Chem. Senses 26: 1211-1219,
2001
© Oxford University Press 2001
Observation of the Swallowing Process by Application of Videofluoroscopy and Real-time Magnetic Resonance Imaging—Consequences for Retronasal Aroma Stimulation
Deutsche Forschungsanstalt für Lebensmittelchemie, Lichtenbergstraße 4, D-85748 Garching and Institut für Röntgendiagnostik, Technische Universität München, Ismaninger Straße 22, 81675 München, Germany
Correspondence to be sent to: Andrea Buettner, Deutsche Forschungsanstalt für Lebensmittelchemie, Lichtenbergstraße 4, D-85748 Garching, Germany. e-mail: andrea.buettner{at}lrz.tum.de
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Abstract |
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The process of eating and drinking was observed in vivo by application of videofluoroscopy, a dynamic X-ray technique, as well as real-time magnetic resonance imaging. The study was aimed at elucidating the timing and performance of the physiological organs involved in mastication and swallowing, mainly the tongue, the pharynx and the soft palate (velum palatinum). It was shown for the first time that effective physiological barriers do exist during food consumption that are capable of retaining volatiles such as helium within the oral cavity. These barriers allow the access of odorants to the nasal cavity only at certain times during the eating process. Their effectiveness is related to the texture of the food as well as the amount of food material present in the oral cavity and, thereby, directly influences retronasal aroma perception.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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In the last 20 years, numerous investigators have studied the temporal aspects of food consumption and their relationships to retronasal aroma perception. Time—intensity profiling has been developed, by which exhaled volatiles were monitored online using various mass spectrometric techniques (Overbosch, 1987
;
Soeting and Heidema, 1988;
Lindinger et al.,
1998; Taylor et al.,
2000). Fitting the obtained release curves to functions
representing the rise and decay of the curves, Overbosch developed a
time-dependent form of the psychophysical function based on Stevens's law,
thereby treating perception of taste and retronasal smell equally.
Recently, computer modelling was used to simulate the release of volatiles
from liquid and solid foods in the mouth
(Harrison and Hills, 1996;
Harrison et al., 1998;
Nahon et al., 2000).
These models incorporated effects such as mass transfer of the volatiles
across the solid—liquid and liquid—gas interfaces, heat transfer,
saliva and air flow, mastication and swallowing (in terms of a withdrawing of
food material and saliva from within the oral cavity). However, no prediction
could be made on the timing and extent of an odorant's transfer to the nasal
cavity, because no detailed information existed on the physiological
constraints influencing this transport to include into the model, such as the
nasal cavity's accessibility for odorants originating from the oral cavity. It
has been implied that this transfer occurs during the tidal flows of breathing
and that odorants are transferred from the oral cavity to the nasal cavity
during exhalation, on condition that they have been released precedingly from
food and saliva. The simulation of flavour release in systems modelling the
mouth was also based on this assumption
(van Ruth et al.,
1994; Roberts and Acree,
1995).
Generally, the transport mechanism of odorants from the oral to the nasal
cavity could not be fully explained. The oral cavity was regarded as a kind of
cave, directly connected to the airways, allowing a free passage of odorants
to the nasal cavity via the retronasal route. It was also assumed that `the
chewing motion causes the mouth to function as a bellows, injecting flavoured
air into the exhaled air, which then passes the olfactory epithelium'
(Haring, 1990), but this could
never be proved by observation. Investigation of both orthonasal and
retronasal odorant detection and identification, involving effects such as
airflow, mucosa and sample presentation, supported the idea that both pathways
(orthonasal and retronasal) lead to the same olfactory sensing system
(Mozell, 1971;
Voirol and Daget, 1986;
Mozell et al., 1991;
Hahn et al., 1994;
Pierce and Halpern, 1996).
Furthermore, it became evident that the efficiency in the delivery of odorants
via either route depends greatly on the techniques used (breathing patterns,
mouth and tongue movement conditions, etc.), so that different groups,
comparing orthonasal and retronasal aroma intensities, gained very divergent
results. However, the physiological reasons for these disparities have not
been elucidated.
In 1994, Land proposed that retronasal aroma stimulation was mainly related
to the event of swallowing, when a small volume of air is exhaled immediately
after swallowing, the so-called `swallow-breath'
(Land, 1994). He determined the
volume of this pulse of air to be 5-15 ml with a soap-film flowmeter. It was
assumed that this pulse should contain the major part of food volatiles that
had been released from the food material prior to swallowing, and should
therefore elicit a retronasal aroma pulse. Recently, we confirmed this theory
by quantifying the exhaled amounts of odorants at time intervals during
swallowing of liquid aroma solutions
(Buettner and Schieberle,
2000).
Oropharyngeal deglutition is a complex process, activating 26 muscle groups
within a very short period of time. Swallowing consists of three phases: (i)
the preparation phase, which includes bolus uptake and chewing, and which is
under voluntary control; (ii) the pharyngeal phase, which starts with the
triggering of the swallowing reflex, lasts 
0.7 s and ends with the
closure of the upper esophageal sphincter; and (iii) the esophageal phase,
where the bolus is transported towards the stomach by primary and secondary
peristalsis. A precise coordination is necessary to avoid aspiration or nasal
penetration of the bolus, especially during the oropharyngeal phase of
swallowing. Figure 1 is a
schematic drawing showing the anatomy of the naso-, oroand hypopharynx in the
sagittal plane.
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Oropharyngeal deglutition can be observed by real-time magnetic resonance
imaging (MRI) as well as videofluoroscopy. Generally, the advantage of
real-time MRI is the direct visualization of soft tissue, while soft tissue
contrast in videofluoroscopy is unsatisfactory and a coating with barium is
needed for good visualization of soft tissue surfaces. Another advantage of
real-time MRI is the free choice of the image plane and the fact that no
ionizing radiation is used. Videofluoroscopy, on the other hand, has superior
temporal and spatial resolution, e.g. for analysis of dysfunctions of the
upper esophageal sphincter. Another general limitation of MRI, unlike
videofluoroscopy, is that examinations cannot be performed in an upright
position, which is essential in patients with swallowing disorders or when
there is a risk of aspiration. However, for analysis of the normal physiology
of swallowing this seems to be of less importance, because once the swallowing
reflex is triggered, the pharyngeal and esophageal stages of swallowing and
bolus transport take place automatically and even against the force of
gravitation. The transit time of a test bolus is slightly less in the tubular
esophagus in a supine position than in the upright position [8.9 rather than
7.7 s (Hannig, 1995)]. Whether
this is also true for the pharyngeal transit time has, to our knowledge, not
yet been investigated. It is hard to quantify, because the pharyngeal stage of
deglutition is very short (average duration = 0.7 s), as already mentioned
above.
To understand the physiological prerequisites of aroma transfer from the oral to the nasal cavity during retronasal perception, the present investigation is aimed at the observation of eating and drinking by videofluoroscopy and real-time MRI.
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Materials and methods |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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Real-time MRI of swallowing
MRI was performed on a Philips Gyroscan ACS NT 1.5 T scanner (Philips, Best, The Netherlands). The gradient system had an amplitude of 23 mT/m in 0.2 s. We used a T1-weighted gradient echo sequence called a `fast field echo'-sequence (T1-FFE). The repetition time was 3.2 ms, the echo time was 0.9 ms, the flip angle was 10°. Temporal resolution was 6 images/s and spatial resolution was 1.2 x 2.4 mm, with a slice thickness of 15 mm. A preparation of gadolinium-DTPA for oral use (magnevist-enteral®, Schering, Berlin, Germany) was diluted with tap water in a ratio of 1:4 and served as fluid contrast medium. As solid contrast medium, cookies coated with magnevist-enteral® were used.
Images were acquired in the sagittal and coronal plane during deglutition of fluid and solid contrast medium, respectively.
Videofluoroscopy
Videofluoroscopy was performed on a conventional fluoroscopy unit (Philips
Diagnost 76) that is used mainly for examinations of the gastrointestinal
tract (e.g. double-contrast studies of the colon or stomach). The unit
consists of an X-ray tube, an examination table for patient positioning and an
image intensifier, which sends the images to a monitor. For videofluoroscopy,
not only a monitor but also a videorecorder is connected to the image
intensifier. Such a unit can be used in two different ways: (i) in the
fluoroscopy mode, where a continous image is obtained, but with a very low
dose and therefore a lower spatial resolution; and (ii) one can make still
images or a series of images with up to 8 images/s with a higher dose, but
also with a higher spatial resolution. The latter mode is used for making
images of high diagnostic quality and for documentation on film. The average
radiation dose for 1 min in the fluoroscopy mode is 3.13 mSv for the lung, 0.8
mSv for the thyroid gland and 0.54 mSv for the bone marrow
(Biegenzahn and Denk, 1999).
This is equivalent to one normal X-ray image of the same region of diagnostic
quality. For the analysis of deglutition, a high temporal resolution is more
important than a high spatial resolution; therefore, the images are acquired
in the low-dose fluoroscopy mode and taped on conventional SVH-videotape for
documentation and reporting. Temporal resolution was 25 images/s. Iotrolan
(Isovist®, Schering, Berlin, Germany) served as fluid oral contrast
medium. Cookies coated with Isovist® served as solid oral contrast medium.
Images were acquired in the sagittal plane during swallowing.
Subjects
All subjects were non-pregnant volunteers of the Technical University of Munich. Five subjects participated in each experiment (two female, three male), and underwent both videofluoroscopy and real-time MRI. Written and informed consent was acquired from all volunteers. The subjects' ages ranged from 20 to 35 years. They exhibited no known illnesses at the time of examination. Also, there was no history of swallowing or eating disorders, or of oropharyngeal surgical interventions or radiation therapy in the past. During experiments, all of the participants exhibited normal olfactory and gustatory function. Subjective aroma perception was normal in the past and at the time of examination in all subjects.
The radiation dose for videofluoroscopy per minute for the radiation-sensitive organs exposed has already been mentioned. Average exposure time was 60 s for all four volunteers, with 30 s for the swallowing studies in the sagittal plane for the female volunteers and 90 s for swallowing studies with the additional helium test or studies for bolus uptake with a straw or spoon for the male volunteers. All volunteers wore a lead apron as protective clothing.
Chemicals
Helium gas (purity 4.6) was from Messer-Griesheim (Krefeld, Germany).
Helium transfer from the oral cavity to the nasal cavity
The experiments were carried out with helium gas as the `ideal' gas: first, helium is a highly volatile gas and so it is readily transferred to the nasal cavity via the retronasal route as long as it is released from within the oral cavity; secondly, it does not exhibit any chemical interactions during its passage to the nasal cavity and so we can exclude losses; and thirdly, it can be easily detected in extremely small amounts in the air expired from the nose.
The experiments were carried out in the following ways: helium (25 ml) was taken into the mouth, with care being taken not to swallow. Subjects were asked to exhale through a nose-piece fitted exactly to the noses of the subjects so that the nostrils were completely sealed (Figure 2). The nose-piece was connected to a glass column and at the tip of the column a helium detector (GL 228, GL Sciences, detection limit: 0.01 ml/min) was positioned in the middle of the exhaled gas stream. The lips were kept closed throughout the entire experiment. The helium was kept for 1 min in the oral cavity while normal respiration was continued. Then, the air present in the oral cavity was deliberately exhaled through the nose. During the entire experiment the velumpharyngeal performances were observed by means of videofluoroscopy. Experiments were carried out in duplicate. To make sure that retardation of helium indeed came from the velum—tongue border and not from helium collecting underneath the hard palate (due to its high volatility), the same experiments were repeated with the head bent down to the chest.
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Results |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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Detection of helium in expired air from the nose—videofluoroscopic observation of the velum—pharyngeal performances
Observation of the oral and pharyngeal segments of a subject by means of videofluoroscopy was performed while the subject kept a `bubble' of helium within the oral cavity (with closed lips and by avoiding any swallowing actions) (Figure 3a). Subjects reported that it did not require any effort to keep the bubble within the oral cavity. During this period, a barrier was formed by the velum and the base of the tongue that prevented the gas from passing the nasal cavity via the retronasal route and from being exhaled from the nose. As a consequence, no helium could be detected in the exhaled air. On the other hand, while performing a swallowing action (Figure 3b), a short opening of the velum—tongue border could be observed, along with an immediate detection of helium in the expired air from the nose. These observations were fully reproducible in every subject and at each repetition of the experiment. Even when the head was bent down to the chest, no transfer of helium to the nasal cavity could be observed as long as no swallowing action or deliberate opening of the velum—tongue border was performed. This clearly demonstrated that the closure of the velum and tongue is a highly efficient barrier even for extremely volatile and inert gases.
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Videofluoroscopy and real-time MRI of the swallowing of liquid foods
In Figure
4a,4d,
the important oral and pharyngeal stages of swallowing are shown in six
pictures from sagittal real-time MRI and videofluoroscopy series with fluid
contrast medium. These stages of swallowing (bolus uptake, beginning of bolus
transport, velopharyngeal closure, triggering of the swallowing reflex,
propulsion of the bolus towards the esophagus, and original position) were
selected according to Hannig (Hannig,
1995) and represent defined steps during the oral and pharyngeal
phase of deglutition which have to be analyzed in the diagnosis of swallowing
disorders.
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After bolus uptake, the bolus is kept in the oral cavity between
the tongue and the hard and soft palates. During rest, there is no connection
to the dorsal oropharynx, the nasopharynx and the airways, preventing leakage
and aspiration of fluids or solid material
(Figure 4a). At the beginning
of swallowing, we can observe an elevation of the tip of the tongue against
the hard palate and an adduction of the soft palate to the base of the tongue
(beginning of bolus transport,
Figure 4b). During the next
step, the soft palate performs a superior and posterior movement in order to
achieve complete velopharyngeal closure to prevent nasal penetration while the
bolus is transported to the hypopharynx (velopharyngeal closure,
Figure 4c). Due to the anterior
and superior movement of the larynx and closure of the epiglottis, the bolus
cannot enter the airways and aspiration is prevented. At the same time, the
contraction of the dorsal wall of the pharynx starts at the height of the
first cervical vertebra, resulting in a propulsion of the bolus in the
direction of the esophagus (triggering of the swallowing reflex,
Figure 4d). In the next
picture, the peristaltic wave of the dorsal pharyngeal wall continues and
reaches the middle and lower parts of the pharynx, and the bolus enters the
esophagus while the upper esophageal sphincter remains open (propulsion of
the bolus towards the esophagus,
Figure 4e). Finally, the bolus
has left the pharynx, the upper esophageal sphincter is closed and the larynx,
epiglottis and velum return to their original positions, followed by a short
pulse of respiration, the so-called `swallow breath'
(Land, 1994)
(Figure 4f). So, when
swallowing liquids, a direct connection of `odorant-loaded areas' to the nasal
cavity, such as the oropharynx, exists only at the instant of the swallowing
breath.
Videofluoroscopy and real-time MRI of the swallowing of solid foods
During mastication, intermittent opening of the connection of the oral cavity to the naso- and dorsal oropharynx can be observed, dependent on the texture and the amount of the bolus (Figure 5a, open oral cavity; Figure 5b, closed oral cavity). In general, the more fluid the texture of the bolus and the greater its volume, the more efficient is the closure of the oral cavity against the nasopharynx and dorsal oropharynx. During this closure no transfer of odorants is possible via the retronasal route to the nasal cavity and the olfactory epithelium.
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Especially after swallowing solid or semi-solid foods, such as yogurt or cottage cheese, one can often observe the formation of a viscous salivary coating on the back (the pharyngeal part) of the tongue, which may contain particles of food along with odorants (Figure 6). This film could possibly induce a prolonged perception of food aroma, acting as a kind of odorant depot, while the main bolus has already left the oral cavity. This coating may still be present even after another swallow of saliva, depending on the texture of the food matrix and its adhesion to the oral and pharyngeal mucosa.
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The drinking of liquids by using a straw
There is an important difference in initial retronasal aroma perception between the conventional introduction of food (liquid and solid) and when using a straw.
During normal introduction of food into the oral cavity, the dorsal opening of the oral cavity (formed by the basis of the tongue and the soft palate) is open (Figure 7a), while when using a straw (Figure 7b), this region must be closed, otherwise no vacuum could be achieved. We suppose that when food material is introduced via the `normal' way small portions of air can enter into the oral cavity along with the food and can proceed into the nasal cavity via the retronasal route through the velopharyngeal portal. For this reason, aroma perception is possible prior to chewing, whereas when a straw is used, the perception of aroma is delayed, and is associated with the event of swallowing.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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Development of a new time—intensity concept based on velum—pharyngeal performances during the consumption of foods
The persistence of perception depends, first and foremost, on the duration
of food material being present within the oral cavity. When swallowing liquid
foods, retronasal aroma perception will be more or less reduced to one main
aroma flash associated with the swallowing event itself. This main event will
be followed subsequently by some minor events induced by the swallowing of
some remaining traces of the liquid and by the delayed release of odorants
from the oral mucosa. When masticating solid foods, there is a series of
retronasal aroma perceptions, mainly related to the swallowing of small
portions of the food material as well as portions of saliva. Generally, we
found that the most compliant textures are swallowed first, e.g. liquids
before solids. As a consequence, the odorants of the liquid phase are exhaled
first. This can be easily observed when masticating e.g. a slice of orange (A.
Buettner et al., submitted for publication). With the first bites,
considerable amounts of juice are present in the oral cavity which induce the
first swallowing actions. Evidently, the full body of the fruity juice aroma
is perceived first, as long as juice is swallowed in portions. Later on, the
pulp will be masticated and swallowed in portions. The eater will perceive a
shift in the aroma profile, from fruity and fresh to a more terpene-like
aroma, because the odorants associated with the pulp differ considerably from
those in the juice (Radford et
al., 1974). The same effect can be observed when masticating
a piece of bread with butter and jam. Small portions of jam and butter will be
swallowed, together with saliva, at the very beginning of the mastication
process, while the last overall impression will be dominated by bread aroma.
As a consequence, the classical time—intensity (TI) curve with
one single maximum of aroma intensity (Imax) cannot be
maintained when we really want to evaluate the series of aroma impressions. We
would, for heterogeneous food systems such as bread with butter and jam, have
to separate into several ImaxS, with each single
Imax being related to one matrix constituent
(Imax-jam, Imax-butter,
Imax-bread). Even more complicated would be the occurrence
of mix-phases as produced during mastication.
We propose that sensory time—intensity investigations, as performed
up to now to follow the temporal dimension of aroma perception
(Lee and Pangborn, 1986;
Overbosch et al.,
1986; Overbosch and DeJong,
1989), need thorough reconsideration. In our opinion, perception
of retronasal smell has to be regarded as a series of `single-peak events'
rather than a `TI curve', and should be carefully separated from the
sensation of taste, temperature and the tactile feelings induced in the oral
cavity during mastication. Particularly the sensations of temperature and
taste should, indeed, exhibit a characteristic time—intensity profile,
and it should be difficult to subtract these mentally from the series of
single-peak events evoked by retronasal odorant perception. We suggest that
this could be explored by very simple experiments: the subjects could, for
example, place a portion of wine in their oral cavity and should first avoid
swallowing actions. They should only perceive the coolness and the taste of
the liquid. Then they could try to open the velum—tongue border
deliberately, by (for example) inhaling small portions of air through the lips
in addition to the liquid, but still avoiding any swallowing action. The more
they succeed in performing such actions, the more retronasal aroma the
panelists should perceive. Finally, they should evaluate the aroma impression
induced by swallowing during the swallow breath.
According to our findings, it becomes evident why retronasal aroma
perception can be considerably different from orthonasal sniffing. Mainly,
when panelists are not aware of how to increase their retronasal perception
either by swallowing or by deliberately opening the velum—tongue border,
the perceived retronasal intensities will be significantly reduced. This
phenomenon has been controversely discussed in previous investigations,
effected by the sample evaluation technique (with or without swallowing), but
could never be fully explained (Voirol and
Daget, 1986; Marie et
al., 1987). The consequence therefore is highly variable
retronasal aroma thresholds and intensity functions from different
laboratories. Many of the difficulties reported previously in producing
consistent sensory TI curves can now be easily explained.
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Conclusions |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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Our investigations showed that aroma perception during drinking and eating depends highly on the velum—pharyngeal performances during mastication and swallowing. Aroma transport to the nose was found to be a series of alternating static and dynamic events such that the oral cavity can be either closed off from the airways by the borders formed by the velum or partially open to the nasal cavity depending on the oropharyngeal actions performed, such as swallowing. This depends strongly on the texture and the amount of food material present in the oral cavity, but also on the behaviour patterns during food consumption. Based on our novel physiological observations, a new approach to interpreting time—intensity data was proposed.
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Acknowledgments |
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We are grateful to Mrs Pamela Gumbrecht and to Mr Daniel Schieberle for participating in our experiments and for excellent technical assistance.
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TopAbstractIntroductionMaterials and methodsResultsDiscussionConclusionsReferences |
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Accepted August 30, 2001
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J. C. Pfeiffer, T. A. Hollowood, J. Hort, and A. J. Taylor Temporal Synchrony and Integration of Sub-threshold Taste and Smell Signals Chem Senses, September 1, 2005; 30(7): 539 - 545. [Abstract] [Full Text] [PDF]
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J. Frasnelli, S. van Ruth, I. Kriukova, and T. Hummel Intranasal Concentrations of Orally Administered Flavors Chem Senses, September 1, 2005; 30(7): 575 - 582. [Abstract] [Full Text] [PDF]
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V. Normand, S. Avison, and A. Parker Modeling the Kinetics of Flavour Release during Drinking Chem Senses, March 1, 2004; 29(3): 235 - 245. [Abstract] [Full Text] [PDF]
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S. Rabe, U. Krings, and R. G. Berger In vitro Study of the Influence of Physiological Parameters on Dynamic In-mouth Flavour Release from Liquids Chem Senses, February 1, 2004; 29(2): 153 - 162. [Abstract] [Full Text] [PDF]
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S. Rabe, R. S.T. Linforth, U. Krings, A. J. Taylor, and R. G. Berger Volatile Release from Liquids: A Comparison of In Vivo APCI-MS, In-mouth Headspace Trapping and In vitro Mouth Model Data Chem Senses, February 1, 2004; 29(2): 163 - 173. [Abstract] [Full Text] [PDF]
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