Abstract
Aim:
This paper investigates the effect of modifying the taste perception on the flow rate of parotid salivary secretion in humans by utilizing Miraculin as a taste modifying agent.
Methods:
Gustatory parotid responses were obtained from seven subjects for several tastants before and after the consumption of a taste modifying protein, Miraculin.
Results:
A noticeable similarity in the effect of a mixture solution of sucrose and citric acid and that of an MCL induced citric acid solution was reported. Sweetness was always the perceived taste for all tastants after the consumption of MCL. The descriptions only differed in the degree of sweetness. Parotid responses for sucrose and citric acid, after the consumption of MCL, reflected taste perception descriptions obtained from subjects particularly with the MCL induced sweet taste after applying the 10× variant of citric acid. The MCL effect suppressed parotid salivary flow rates when both sucrose and the less concentrated acid solutions were applied. The opposite occurred when the more concentrated acid solution was applied. A mixed solution of sucrose and citric acid 10× had a comparable effect on the mean parotid salivary flow rate to that of the MCL induced effect of citric acid 10× alone. While before the consumption of MCL the mixed solution evoked mean parotid responses that exceeded those for citric acid 10×
alone. Yet, this was not the case with the less concentrated variant of citric acid.
Conclusion:
MCL‑induced sweetness has evoked a significant increase in the parotid salivary flow. Further work needs to be carried out to characterize and more fully understand taste perception, quality coding, and binary taste interactions.
Introduction
Taste sensation is characterized by a special and powerful chemosensory system which has an important role in drink and food physiological assessment and evaluation. Unlike olfaction, audition, vision, and touch, which serve in various behavioral conditions, related to behavior; taste sensation has developed to function as a major controller and driver of eating/drinking behavior. Gustatory systems are responsible for detecting “unsafe” substances in foods and drinks and therefore generate an innate (native) defense system that approves or denies potential food sources. Gustatory‑salivary reflex yields a high flow rate secretion of stimulated saliva, rich in bicarbonate, in response to sour or low pH stimulation of taste receptor cells. Among all the five basic human tastes, sour taste has the greatest impact on the salivary flow. Miraculin (MCL) is a homodimeric protein that has a distinctive feature of altering the perception of sour tasting foods into sweet. The protein is extracted from berries, similar in size to olives, which grow on a tropical plant called Richadella dulcifica, a member of the Sapotaceae family found in West Africa. In general, when the taste buds are exposed to MCL, any sour foods and beverages containing citric or acetic acid will taste pleasantly sweet when consumed after this protein. This effect would last for 30–60 min. The molecular entity of the sweet taste receptor has been identified to be “human taste type 1 receptor 2 and 3” hT1R2‑hT1R3. It is generally believed that MCL’s mechanism of action is closely bound to the hT1R2‑hT1R3 receptor. MCL has a huge molecular weight;
therefore, it is unlikely that the taste‑modifying protein penetrates into the taste bud.
Moreover, MCL once bound to the receptor remains bound and able to activate/reactivate it depending on the pH state. This relationship can last for a period of around 1h. However, this phenomenon and its effects have received limited research attention and the scientific findings in the field are highly varied and inconclusive. The aim of this work was to more fully understand the mechanism of action and subsequent effects of MCL and assess the effect of modifying the taste perception on the flow rate of parotid salivary secretion in humans by utilizing MCL as a taste‑modifying agent.
Subjects and Methods Tastant preparation
Citric acid (Sigma‑Aldrich Ltd., Gillingham, Dorset, UK) in its pure form was selected as the sour taste stimulant and sucrose (Silverspoon, Sugar Way, Peterborough, UK) as the sweet taste stimulant. Three different concentrations of citric acid were assessed, 4.4, 2.2, and 0.44 mg/ml. These values represent ×10, ×5 and ×1 the taste recognition
threshold, respectively. In addition, three concentrations of sucrose were assessed; 34 mg/ml, 17 mg/ml, and 3.4 mg/ml which represent ×10, ×5, and ×1 the taste recognition threshold of sucrose in human taste buds, respectively. Following a pilot experiment (not presented), it has been decided to exclude tastants that were not fully recognized by participants, namely, citric acid ×1, sucrose ×1, and sucrose ×5. This will ensure that subjects will fully recognize all tastants as it was not the scope of this experiment to observe the effect of different tastants at sub‑recognition concentrations recognition
thresholds according to Pfaffmann et al., 1971.
Subjects
This experiment was conducted on seven adult participants (five males and two females). The volunteers involved were nonsmokers, had no history of systemic
diseases, were on no medication known to alter salivary flow and function and had no history of the head‑and‑neck radiotherapy. After ethical approval was obtained from the
Local Ethics Committee, the procedure was fully explained to the participants and written informed consent was obtained from each participant and a detailed questionnaire about food allergies. The questionnaire ensured that participants with a history of food allergy were excluded from the study.
Saliva collection
The participants were asked to sit in a relaxed upright position. To obtain the salivary secretion of the parotid gland, sterile Lashley/Carlson‑Crittenden cups (Stone Machine Company, Colton, CA, USA) were used according to the manufacturer’s instructions. The flow rate in ml/min was calculated by dividing the volume of saliva obtained over the collection period.
Before the consumption of miraculin
• Unstimulated saliva collection was undertaken over a period of 6 min
• Measurement of the stimulated parotid salivary flow rate was undertaken by introducing 0.5 ml of each tastant every 20 s for 2 min via a sterile syringe. For each stimulus (i.e., tastant), the measurement was repeated three times at 3 min intervals.
During the consumption of miraculin
The participants were asked to consume a commercial MCL vehicle which comes in the form of tablets (Classic Miracle FrootiesTM). They were therefore instructed to roll
around on their tongue one tablet for 2 min, allowing it to dissolve. The volume of saliva obtained over the 2 min was used to calculate the parotid salivary flow rate during the
consumption of MCL.
After the consumption of miraculin
Once the tablet was completely dissolved, the same tastants were used again as gustatory stimuli using the same technique described above. During the application of
different tastants, the participants were asked to describe the perceived taste for each tastant after the consumption of MCL using their own words.
Results
The conventional statistical significance of P < 0.05 was used throughout. The statistical software package, GraphPad Prism 6 (GraphPad Software Inc., San Diego, USA) was
used. D’Agostino and Pearson omnibus normality test demonstrated that the data were distributed normally. To investigate any effects of different tastants with or without MCL on the salivary flow rate, statistical analysis using one‑way analysis‑of‑variance (one‑way ANOVA) on this data was undertaken. This revealed very highly statistically significant effects P < 0.0001.
Before the consumption of miraculin
Salivary stimulation by citric acid ×10 was considerably higher relative to citric acid ×5 and sucrose solution. This difference was found to be very
highly significant (P < 0.001) (ANOVA). The mixed solution of sucrose and citric acid ×10 resulted in a higher stimulated salivary flow compared with that of any other tastant. This difference was also found to be very highly statistically significant (P < 0.001) (ANOVA).
After the consumption of miraculin
After consuming MCL, the overall trend remained comparable with that before MCL consumption. An increase in stimulated salivary flow rate was reported for citric acid ×10 and the mixed solution of sucrose and citric acid ×10 groups after consuming
MCL. However, the salivary flow rate was reduced for all other groups (i.e., sucrose, citric acid ×5, and the mixed solution of sucrose and citric acid ×5) after the consumption of MCL. The mixture of sucrose and citric acid ×10 solution’s effect on
stimulated salivary flow rate was comparable to that of citric acid ×10 after the consumption of MCL (P > 0.05) (ANOVA). No effect of MCL consumption was reported on all tastants except citric acid ×10 which demonstrate statistically very highly significant difference (before vs. after) (P < 0.001) (ANOVA). However, this difference was found to be nonsignificant for the citric acid ×5 group. Resting salivary flow rate was found to be stable at 0.14 ml/min regardless before or after the consumption of
MCL [Figure 3]. The stimulated salivary flow rate during the consumption of MCL was 0.70 ± 0.13 ml/min.
Discussion
Sucrose solution evoked the least parotid flow rate among other tastants. This result was expected because sweet taste evokes the least human salivary stimulation compared to other taste modalities. Interestingly, of the 35 taste perception descriptions obtained for the five tastants after the consumption of MCL (i.e., 7 subjects × 5 tastants), sweetness was always the perceived taste. The descriptions only differed in the degree of sweetness.
Nonetheless, other potential non gustatory inputs can also have an effect on parotid reflexes and the salivary flow, such as the oral irritation caused by acids that activate somatosensory afferents supplied by the trigeminal nerve.
Conclusion
This paper has focused on the effects of modifying the taste perception on the parotid salivary reflex in man by altering the perception of sour to sweet. As expected, MCL‑induced sweetness has evoked a significant increase in the parotid salivary flow. Further work needs to be carried out to characterize and more fully understand taste perception, quality coding, and binary taste interactions.
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