Notes
Evaluation of Taste Solutions with or without Aromas Based on the Relationship between Individual Resting and Stimulated Salivation
2020 Volume 26 Issue 3 Pages 451-457
Details
2020 Volume 26 Issue 3 Pages 451-457
The phenomena of taste and aroma are difficult to express in words. To develop a method for evaluating tastes and aromas without language, we focused on salivation, which is involved in eating behavior and can be noninvasively examined. We analyzed correlations between resting salivation and salivation stimulated by individual basic taste solutions at recognizable concentrations. We found that salivation induced by basic tastes was significantly correlated with resting salivation. Interestingly, salivation by sour taste showed a significantly lower correlation than other basic tastes. We also analyzed salivation induced by sweet or umami tastes with azuki, matsutake, or dried bonito-aromas and found that some combinations had a lower correlation, and the preferences for these combinations varied relatively widely. Our results could lead to a novel analytical method for evaluating the quality and preference of taste solutions with aroma based on the relationship between individual resting and stimulated salivation.
The appreciation of tastes and aromas depends on sensory evaluation; however, the communication of those sensations to others is a verbal activity, and the sharing of taste and aroma evaluation results between countries with different languages is challenging. As a potential alternative, we examined evaluation methods that use human physiological responses.
Salivation is a physiological response that is closely related to food intake (Humphrey and Williamson, 2001). Saliva works primarily as a digestive juice, catabolizing starch into maltose, and moisturizing food in order to form a bolus suitable for swallowing. When food is placed in the mouth, secretion of saliva is stimulated by various peripheral factors such as taste, olfaction, temperature, and mastication. Thus, salivation arises from a variety of stimuli.
This study aimed to elucidate the relationship between salivation arising from the basic tastes with or without aromas and the preference for these. We first focused on salivation stimulated by five basic tastes: sweet, bitter, sour, salty, and umami. Basic tastes consist of the senses indicating what is good to ingest such as sweet and umami tastes, and what should be avoided such as bitter and sour tastes (Breslin, 2013). As the body responds differently to each taste, it is expected that a salivation system depending on each taste quality exists. Therefore, we analyzed and compared all the basic tastes.
The effect of aroma combined with taste is also examined in this study. The mechanism of salivation can be roughly divided into two pathways: via the cerebral cortex or not (Proctor, 2016). Signals arising from taste, temperature, mastication, and touch stimuli do not pass through the cerebral cortex. On the other hand, salivation based on aroma, auditory, visual, etc. stimuli go through the cerebral cortex (Griffiths, 2015). Aroma is the unique sense that induces cerebral cortex-mediated salivation when food is recognized in the oral cavity. Therefore, salivation elicited by a stimulus of a mixture of taste and aroma could be a model for analyzing the interaction between sensory systems with and without cerebral cortex involvement. However, there is a scarcity of information on the relationship between salivation by taste and aroma (Schiffman and Miletic, 1999).
In this study, we also examined the relationship between resting salivation and stimulated salivation of individuals. This is because salivation by oral stimulation has been analyzed using group averages, with no consideration of individual differences in many studies (Bonnans and Noble, 1995; Heinzerling et al., 2011; Hodson and Linden, 2006). We individually analyzed the relationship between resting salivation and stimulated salivation by taste and aroma, with results showing possible correlations, indicating variance in the preference for flavored taste solutions.
Taste and aroma materials Sucralose was used to represent sweet taste (gifted by San-Ei Gen F.F.I., Inc., Osaka, Japan). Caffeine was used to represent bitter taste (gifted by Ogawa & Co., Ltd, Tokyo, Japan). Citric acid, table salt and monosodium glutamate (MSG) were purchased from a local market and were used to represent sour, salty and umami tastes, respectively. Azuki bean aroma essence was purchased from T and M Co., Ltd. (Funabashi, Japan). Matsutake mushroom aroma essence was purchased from Tomizawa Shoten Co., Ltd. (Tokyo, Japan). Dried bonito (dried, fermented, and smoked skipjack tuna) aroma essence was gifted by Nagaoka Co., Ltd. (Osaka, Japan).
Flavored taste solutions Experiment 1: All taste solutions were prepared using water passed through an activated carbon filter. Solutions of 0.5 mM sucralose, 100 mM MSG, 150 mM NaCl, 40 mM caffeine, and 100 mM citric acid were used for sweet, umami, salty, bitter, and sour taste solutions, respectively. The taste solutions were prepared at concentrations within the ranges found naturally in foods.
Experiment 2: Solutions of 0.5 mM sucralose and 100 mM MSG were prepared using filtered water for sweet or umami taste solutions, respectively. Aroma essence (azuki bean, matsutake mushroom, or dried bonito) was added to the taste solutions to a concentration of 0.1% (v/v). The test solutions were prepared to produce “normal” or “abnormal” combinations of tastes and aromas. The normal combinations are azuki aroma + sweet taste, and matsutake or dried bonito aroma + umami taste. The abnormal combinations are the opposite: azuki aroma + umami taste, and matsutake or dried bonito aroma + sweet taste.
Sensory tests in human subjects Subjects: All experiments were performed in accordance with the standards set out in the Declaration of Helsinki. The experimental protocol was approved by the ethics committee at the Food Research Institute, National Agriculture and Food Research Organization, Japan (No. 28NFRI-0003). The experimental procedure was provided to all subjects in writing, after which subjects provided written consent to participate. Subjects had the right to cease participation after agreeing to take part in the study. The total number of subjects in the experiments was 29 (10 males and 19 females). All subjects were nonsmokers in good general health. Depending on the subjects? availability, they participated in both Experiments 1 and 2 (5 males and 14 females), only in Experiment 1 (4 males and 3 females) or only in Experiment 2 (2 males and 1 female). In one experiment, some subjects did not complete all samples.
Procedure: Prior to the study, all subjects were seated on a chair in a quiet laboratory. They received instructions on the experimental procedure. The procedure was as follows. Before the tasting, the subject's saliva was collected for 1 min. After a break of 50 s, the subjects tasted the taste solutions for 10 s and swallowed. Taste and aroma solutions were dropped onto the tongue using medicine droppers. After swallowing, saliva was continuously collected every minute for 5 min. The subjects then recorded their evaluations of their preferences using a structured line scale. After the saliva was collected, the subjects rinsed their mouths with activated carbon-filtered water. The next sample was tested at least 2 h later.
Saliva collection We collected the subjects' saliva in 2-oz plastic cups. The amount of saliva was determined by weighing the cups on a scale before and after saliva collection. Salivation amount for the 1 min immediately after swallowing was used for the stimulated salivation amount for tastes and aromas. Salivation amount for the 1 min immediately before tasting was used for the resting saliva amount. The individual resting salivation values were calculated as the average of saliva weight values measured prior to the stimulation from the various samples.
Psychophysical scaling We analyzed preferences using a structured line scale. The line was 196 mm in length, and ranged from −1 (dislike), through 0 (midpoint), to +1 (like), following the design of Kemp, Hollowood and Hort (2009).
Statistical processing The relationships between resting salivation and salivation by taste and aroma were plotted on graphs and correlation values were calculated using Microsoft Excel (Microsoft Corp., Redmond, WA, USA). Probability values for the correlation were calculated using a t distribution with (n−2) degrees of freedom. Statistical significance among correlation values was determined with Bonferroni's correction. Values of average, variance, and standard error of salivation and preference scores were calculated using Microsoft Excel. Statistical significance was calculated using analysis of variance (ANOVA) followed by Tukey's test with SigmaPlot software (Systat Inc., Chicago, IL, USA).
Individual differences in salivation We examined the relationships between resting salivation and stimulated salivation by basic tastes (Experiment 1) and flavored tastes (Experiment 2), individually. The subjects were primarily women in their 20s. The average amount of salivation before stimulation was used for the individual resting salivation. Resting salivation amount varied widely, with the majority of subjects being in their 20s (Table 1, Fig. 1A). There was no significant difference in resting salivation between the subjects who were 20–29 years old and those who were ≥30 years old and between males and females (Fig. 1).
| Age (years) | Male (n) | Female (n) | Total (n) |
|---|---|---|---|
| 20–29 | 7 | 12 | 19 |
| 30–39 | 0 | 3 | 3 |
| 40–49 | 2 | 3 | 5 |
| 50–59 | 0 | 0 | 0 |
| 60 and above | 1 | 1 | 2 |
| Total | 10 | 19 | 29 |
Distribution of individual differences in resting salivation. The histograms show the distribution of resting saliva for all participants by age (A) and sex (B) (n = 29, Table 1). The average amount of salivation before stimulation was used for the individual resting salivation.
Experiment 1: Relationship between resting salivation and stimulated salivation by basic tastes Results for stimulated salivation by basic tastes were averaged and are shown in Table 2. Basic taste solutions were prepared in concentrations that allowed clear distinguishing of the quality of tastes. The concentration of the sour taste solution was set a level that is typically avoided (100 mM citrate acid). As per the results, the sour taste induced significantly more salivation than the other basic tastes, reflecting previous reports (Hodson and Linden, 2006). Other tastes induced almost equal salivation. Next, correlations between resting salivation and stimulated salivation by basic tastes were analyzed individually (Table 2). Although stimulated salivation was significantly correlated with resting salivation for every basic taste, interestingly, salivation by the sour taste showed a significantly lower correlation than all the other basic tastes. The scatter plots showed the relationship between stimulated salivation (y-axis) and resting salivation (x-axis) (Figure 2). Linear regression analyses using these plots showed a characteristic feature of salivation by sour taste stimulation. The regression lines (Figure 2) were shown as y = y0 + ax (y: salivation by taste stimuli, x: resting salivation, y0: basal salivation stimulated by taste, a: proportional constant of stimulated salivation depending on the resting salivation). The value of “y0” for sour taste was remarkably higher than that for the other tastes, indicating that subjects with low resting salivation had relatively higher salivation by sour taste stimulation than other taste stimulations.
| Salivation | ||||||
|---|---|---|---|---|---|---|
| Salivation (g/min) | Correlation between salivation by taste with or without aromas and resting salivation | Preference | n | |||
| Experiment 1: Basic taste | mean | r-value | p value | mean | variance | |
| Sweet | 0.970 | 0.890 | <0.005 | 0.227 | 0.170 | 20 |
| Umami | 1.080 | 0.806 | <0.005 | 0.334 | 0.120 | 10 |
| Salty | 1.000 | 0.872 | <0.005 | −0.042 | 0.030 | 18 |
| Bitter | 0.930 | 0.926 | <0.005 | −0.750 | 0.047 | 10 |
| Sour | 3.020 | 0.555 | 0.0169† | −0.088 | 0.114 | 18 |
| Experiment 2: Taste + Aroma | mean | *r-value | p value | mean | *variance | |
| Sweet | 1.200 | 0.840 | <0.0001 | 0.339 | 0.231 | 17 |
| Sweet + Azuki | 1.090 | 0.840 | <0.0001 | 0.297 | 0.190 | 16 |
| Sweet + Matsutake | 1.360 | 0.613 | 0.010 | −0.079 | 0.253 | 14 |
| Umami | 1.280 | 0.876 | <0.0001 | 0.256 | 0.090 | 14 |
| Umami + Azuki | 1.200 | 0.910 | <0.0001 | −0.358 | 0.090 | 14 |
| Umami + Matsutake | 1.360 | 0.936 | <0.0001 | −0.289 | 0.041 | 8 |
| Umami + Dried Bonito | 0.900 | 0.608 | 0.031‡ | 0.160 | 0.163 | 10 |
Relationship between resting salivation and the stimulated salivation by basic tastes individually. The graphs show two-dimensional relationship between resting salivation and the stimulated salivation by basic tastes. The details are shown in Table 2. Each equation of a regression line is written in the graph.
y: Salivation by taste stimuli, x: Resting salivation
Experiment 2: Effects of aroma addition to taste solutions on salivation and preferences Azuki bean and matsutake mushroom aroma were selected as sweet taste-related and umami taste-related aromas, respectively. Azuki beans are used in Japanese sweets and are rarely used in umami taste foods. Conversely, matsutake mushrooms are used in Japanese soups and cooked rice but not in sweet foods. However, as the aroma and essence of matsutake mushroom were not recognized by many in their 20s, an alternative umami taste-related aroma, namely dried bonito (Katsuobushi; a popular aroma used in the Japanese soup dashi), was also used. Results for salivation were averaged and expressed as a ratio relative to resting salivation (Table 2). The addition of aroma did not significantly alter salivation, regardless of whether the aroma-taste combinations were normal (sweet + azuki, umami + matsutake or dried bonito) or abnormal (sweet + matsutake, umami + azuki). Results for evaluations of preferences showed that preferences for abnormal combinations were significantly reduced, as expected. On the other hand, unexpectedly, preferences did not increase for normal combinations.
Next, salivation by flavored taste solutions was examined by correlations between resting salivation and stimulated salivation, as in Experiment 1 (Table 2). Salivation by a dried bonito flavored umami taste solution showed a significantly lower correlation, and that by a sweet taste solution flavored with matsutake showed a low correlation (not significant). This result indicated that it was irrelevant whether the combination of taste and aroma was normal or abnormal, preferred or not preferred. The scatter plot and linear regression analyses were carried out as in Experiment 1 (Figure 3). None of the factors correlated with the values of “y0”, which were relatively wide from −0.456 to +0.810. Moreover, it is unclear how the values of “y0” could be less than 0. Then, we focused on the variance of preferences for the flavored taste solutions (Table 2). Interestingly, it was observed that the combination of taste and aroma, for which preference variance is high, tended to lower the correlation between resting salivation and stimulated salivation. The correlation between the variance value of preference and the r value for salivation was −0.659 (p = 0.0538) (Table 2). On the other hand, there was no significant correlation between the mean salivation and the mean preference values (Table 2).
Relationship between resting salivation and the stimulated salivation by sweet or umami tastes with aroma individually. The graphs show two-dimensional relationship between resting salivation and the stimulated salivation by sweet or umami taste with aroma. The details are shown in Table 2. Each equation of a regression line is written in the graph.
y: Salivation by taste with aroma stimuli, x: Resting salivation
This study shows that the correlation coefficient between resting salivation and stimulated salivation by tastes and aromas might be useful as a marker to evaluate food quality, and we propose a novel viewpoint for the quality and preference evaluation of taste solutions with aroma.
In this paper, we focused on individual differences in salivation. In salivation research, individual differences in resting salivation of healthy people have not been emphasized. When the resting salivation is extremely low (<0.1 g/min), it is taken as a subjective symptom of dry mouth (Sreebny and Schwartz, 1997). On the other hand, it is generally said that salivation is about 0.3 g/min (Humphrey and Williamson, 2001). Therefore, salivation induced by stimuli such as taste, olfaction, and mastication has been reported by simple averaging, without taking into account individual differences (Neyraud et al., 2009; Hodson and Linden, 2006; Polland et al., 2003). By averaging, individual differences between resting salivation and salivation by oral stimulation have been treated as the range of error. Therefore, this study clarifies a phenomenon that has, to date, been recognized as a background phenomenon caused by averaging.
At first, we analyzed stimulated salivation by basic tastes involving non-cerebral cortex pathways. Each basic taste was classified as a positive taste for the body or a taste to be avoided. However, particularly in humans, unfavorable tastes may not be avoided at low concentrations. Therefore, in this study, we set the concentrations of the taste solutions so they could be clearly recognized, and sour and bitter tastes were avoided. In our study, salivation by sour taste was featured from the viewpoint of the correlation between resting salivation and stimulated salivation. Salivation by sour taste might not only be a result of taste stimulation but may also act as a buffering system to maintain pH in the oral cavity. In many studies, citric acid, which was used as the sour taste in our study, has been used as a substance to lower pH (Brorsson et al., 2014; Li-Hui et al., 2016) rather than as a taste stimulant. Christensen et al. (1987) divided subjects into groups with low and high levels of resting salivation, and showed that the ratio of acid-induced salivation to resting salivation was higher in the low than in the high resting salivation group. They also suggested that the difference did not relate to the sour taste sensitivity, but might relate pH maintenance. The greater salivation seen in the low resting salivation group in Christensen's study might be related to the low correlation of the sour taste seen in our study. We caution against simple comparison of results since the concentration of taste substances and the intensity of tastes may vary between studies. Future experiments involving taste solutions at multiple concentrations are required.
Next, we analyzed changes in salivation by adding aromas to the taste solutions. There was no correlation between salivation and preference of the solutions. On the other hand, we found that some aroma and taste combinations decreased the correlation between resting and stimulated salivation, and that the decrease in correlation is related to variation in the preference of solutions. Emotion has been employed as a keyword for relationships among aroma, preference, and salivation. Emotion acts on the autonomic nervous system to affect salivation (Ekman et al., 1983). Aroma induces salivation via the cerebral cortex, an area related to emotion (Proctor, 2016). The relationship between emotion and salivation is complex, as emotion is associated with pleasure, which reflects a favorable feeling, and arousal, which reflects a feeling of excitement (Reisenzein, 1994). Pleasure and arousal are considered to act separately on the autonomic nervous system (Bensafi et al., 2002), suggesting that salivation might be affected by both pleasure and arousal. In this context, the evaluation of both pleasure and arousal induced by the flavored solutions might be necessary to understand aroma-induced salivation. However, our evaluation of preference might be a satisfactory substitute for pleasure, though not for arousal. An evaluation that lacks consideration of arousal might lead to complex results, whereby some aroma and taste combinations decrease the correlation independent of the relationship between salivation and preference. Employing sensory evaluation with an arousal rating might improve the accuracy of evaluating the relationship between salivation amount and sensory evaluation.
The results of this study are somewhat limited because only three aromas were analyzed and the concentration of the taste solution was constant. Thus, future experiments involving some of the wide array of other aroma types that exist would be beneficial.
Unfortunately, the azuki and dried bonito flavored solutions we chose recalled foods such as Japanese sweets and soups, but did not increase palatability, precluding analysis of the relationship between increased preference and salivation. Other taste qualities (such as salty taste) or sensations (such as texture) might be needed to increase preference. In addition, the combination of matsutake aroma and umami was unfamiliar to young subjects, and was probably the cause of the reduced palatability. The ingredients MSG and nucleic acid bring about umami flavors; however, both umami tastes of matsutake mushroom and dried bonito are not from MSG but from nucleic acids. We do not deny that the flavor of nucleic acids differs from that of MSG, which is why the aromas of matsutake and dried bonito did not increase palatability. However, the quality of their umami taste should be the same since MSG and nucleic acids bind to the same umami receptor, T1r1/T1r3 (Zhang, et al., 2008).
In future studies, it would be advisable to separately investigate the appropriate combinations of aromas and tastes in advance in light of our constantly changing dietary environment.
We compared the correlation coefficients of resting salivation and stimulated salivation by taste solutions with or without aroma individually. We found that salivation by basic tastes was significantly correlated with resting salivation, while the correlation was decreased by sour taste and some taste/aroma combinations. There are multiple neurological pathways that induce salivation from food stimulation, and the relationship between resting salivation and stimulated salivation is considered to be one of the factors needed to understand their interaction. As with salivation, food also stimulates various digestive organs via the cerebral cortex or not. Therefore, studying the relationship between salivation by taste and aroma could help to elucidate the relationship between food stimulation and the digestive system.
Acknowledgments We thank San-Ei Gen F.F.I., Inc. for providing the non-nutritive sweeteners, Ogawa & Co., Ltd. for a bitter substance, Nagaoka Co., Ltd. for an aroma substance, and Ms. Shindo for providing technical assistance. This work was supported by The Japan Food Chemical Research Foundation, and KAKENHI (16K00840, 19K02368).
