absorbed

and metabolized.

The underlying

physiological pathways that facilitate the relationships between flavour and nutrient metabolism are neural connections among the oropharyngeal

region, the brain and periph-

eral tissues. Recent studies have shown that the flavour of food can improve nutrient metabolism in human subjects. Future work improve

should

nutrient

identify

metabolism

how

individual

flavours

in both normal

can

and clinical

populations.

Food flavour is an essential determinant of what an individual chooses to purchase and consume. Thus, improving and enhancing the flavour of food are of primary concern to the food industry. What may be less apparent is that, in addition to contributing to the hedonic value of a food, flavour has the potential to influence human physiological function. The relationship between flavour and physiology has been recognized since the turn of the century, when Karen 1. Teff is at the Monell Chemical Philadelphia,

PA 19104,

Senses Center,

USA (fax: +l-215-898-2084;

3500 Market e-mail:

upennedu).

448

Street,

kteff@pobox.

Copyrlght01996, Elsevier Science Ltd. All rights reserved. 0924 -2244/96/$15.00 PII: 50924.2244(96)10047-9

Pavlov conducted his ground-breaking experiments demonstrating that the presence of food in the oral cavity enhanced protein digestion in dogs’. Similar effects have been demonstrated in humans; Wolf and Wolff* reported on an individual who, in 1895, required a gastrostomy (creation of an artificial opening in the stomach) after burning his oesophagus. Through trial and error, the individual learned that in order to gain weight and satisfy his appetite, he had to taste and chew the food before its insertion into his stomach. These early studies imply that food flavour improves nutrient metabolism and influences satiety. The aim of this review article is to discuss the mechanisms that enable flavour to influence nutrient metabolism and to review the evidence establishing a role for flavour in human physiology.

Physiological pathways mediating the effects of flavour on nutrient metabolism The underlying physiological pathways that facilitate the relationships between flavour and nutrient metabolism are neural connections among the oropharyngeal region, the brain and peripheral tissues (Fig. 1). Stimulation of receptors in the mouth, nasal cavity and throat by food-related sensory stimuli activates neural fibres leading to the central nervous system, where specific brain areas are used as relay stations, integrating sensory information and initiating appropriate responses. The nucleus of the tractus solitarius is the primary site that receives information concerning taste, and when activated it sends a message to the dorsal motor nucleus of the vagus, where the efferent fibres of the vagus nerve originate3. The vagus nerve, part of the parasympathetic nervous system, branches extensively and innervates many of the tissues involved in nutrient metabolism including the stomach, intestine, pancreas and liver. Vagal activation releases biologically active substances from the innervated tissues. Saliva4, gastric acid5, enzymes from the exocrine pancreas6 and hormones Trends

in Food Science

& Technology December 1996 [Vol. 71 Special Issue on Flavour Perception