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Influence of Bitter Taste on Mastication Pattern

¹ Station de Recherches sur la Viande, INRA-Theix. 63122 Saint Genes-Champanelle, France; and
² Estanción Tecnológica de la Carne, Instituto Tecnológico Agrario, 37770 Guijuelo (Salamanca), Spain;

^* corresponding author’s current address, Wageningen UR, Agrotechnology and Food Innovations, Bornsesteg 59, 6700 AA Wageningen, The Netherlands; eric.dransfield{at}wur.nl

	ABSTRACT

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Mastication is a rhythmic activity that can be modified by peripheral information generated in the mouth. To study whether taste cognition could influence the way in which a food is broken down in the mouth, subjects masticated firm, sugar-based gelatine gels with differing concentrations of quinine, up to 1500 µmol/kg, while electromyography (EMG) of masticatory muscles was recorded. Taste intensity and composition of saliva were measured. With increasing quinine concentration, the average number of chews for nine subjects decreased from 30 to 22, and their average clearance time increased from 7 to 14 sec. Quinine concentration had no effect on chewing frequency (1.3 Hz) or on the rate of salivation (5.5 g/min). Bitterness increased, while acceptability and sweetness decreased, with increasing concentration of quinine in the gel and in saliva. Taste cognition could therefore modify food breakdown in the mouth.

KEY WORDS: mastication • salivation • sweetness • bitterness • salivary composition

	INTRODUCTION

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Mastication is a rhythmic activity controlled by the brain stem (Lund, 1991) that can be overridden by the higher centers and modified by peripheral information (Inoue et al., 1989; Ottenhoff et al., 1992). Mastication consists of the bite, stage 1 transport, and the chewing and clearance stages (Hiiemae et al., 1996). In the chewing stage, muscular activities and number of chews increase with increasing food hardness (Möller, 1966; Horio and Kawamura, 1989; Brown et al, 1994; Van der Bilt et al., 1995; Schindler et al., 1998; Lassauzay et al., 2000; Peyron et al., 2002). Other peripheral information from temperature and taste receptors is sent simultaneously to the brain stem and could affect mastication (Schwartz and Lund, 1995). This information is also transmitted to the higher centers and can result in pain, pleasure, or acceptability. So, cognition, in addition to reflex control, could influence food perception and breakdown in the mouth. Using model gels, investigators have showed a mild bitter taste (Alfonso et al., 2002) to have no effect on chewing muscle activity and chewing frequency, but, in a meal situation, chewing time was shorter and fewer chews were made when more palatable foods were eaten (Bellisle et al., 2000). Thus, mastication may depend on the type of food and taste intensity or its degree of acceptability.

The aim of the present work was to study whether a bitter taste could affect the chewing pattern. For this, visco-elastic gels differing in quinine concentrations were chewed while electromyography (EMG) simultaneously recorded mastication characteristics. Sensory bitterness and acceptability were assessed, as well as the concentrations of quinine and other components in saliva.

	MATERIALS & METHODS

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Gelatin-based Gels
Forty-five gelatin-based gels—containing sugar and glucose-syrup (incorporated for texture and for palatability)—were made containing quinine (Sigma-Aldrich Chimie, Lyon, France) and set in cylindrical moulds (1 cm high and 2 cm in diameter) in one batch (Lassauzay et al., 2000; Alfonso et al., 2002). Six concentrations were made: 0, 241, 362, 482, 723, and 1446 µmol quinine/kg of gel, with about 7 cylindrical gels of each concentration made in each batch. In this study, 4 batches were made for rheology and mastication studies and 4 for the sensory testing and assays of saliva. The batches used for mastication, sensory, and saliva studies for each subject are listed in Table 1.

Detection thresholds for quinine were determined 4 times by means of a series of increasing concentrations of quinine hydrochloride in de-ionized water, with 2 or 3 samples of water presented first.

In 4 other sessions, performed after mastication was recorded, the nine subjects evaluated the gels for bitterness, sourness, sweetness, firmness, and acceptability. They marked non-structured, 10-cm lines, labeled "low intensity" at the left end and "high intensity" at the right end. The distance (cm) from the left end of the scale was recorded as the intensity score.

Six of the nine subjects chewed the quinine-containing gels and assessed sourness, sweetness, and bitterness, in that order, on similar non-structured line scales. After 15 sec, they spat out into plastic pots. They were asked to rinse their mouths and then wait 10 min before assessing the 2nd sample similarly. They continued in the same way to complete assessments of all 6 samples. On a second occasion, they repeated the assessments of sourness, sweetness, and bitterness on the 6 gels. For all tests (6 samples, six subjects, 2 replicates), the order of presentation was given in a Latin-square design (Wakeling et al., 2001), in which every subject tested each sample once, and this was repeated with a reverse order of presentation.

Rheological Measurements
The shear strength (kPa) was assessed with a punch-and-die test, where the punch forces a center cylinder of the gel through the die. Five replicates were determined on each of the quinine concentrations (Alfonso et al., 2002). The punch and die were mounted in an Instron Universal Testing Machine (model 4501, High Wycombe, Bucks., UK). The flat-ended punch had a diameter of 10 mm, and the die had a 10.1-mm-diameter hole; thus, the distance between the punch and the die was 0.05 mm. The punch speed was 1 mm/sec, and the data acquisition rate was 50 points/sec. Maximum force (N) occurred just before the punch entered the die. We calculated gel strength (Pa) by dividing the maximum force by the sheared surface area, equal to 2rh, where r is the radius of the punch and h the thickness (height) of the gel.

Masticatory Recordings
The nine subjects chewed and swallowed the gels during EMG recording. Each subject was given half a cylindrical gel without quinine to chew and to familiarize him/herself with the EMG recording system and the environment. Testing then proceeded (see sensory evaluation, above). At the start of a recording session, lasting about 1 hr, each subject was asked to close his/her eyes while the gel was placed onto the distal part of the tongue by the experimenter. EMG recording continued until the disappearance of the residual EMG signal, and the subject rested.

EMG activity was recorded from both left and right masseter and temporal muscles by means of surface bipolar electrodes. The subject clenched his/her teeth, and the site for electrode positioning was found by palpation. The skin was cleaned with soapy water, and the 2 electrodes, coated with conductive gel, were fixed to the skin over each muscle. An additional ground electrode was attached to the wrist. For the masseter, one electrode was placed midway between the anterior and posterior ends of the muscle, and the second was placed above, over the muscle insertion point. For the anterior temporalis, one electrode was placed at the side of the eyebrow and the other below the hairline. EMG was recorded with the use of a Grass Polygraph (Model 7p511, Grass Instruments, Astromed, Trappes, France). A to D conversion was done at 1000 Hz by means of a CED 1401 converter (Cambridge Electronic Design, Cambridge, England). Data were collected and analyzed with Spike2 software (Cambridge Electronic Design, Cambridge, England). The beginning and end of each burst were defined when the EMG signal reached a level of 10% above or below the mean area of the baseline at 0 mV (Peyron et al., 2002). The rectified EMG record was divided into the initial part (chewing stage), showing regular periodic activity, and the latter part (clearance stage), showing irregular activity. Masticatory characteristics, determined during the chewing stage, were: number of chews, the number of EMG peaks during chewing stage; muscle effort (mV.sec), the sum of the areas under the rectified EMG curves of all 4 muscles of the whole sequence; average chewing frequency (Hz), the number of chews divided by the duration of the masticatory stage; and clearance time (sec), the time after the end of the last chew until the disappearance of the residual EMG signal.

Composition of Saliva
After chewing ceased, the chewed gels and saliva were centrifuged (Eppendorf, 5804R, rotor F34-6-38) at 5000 rpm for 1 min, and the liquid (saliva) was then removed and weighed. The assays were of UV absorption (1 cm light-path at 280 nm), osmolarity (by depression of freezing point; Roebling Osmometer, 13DR; mOsmoles), pH, sugar (as glucose; Euroflash, Johnson & Johnson; mM glucose), and quinine (in 0.1 M NaSO₄, by fluorescence at 335 nm excitation and 440 nm emission, with an added internal standard) and were completed within 2 hrs.

Statistical Analysis
A mixed-model analysis was performed with the use of a General Linear Model (GLM, SAS 6.12 software) with the effect of sample type, subject, and the interaction term. Student-Newman-Keuls test (5% risk) was used for comparison of the means of the 6 gel types. Linear correlations were performed between measurements on averages of all subjects and on original and transformed data from each individual.

	RESULTS

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Rheological Measurements
Shear strength mean values of the 6 types of gels ranged from 22 to 44 kN/m² (Table 2). Including samples from 4 batches tested, strength was not related systematically to quinine concentration (Table 2).

View larger version (26K): [in this window] [in a new window]   Figure. Rectified surface EMG recordings over the masseter and temporalis muscles from (a) subject having a long chewing stage and a short clearance time stage and (b) from another subject having a short chewing stage and a long clearance time stage.

The clearance time increased (F = 4, p < 0.01) from 7 sec at 0 µmol/kg to 12 sec at 241 µmol/kg and up to 14 sec at 1446 µmol quinine/kg gel.

Variations in all mastication measurements were often larger among subjects than among quinine levels. Five-fold variations among subjects were found in muscle effort (from 47 to 238 mV.sec) and number of chews (from 9 to 49). Frequency varied least from 0.88 to 1.80 Hz, and average clearance times varied from 4 to 23 sec among subjects.

For each subject individually, the average of all samples was calculated for each EMG characteristic. These averages were used to calculate the (coefficient of) variation (100 * mean/standard deviation) among all subjects. The variations among subjects were 52, 67, 15, and 22% for the number of chews, muscle effort, frequency, and clearance time, respectively.

Sensory Evaluation
The average gel firmness varied significantly, but inconsistently, with quinine concentration (Table 2). Average bitterness ratings increased progressively with increasing quinine concentration, from 1.0 without quinine to 8.1 at 1446 µmol quinine/kg. The average sweetness rating was halved with increasing quinine concentration. Ratings of sourness were low and not affected by quinine concentration (Table 2). Average acceptability ratings decreased significantly with increasing quinine concentration, but there was a statistically significant interaction between subject and quinine concentration, with acceptability of the strongest quinine gel varying from 0 to 3 among subjects.

Salivary Flow and Assays
The glucose content was not affected by subject or by quinine concentration in the gels (Table 2). Average flow rate (5.5 g/min), UV absorption (8.8), osmolarity (969 mOsmoles), and pH (4.2) varied significantly among subjects, but not among the 6 quinine gels. Among subjects, salivary flow varied from 3.4 to 6.0 g/min, pH from 4.1 to 4.3, glucose from 199 to 272, quinine from 3.2 to 8.9, UV from 5.9 to 12.9, and osmolarity from 635 to 1894. Those subjects having a high level of quinine in saliva also tended to have high glucose (r = 0.74), high osmolarity (r = 0.70), high UV (r = 0.64), and low pH (r = –0.67).

Average quinine concentration in saliva increased up to 13 µM in direct proportion (r = 0.98) to the quinine concentration in the gels.

Relationships between Sensory Ratings and Mastication
Bitterness was well-correlated with quinine concentration for each subject. Good correlations were also observed between acceptability and quinine concentration, except for one subject. For two other subjects, bitterness and acceptability were not related to, and there was no change in the number of chews with, quinine concentration.

Among the nine subjects, average bitterness increased with increasing quinine concentration in gels (r = 0.76) and in saliva (0.6), which were themselves related (r = 0.75). Sweetness decreased with increasing quinine in the gels (r = –0.52), but was not related to glucose concentration in saliva (r = 0.18). Those subjects who chewed for longer gave higher bitterness (r = 0.72) and lower acceptability (r = -0.72) ratings, those who used more muscle effort found the gels sweeter (r = 0.69), and those with higher concentrations of quinine in the saliva tended to rate (r = 0.66) the gels as more bitter.

	DISCUSSION

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Unlike the analogous rhythmic behaviors of respiration and locomotion, mastication is designed to break up the food. A behavioral model for food transport in the mouth predicts 3 stages. In stage 1, the food bite is transported to the post-canine area; in stage 2, chewing, the number of cycles is related to the bite volume and consistency; and in stage 3, the triturated food is transported for deglutition (Hiiemae et al., 1996).

In this work, in which the gels were placed in the mouth, only the chewing and clearance stages were measured. While these stages varied between subjects, the total time remained approximately constant but with a shorter time of mastication (fewer bites, less muscle effort) and a longer clearance time as bitterness increased. It appeared that subjects made a conscious effort to reduce the time the bitter gels stayed in the mouth. So, taste compounds could influence mastication, but they are rarely taken into account in studies on mastication.

Little information is available on this during natural eating (Neill, 1982). Recordings of the distension of the cheek and throat movement (Bellisle et al., 2000) showed that highly palatable foods were swallowed after minimal chewing, although the meal duration was longer with more palatable foods. The separate stages of mastication were not identified, and the results appeared to be food-specific. In the present work, frequency during the mastication stage was not affected by bitterness or acceptability, suggesting that the pattern generator (Lund, 1991) for mastication was not affected by peripheral taste perception or cognition.

Many taste compounds stimulate salivation (Humphrey and Williamson, 2001), derived from the first-order taste relay (solitary nucleus), although stimulation of the second order-relay (parabrachial taste area) induces salivation. Taste aversion information appears to arrive from the parabrachial taste area to the salivary secretion center via the reticular formation ventral to the parabrachial nucleus (Matsuo et al., 2001). However, there is limited information on their influence on salivary composition, which itself may influence taste perception. The level of quinine incorporated was up to 1500 µmoles quinine/kg gel and, after 15 sec of chewing, gave approximately 1% (15 µM) in saliva, with a minimum perception at about 1 µM in saliva. These values compare with a threshold, determined in aqueous solutions, of about 8 µM, suggesting an approximately 10-fold salivary dilution. Salivary flow was high (5 g/min) and was not affected by the presence of quinine. This is different in rats, where taste aversion and rejection behavior to quinine would give a 10-fold increase in (submandibular gland) salivary flow (Matsuo, 2000). So, in these studies in man, the near-maximal salivary flow induced both by the high gel shear strength and by the taste compounds was not increased further by the presence of quinine.

	ACKNOWLEDGMENTS

 
EN and CV were, respectively, recipients of scholarships from the French National Education, Research and Technology Ministry, France, and from the National Institute of Agricultural Research and Food Technology, Ministry of Science and Technology, Spain.

Received April 8, 2004; Last revision November 17, 2004; Accepted December 12, 2004

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