Chapter 30
The Relationship between Carvone Release and the Perception of Mintyness in Gelatine Gels Flavor Release Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 08/05/18. For personal use only.
T. A. Hollowood, Rob S. T. Linforth, and Andrew J. Taylor Samworth Flavour Laboratory, Division of Food Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD, United Kingdom
A sensory panel rated the intensity of minty flavor in a 6% gelatine gel, containing varying concentrations of carvone. The flavor was assessed using Magnitude Estimation and Time Intensity Methods. In addition, the quantity of carvone released from the gel and reaching the assessor's nose was measured, breath by breath during eating, using the M S Nose™. The results showed that the quantity of volatile delivered to the nose was directly proportional to the concentration in the sample, however, the absolute quantity varied greatly between individuals. Furthermore, the relationship between perceived intensity and sample concentration was linear for both types of sensory data. Neither the speed of eating nor the concentration of volatile reached in-nose, affected an individuals ability to judge intensity. There was evidence to suggest, however, that the speed of eating affected the level of adaptation to the carvone stimulus.
Flavor can be defined as a complex pattern derived from the interaction of volatile, non-volatile, trigeminal and textural properties of food (I). Aroma delivery is arguably the most important aspect of flavor and, therefore, it is important to understand its impact on consumer perception. The type of volatile compounds present, their concentrations, and their interactions with non volatile components are important factors. Historically, Stevens Law dictated that the relationship between perceived and actual intensity of a stimulus was governed by the power law I=k(S-S*) (2). This included terms for stimulus intensity (S) and perception threshold (S*) but did not account for adaptation to a stimulus during prolonged exposure, nor the effect of varying the stimulus over time (3). Later derivations of the original equation included a term for the adapted perception threshold (4-5). Volatile components in food interact with non volatile components to produce a heightened response. For example, the perception of n
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mintyness has been found to correlate with the release of sugar from chewing gum rather than simply the release of menthone (6). The means by which the components of a foodstuff become available to the sensory receptors is another controlling factor in perception. The breakdown of the food matrix during mastication coupled with the transport of gaseous volatiles to the olfactory epithelium, are governed by aspects of human physiology, behaviour, and anatomy; not to mention food structure, ingredients, and interactions. Early investigations of perception and adaptation used olfactometry and other orthonasal sniffing methods (7). Unfortunately sniffing a volatile compound cannot compare with the complexities of eating as described above, given that contact with the olfactory epithelium is greater for expired air than inspired air (8). Further work showed that sniffing and inhaling citral and vanillin solutions were consistent with Stevens Law, while sipping gave a very different response due to the influence of adaptation and trigeminal stimulation (9). It has been demonstrated using gels of varying gelatine concentration, flavored with furfuryl acetate, that the rate of volatile release has more influence on the intensity of perception than the absolute quantity released (JO). The variation among individuals makes it difficult to provide more than general trend information about how volatile release will be affected by certain parameters. Variation observed during previous breath by breath analysis and release profile studies was probably due to differences in the rate of chewing, frequency of swallowing, saliva flow, rate of breathing,and anatomy (11-12). Detailed work looking at the effect of mastication patterns has shown that perception of flavor is linked to an individuals chewing style (13). The following paper looks at the aspects of aroma delivery from a model foodstuff containing volatile, non volatile and textural properties. It seeks to find the differences that occur among individuals and how do these affect their perception.
Materials Sensory Panel A sensory panel consisting of 4 men and 10 women aged between 25 and 60 were selected on the basis of their ability to discriminate between samples of different intensity. Selection procedures included the ranking of citric acid solutions of differing concentrations and magnitude estimation (14) of orange flavor intensity using a cordial diluted to different strengths. Training of the panel, during 8x 3hour sessions, and subsequent experience focused on Time Intensity (TI) as a means of continually assessing the intensity of specific attributes (e.g. mintyness or sweetness) during eating and thereafter. Samples The base gel mixture was prepared using 30% granulated sugar, 35% water, 40% glucose syrup, 6% gelatine (Type A - US mesh 20, 250 bloom), and 1% citric acid.
372 A l l quantities were on a w/w basis. The molten gel was mixed with quantities of carvone dispersed in propylene glycol to give final volatile concentrations of 125, 250, 500, 750 and 1000 ppm (mg/Kg). Individual samples were cut to produce cubes of gel weighing 6g +/- l g . Samples were stored at 4°C but allowed to equilibrate to room temperature (18-20°C) prior to eating. The selected concentration range fell between the individuals recognition and terminal thresholds.
Equipment
In-nose volatile concentration was monitored by the M S Nose™ (Micromass Manchester U K ) using an Atmospheric Pressure Chemical Ionisation - Mass Spectrometry source (APCI-MS) Breath was sampled into the A P C I - M S from a plastic nosepiece inserted into one nostril. Volatile compounds present were ionised and detected on the basis of their characteristic ion masses. Flavor intensity was measured by means of a pivoting lever set against a 10-point scale. The positioning of the lever and the speed of measurement corresponded to the changing flavor intensity perceived in-mouth. The lever was attached to a 9-volt battery and performed as a potentiometer allowing more or less current to flow, dependent on its position. The output from the TI lever was interfaced to a computer and the electrical signal was converted into a trace showing perceived changes in flavor intensity in real time.
Procedure Magnitude Estimation
Five samples containing different carvone concentrations were presented simultaneously to the panel. To reduce a sampling order effect, each assessor received the samples in a different random order. The panel was instructed to assess the first sample and attribute a score of 100 to its perceived minty flavor. A l l subsequent samples were scored for minty flavor relative to this sample. Mineral water and unsalted crackers were provided as palate cleansers.
Time Intensity and In-nose Volatile Release
TI and in-nose measurement of volatile release were performed simultaneously. Individual assessors were given time to relax and become accustomed to breathing through the plastic nosepiece. The pattern of acetone released on the breath was monitored to ensure that relaxed, regular breathing was achieved before a sample was introduced. The samples were presented to each assessor in a different random order. Individuals were instructed to chew the gel samples with their mouths closed, breathe
373 regularly, swallow as necessary, and record the perceived carvone concentration using the lever. The assessment was complete when no more minty flavor was perceived. Assessors received a 15 min break between samples. The M S Nose ™ was calibrated by direct comparison of the peak height for carvone released on each breath against the peak height for a known concentration of volatilised carvone injected directly.
Results and Discussion
The Effect of Increasing Carvone Concentration on Volatile Release
It is reasonable to assume that increasing the concentration of volatile in the sample results in a proportional increase in volatile delivered to the olfactory epithelium. Non linear behaviour would, however, occur if there were changes in the way in which the volatile was distributed in the food matrix (e.g. formation of droplets at high concentrations). The results in Figure 1 show the maximum quantity of carvone achieved in-nose (Imax-ins) for each sample concentration, averaged across the panel. The clearly linear relationship is supported by an R value of 0.9842 2
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Figure J. The relationship between carvone concentration in sample and the maximum volatile measured in-nose (average panel results)
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374 The standard deviation, shown as error bars in Figure 1, highlights a significant variation in quantity of volatile delivered to the nose when comparing individual panellists.
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Figure 2. Relationship between carvone concentration in sample and the maximum volatile measured in-nose (individual panellists) The quantities in-nose achieved by Carole were 3 times greater than those seen from Kay (Figure 2). This variation may have arisen from differences in their human physiology or may have been due to the mechanics of their eating, swallowing and breathing during eating. These differences were, however, consistent across the entire range of samples studied.
The Relationship Between Stimulus and Perception The maximum perceived flavor intensity extracted from the TI data (Imax-sen) was averaged across the panel for each sample concentration. As long as all the data is gathered in the same way, the pooling of data from individual curves is a more robust measure of sensory effects from changes in stimuli (15). The relationship between perceived mintyness and sample carvone concentration was linear for the concentration range used giving an R value of 0.972 (Figure 3). This linear relationship is most likely due to the exponents (0.2-0.9) for volatiles in Stevens Law, which tend to give linear relationships over limited concentration ranges. Magnitude Estimation data in the same way supported this relationship (Figure 4). 2
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Figure 3. The relationship between maximum perceived mintyness determined by TI, and sample concentration.
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Figure 4. The relationship between perception of mintyness as determined by Magnitude Estimation, and sample concentration.
376 Differences Within the Panel
The results presented above utilise averaged data across the panel, however, some individuals did not exhibit a truly linear response for perception in relation to stimulus. Figure 5 illustrates the effect of increasing the in-nose concentration of carvone on the maximum perceived mintyness (Imax-sen) for two assessors. Comparing the two individuals, Mike achieved a lower maximum concentration innose (Imax-ins) and a poorer linear correlation between perception and stimulus (R = 0.821) whereas the in-nose concentration and linear correlation for Sally was much higher (R = 0.942). 2
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Figure 5. The effect of changing stimulus on perception - a comparison of two assessors.
Does this suggest that the speed of eating and/or the efficiency of volatile delivery to the nose affect the linearity of the response? To investigate this possibility further the R values were calculated for each individual panellist as a measure of their ability to perceive changes in stimulus concentration. The speed of eating was identified using the time to reach maximum carvone concentration in-nose (Tmax-ins). It is reasonable to assume that later Tmax values occur when food was kept in the mouth for longer. Tmax-ins values were very similar for an individual regardless of the sample concentration. Therefore, values for all samples were averaged for each assessor giving a more robust measure of their eating speed. 2
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Figure 6. The effect of eating speed on the linear correlation of stimulus and perception.
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Figure 7. The effect of the maximum breath volatile concentration (lOOOppm gels) on the linear correlation between stimulus and response.
378 Figures 6 and 7 showed there was clearly no effect of speed of eating or efficiency of volatile delivery on the R values for the relationship between stimulus and perception. Therefore, panellists who ate quickly or who had greater breath Imaxins values, performed no better or worse than the other panellists. Furthermore, Figure 8 showed that speed of eating and the delivery of volatile to the nose were independent variables. The maximum quantity achieved in nose for the 1000 ppm gel did not increase just because the food remained in the mouth for longer. 2
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Figure 8. The effect of eating speed on the maximum [carvone]measured in-nose (lOOOppm gel).
Adaptation When comparing the time to maximum concentration in-nose (Tmax-ins) and the time to maximum perception (Tmax-sen) there appeared to be a difference in the timing of the two events. For several assessors, Tmax-sen occurred before Tmax-ins suggesting adaptation. Previous work has shown that for volatiles released slowly from a food system, the perceived maximum intensity occurs before the in-nose maximum due to adaptation to the stimulus. Conversely, volatiles released quickly from a food system tend to show a perceived maximum intensity after that measured in-nose (16). Could the adaptation effect observed for the carvone data be linked to the eating process? The effect of adaptation was taken as the time difference between T50-ins and T50-sen, that is, the time to fall to 50% of the maximum intensity for stimulus and
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perception (Figure 9). Figure 10 shows the effect of eating speed on the level of adaptation. There was evidence to suggest that the slower the eating event, the greater the adaptation to the stimulus. The relationship gave an R value of 0.50 and the linear regression was significant at a 0.05 probability. 2
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Figure 9. The use ofT50 values from sensory and volatile release curves as a means of measuring adaptation.
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Figure 10. The effect of eating speed on adaptation.
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Conclusion The investigation into the effect of increasing the carvone concentration on the perception of minty flavor in a 6% gelatine gel revealed that, on average, the relationship between stimulus and perception was linear for the system used. Some individual assessors showed more linearity in this relationship than others. Further analysis of the data revealed that the speed of eating and the maximum quantity of volatile delivered to the nose had no effect on the linear correlation. However, there was some evidence that the speed of eating affected the level of adaptation to the stimulus.
Acknowledgements This work was funded through a B B S R C Link Scheme which included Firnenich and Nestlé as industrial partners.
Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
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