Original papers
Analysis of Active Components on Oral Fat Sensations in Oolong Tea
2017 年 23 巻 1 号 p. 71-78
詳細
2017 年 23 巻 1 号 p. 71-78
This study was conducted to identify key components that attenuate oral fat sensations in oolong tea. Oolong tea is often consumed with fatty food because of its “refreshing” effects during the course of a meal. We hypothesized that oolong tea gives a cleanness sensation via psychological and physical effects. We showed through a sensory study that oolong tea consumption significantly reduces oral fat sensations as compared to water. Interfacial tension was measured to evaluate the emulsifying properties of teas and its components. Oolong tea and tea-leaf saponins were significantly lower when compared with other teas and major tea components. Furthermore, the emulsion made from tea-leaf saponins and corn oil was stable after 24 h. Moreover, polyphenol enhanced oolong tea showed significantly decreased oral fattiness when compared with oolong tea. Thus, tea-leaf saponins appear to be key components in reducing oral fat sensation.
Appropriate pairing of beverages and food during a meal enhances the dining experience. Pairing of an astringent beverage and fatty food is traditionally seen in many cultures. For example, red meats are typically served with red wines in western cuisine, while unsweetened teas are often consumed with fatty food in Asian cuisine. These pairings have long been thought to have come from experience and learning which the past experience leads to expectations about linked combinations (Gall and Barnett et al., 1987; De Houwer and Thomas et al., 2001). In recent years, studies have reported the interactions between food pairing, taste stimuli and food ingredients from the viewpoint of human taste and flavor perception (Ahn and Ahnest et al., 2011; Peyrot des Gachons and Mura et al., 2012).
In Asian countries, various unsweetened teas are selectively consumed with food. For instance, in Japan, green tea is consumed with confectionery and oolong tea is consumed with fatty foods. One of the reasons for this phenomenon is considered to be its multiple beneficial effects for health. Both green tea and oolong tea are made from the tea leaves of Camellia sinensis. Because green tea is classified a non-fermented tea, green tea contains more monomeric catechins such as epicatechin, gallocatechin and epigallocatechin than fermented tea (Balentine and Wiseman et al., 1997). These compounds show an inhibitory effect on blood sugar elevation (Iso and Date et al., 2006) and an anti-cariogenic effect (Hamilton-Miller 2001). In contrast, oolong tea, which is classified as a fermented tea, contains high levels of oligomeric catechins and polymerized polyphenols (Hashimoto and Nonaka et al., 1987; Hashimoto and Nonaka et al., 1989). These compounds have an antiobesity effect (Murase and Nagasawa et al., 2002; Han and Kimura et al., 2001) and an inhibitory effect on pancreatic lipase (Shibata and Mitsunaga et al., 2005). In terms of nutritive functional regulation, these differences in pharmacological effects affect the choice of tea during the course of a meal.
In terms of sensory characteristics, “refreshing effects” on our palates are thought to be the reason for the combination of oolong tea and fatty foods. However, it remains unknown how oolong tea exerts such effects. To understand the refreshing effects of oolong tea, there have been scientific studies into the physiochemical properties of tea. Tea tannins have emulsifying properties (Figueroa-Espinoza and Zafimahova et al., 2015) and oolong tea is useful for cleaning oil from endoscopic lenses after transnasal esophagogastroduodenoscopy (Komazawa and Amano et al., 2010). Given these reports, it is hypothesized that oolong tea emulsifies and removes fat contents from the oral cavity during meals.
Here, we show through a sensory study that oolong tea consumption significantly reduces oral fat sensations when compared to water. The study focused on modulation by beverage consumption of perceived oral fattiness elicited by diverse fatty foods. Furthermore, we show through the measurement of interfacial tension and emulsification stability that tea-leaf saponins are one of the active components responsible for reducing fat sensations in the mouth.
Materials Oolong tea, polyphenol enhanced oolong tea, green tea, black tea and natural mineral water (hardness: 30 mg/L) were used as commercial products. Oolong tea leaves were sourced from Suntory (Osaka, Japan). Sunphenon XLB-100 (contains 54.5% (-)-epigallocatechin (EGC)) and Sunphenon EGCg (>90% (-)-epigallocatechin gallate (EGCG)) were sourced from Taiyo Kagaku (Tokyo, Japan). Corn oil, Caffeine, Cerium Sulfate, Sep-Pak C18 cartridge and Silicagel 70 F254 Plate were purchased from Wako (Osaka, Japan), Cosmosil 17C18-opn column from Nacalai tesque (Tokyo, Japan), Polyvinylpyrolidone (PVPP) from ISP Technologies, Inc. (Wayne, NJ).
Sensory Evaluation Multi-sip condition: Fifty-five healthy Japanese adults (female=28, aged 20 – 59 years) participated in the test. The time schedule for multi-sip conditions is shown in Figure 1A. Subjects exposed their mouth to 5 g of fatty food and 3 cups of 5 mL of beverage bilaterally for 30 s. Subjects then exposed their mouth to 5 g of fatty food again and 5 mL of beverage bilaterally for 30 s. Subjects rated their sensation 6 times for fattiness and astringency on a 20 point scale. Dry meat was used as a fatty food. Two beverages were used: oolong tea and natural mineral water. Data was analyzed by paired two-way ANOVA. Post-hoc analysis was conducted by Bonferroni test (SPSS Statistics 21).
(A) Time schedule of multi-sip condition. (B) Time schedule of single-sip condition.
Single-sip condition: Twenty-nine healthy Japanese adults (female = 6, aged 20 – 21 years) participated in the test. The time schedule for single-sip conditions is shown in Figure 1B. Subjects exposed their mouth to 5 g of fatty food. After 30 s, subjects drink a cup of beverage. Subjects rated their sensation 3 times for fattiness and astringency on labeled magnitude scale. Whipped cream was used as a fatty food. Two beverages were used; oolong tea and polyphenol enhanced oolong tea. Data were analyzed by paired two-way ANOVA. Post-hoc analysis was conducted by Bonferroni test (SPSS Statistics 21).
Measurement of Interfacial tension The interfacial tension and dynamic interfacial tension of the oil interface were determined at 32°C by the pendant drop method, which was performed using an automatic interfacial tensiometer (Teclis, Lyon, France). The sample drop was formed in corn oil at the tip of a syringe by pressing the solution out by means of a setscrew. The interfacial tension was calculated at 10 s after making drops. The dynamic interfacial tension was analyzed by obtaining mean of every 10 s for 300 s. Each compound was dissolved in deionized water. Corn oil was used as an oil layer. Data was analyzed by ANOVA followed by Bonferroni adjustment (SPSS Statistics 21).
Separation of water-soluble pectin and tea-leaf saponins in oolong tea A solid phase extraction method was used for the separation of water-soluble pectin and tea-leaf saponins from tea (Shimada 2003). Two milliliters of oolong tea were injected into the column and eluted gradually with 8 mL of distilled water to recover the fraction containing water-soluble pectin, 15 mL of 20% methanol in distilled water and then 5 mL of 80% methanol in distilled water to recover the fraction containing tea-leaf saponins. These fraction were detected by TLC using butanol:acetic acid:water at 4:1:1.5, and detection was performed by spraying with 1% cerium sulfate in 10% sulfuric acid with subsequent heating.
Extraction of tea-leaf saponins Extraction was performed as reported previously (Sagesaka and Uemura et al., 1996). One kilogram of oolong tea leaves were steeped for 15 min at 90°C with 10 L of water. Tea leaves were extracted 3 times in order to remove caffeine and water-soluble compounds. Leaves were immersed with 10 L of 50% ethanol at r.t. for one day. The ethanol extract was concentrated under reduced pressure and lyophilized. The extract was suspended in 3 L of water and mixed with 3 L of cyclohexane. The water layer was treated with 80 g of PVPP twice to remove polyphenols. After filtration, the filtrate was extracted with water-saturated butanol. The butanol layer was evaporated, and the extract was fractionated by reversed-phase column chromatography (0, 40, 60, 80, 100% methanol). Crude saponins (0.814 g) were obtained from 80% methanol. The saponins fraction was assessed by TLC using butanol:acetic acid:water at 4:1:1.5, and detection was performed by spraying with 1% cerium sulfate in 10% sulfuric acid with subsequent heating.
Quantitative analysis of tea-leaf saponins in beverages The concentration of tea-leaf saponins was determined by the phenol-sulfuric acid methods (Dubois and Gilles et al., 1956). Five milliliters of concentrated sulfuric acid and 1 mL of 5% phenol is added to 1 mL of tea-leaf saponins that were lyophilized and dissolved in 16 mL of distilled water. The solution was then heated for 30 min at 80°C, and was left to cool to room temperature. Absorbance was measured at 490 nm using a spectrophotometer. Standard calibration curves were derived from PVPP processed tea-leaf saponins extracted from tea leaves (0, 50, 100, 150, 200 ug/mL) using a 490-nm area.
Evaluation of Emulsion Stability Thirty milliliters of beverage and 3 mL of corn oil were homogenized for 3 min at 19000 rpm, then sonicated for 3 min. This method was determined to be sufficient to homogenize oil and liquid. Emulsions were placed at room temperature for 24 h. Emulsion stability was assessed the state of segregation by visual observation. The concentrations of the samples were as follows: EGCG, 280 ppm; EGC, 280 ppm; caffeine, 280 ppm; pectin, 75 ppm; and tea-leaf saponins, 100 ppm.
Fattiness perception over multi-sip conditions Sensory evaluation was conducted in order to understand the changes in oral sensations during a meal. To pursue oral sensations during an actual meal, the protocol mimicked eating behavior, repeatedly alternating beverages with food (Fig. 1A). Oolong tea was used as a test beverage and water was used as a control.
Figure 2 shows the results of multi-sip conditions. The scores determined after eating fatty foods are indicated on F1–F2 in this figure. Scores determined after drinking beverages were indicated on B1–B4.
(A) Perceived fattiness ratings over multi-sip condition. (B) Perceived astringency ratings over multi-sip condition. The white square indicate water drinking. The solid square indicate oolong tea drinking. The ratings after ingesting fatty food were shown in F1 and F2. The ratings after drinking beverages were shown in B1 – B4. Value represent mean ± SEM (n = 55). Asterisk: p < 0.05, water versus oolong tea (paired two-way ANOVA).
Oral fattiness after drinking oolong tea at the time of B1 and B2 was significantly lower than drinking water (Fig. 2A). Simultaneously, the oral astringency after drinking oolong tea was significantly higher than drinking water (Fig. 2B). Therefore, it appears that oolong tea reduced the fat sensation to a greater degree than water during meals because of its astringency. Astringency is considered to be a tactile sensation and to elicit rough sensations (Breslin and Gilmore et al., 1993; Lee and Lawless 1991). In terms of tribology during oral processing, astringency indicates a loss of lubrication on the oral surface and fattiness indicates low friction, which reflect food properties and oral surface properties (Stokes and Boehm et al., 2013). Hence, fattiness is reduced by the astringent stimulus based on the oral tribological spectrum between fat and astringency. In addition, given the perception mechanisms of fat sensation, which is sensed through somatosensory systems (Grabenhorst and Rolls et al. 2014) and fatty acid sensors such as GPR40 and GRP120 (Matsumura and Mizushige et al. 2007), it is possible that astringent compounds interact with fatty acid sensors.
Although the oral astringency of oolong tea at B3 was significantly higher than that of water (Fig. 2B), there were no differences in oral fattiness between oolong tea and water at B3 (Fig. 2A). This result suggests that fat is physically removed from the mouth through multiple drinking, regardless of beverage.
Comparing the oral fattiness after the second eating showed no differences between oolong tea and water (Fig. 2B, F2). Although the oral astringency of oolong tea at B3, indicating the oral state before rating F2, was significantly higher than water (Fig. 2B, B3), there was no difference in oral fattiness at F2 (Fig. 2A). This suggests that pre-rinsing with of astringent stimuli has no effect on fattiness perception. Thus, oral fattiness is perceived regardless of the preceding state of oral astringency. In addition, the oral astringency of oolong tea at F2 did not differ from that of water (Fig. 2B). This suggests that oral astringency of oolong tea was nullified by eating fatty food. It is known that astringency decreases with lubricating rinses (Breslin and Gilmore et al., 1993); thus, oral astringency may balance with the lubricating sensation of fatty food.
Interestingly, fattiness after drinking oolong tea at B4 was significantly lower than that after drinking water (Fig. 2A). Thus, fattiness remained low without drinking at 120 s. The oral tribological spectrum between fat and astringency may keep affecting the oral fat perception for two minutes, while oolong tea physically removes fat from the oral surface.
Emulsifying properties of tea and its components Based on the results of sensory evaluation, we hypothesized that oolong tea emulsifies and removes fat from the oral cavity. To investigate this hypothesis, we measured the emulsifying properties of teas by measuring interfacial tension. Interfacial tension is a quantitative measurement of the emulsifying properties between oil and liquid. It shows the adhesive force between two layers, and the lower interfacial tension indicates that two layers strongly adapt to one another. The interfacial tension was measured at 32°C, which assumed oral temperature after consuming food and beverages. Corn oil was used as an oil layer instead of animal fat, which was used in sensory evaluation, as animal fat is a solid at the measurement temperature, and the emulsifying ability of beverages with vegetable oils such as soy bean oil, olive oil or palm oil show the same trends by the Wilhelmy method, regardless of fatty acid composition (data not shown). Although odd-chain fatty acids such as myristoleic acid, heptadecanoic acid and heptadecenoic acid are not present in corn oil, further studies are needed to understand the effects of composition on the emulsifying ability of beverages.
Figure 3A shows the interfacial tension of commercial unsweetened teas. Among mineral water and teas, green tea and oolong tea showed significantly lower tension than mineral water. Thus, it appears that these teas are able to emulsify oils. The difference between mineral water and black tea did not reach significance. However, the interfacial tension of black tea tends to be low when compared to water ( p = 0.059). These teas were prepared from the tea leaves of Camellia sinensis. Therefore, this lower interfacial tension is thought to be caused by the components commonly present in tea.
Interfacial tension of commercial beverages and tea components. A, Commercial unsweetened beverages; B, Caffeine; C, EGCG; D, Water-soluble pectin; E, Tea-leaf saponins. Value represent mean ± SD (n = 3). Asterisk: p < 0.05, versus water (ANOVA Bonferroni adjustment).
We then measured the interfacial tension of major tea components. Caffeine, EGC, EGCG, water-soluble pectin and tea-leaf saponins were measured with the same protocol. Water-soluble pectin and tea-leaf saponins were extracted from tea leaves for this measurement. Concentrations were based on the range of concentrations thought to be present in commercial tea. (Balentine and Wiseman et al., 1997; Wang and Provan et al., 2000; Shimada 2003).
The results are shown in Figure 3B–3E. The interfacial tension of caffeine was almost constant with concentration and there were no significant differences compared to water (Fig. 3B). Similarly, EGCG was almost constant and there were no significant differences compared to water (Fig. 3C). The interfacial tension of EGC did not differ from that of water using the Wilhelmy method (data not shown). The chemical structure of EGCG is similar to EGC, but EGCG has a galloyl group attached by an ester bond at the 3-position of the hydroxyl group of EGC. Because the steric structure of EGCG outwardly projects the ester moiety, the hydrophobicity of EGCG is higher than EGC (Hayashi and Ujihara et al., 2005; Kajiya and Kumazawa et al., 2001). This structural difference is believed to affect to the partition coefficient (P) values and emulsifying ability. Although there were some exceptions, it is known that compounds with high and low P values were not good emulsifiers. The log P value of EGCG was reported to be 0.39 and EGCG did not form an emulsion with n-tetradecane in a concentration range from 0.1 to 1 mM (Luo and Murray et al., 2011). Given these results, EGCG and EGC do not have emulsifying ability, regardless of the differences in hydrophobicity.
On the other hand, water-soluble pectin showed low interfacial tension compared to water (Fig. 3D). Our results agree that pectin is a thickening agent and protein stabilizer. It is known that the emulsifying ability of sugar beet pectin and soybean soluble polysaccharide increase in a concentration-dependent manner (Nakauma and Funami et al., 2008). Thus, the interfacial tension of water-soluble pectin from tea leaves would also depend on concentration, as its structure is similar to other plant-derived pectin (Ele-Ekouna and Pau-Roblot et al., 2011; Yapo, 2011). Although pectin forms complexes with the gallate-type catechins such as EGCG (Hayashi and Ujihara et al., 2005), it is unclear that whether pectin actually exerts an emulsifying effect on tea.
The interfacial tension of tea-leaf saponins was also significantly lower than that of water at all concentrations tested, similarly to water-soluble pectin (Fig. 3E). The structure of tea-leaf saponins, such as Theasaponin B1, includes an aglycone and a sugar chain (Nakai and Fukui et al., 1994; Sagesaka and Uemura et al., 1994). The aglycone comprising sapogenin is hydrophobic, and the sugar chain which has numerous hydroxyl groups is hydrophilic. This non-polar (sapogenin) and polar (sugar) group gives high emulsifying ability. Saponins are almost exclusively present in plants such as soybeans, peanuts, theaceous plants and many are used as surfactants because of their amphiphilicity (Oakenfull 1981; Güçlü Üstündağ and Mazza 2007; Chen and Yang et al., 2010). For example, soybean saponins increase the stability of whey proteins (Shimoyamada and Ootsubo et al., 2000) and quinoa saponins increase the stability of emulsions (Chauhan and Cui et al., 2009). Although the strength of surface/interfacial properties differs with their structure, tea saponins have higher dilatational and shear elasticity than ginsenosides (saponins from Panax ginseng) but lower dilatational and shear elasticity than Quillaja saponins (Pagureva and Tcholakova et al., 2016).
When compared to the interfacial tension of oolong tea, caffeine, EGCG and water-soluble pectin showed significantly higher interfacial tension (p < 0.01 respectively). On the other hand, 50 – 100 ppm tea-leaf saponins showed significantly lower interfacial tension than oolong tea (p < 0.01). The concentration of tea-leaf saponins in commercial unsweetened beverages measured by phenol-sulfate acid method was as follows: black tea, 17.3 ppm; and green tea, 17.8 ppm. The oolong tea which used in the sensory evaluation had a concentration of 53.1 ppm. Thus, oolong tea contains 3 – 4 times more tea-leaf saponins than green tea and black tea. Moreover, the amount of saponins in oolong tea was sufficient to decrease interfacial tension. Therefore, tea-leaf saponins may play a role in the emulsifying ability of oolong tea and the attenuation of fattiness during a meal.
Emulsion stability of tea-leaf saponins Further emulsifying properties of tea-leaf saponins were analyzed by the dynamic interfacial tension. Figure 4 shows the temporal changes of interfacial tension of tea-leaf saponins. The interfacial tension decreased rapidly in the first 50 s, indicating an adsorption process. After 300 s, the interfacial tension reached equilibrium at the same value, regardless of its concentration. The speed of equilibrium at 50 ppm saponins was slower than at 100 ppm. Because low concentrations contain smaller amounts of molecule in the water phase, the coagulation of molecules in the water phase at the liquid-oil interface decelerate toward the low concentration.
Dynamic interfacial tension of 50 ppm (black Square), 70 ppm (white square) and 100 ppm (black circle) of tea-leaf saponins solutions from oolong tea.
In order to evaluate the emulsification stability of tea-leaf saponins, the detachment process of emulsions made by homogenization of tea compounds and oil was tracked by visual observation at 24 h. Figure 5 shows the detachment process of emulsions. All emulsions were completely emulsified by homogenization and sonication just after emulsification. At 5 min after emulsification, emulsions made from water, caffeine, EGCG, EGC and water-soluble pectin formed oil drops in the emulsion layer. However, emulsions made from tea-leaf saponins did not form any oil drops. After 24 h, the oil layer was specifically segregated from the emulsion layer made from water, caffeine, EGCG, EGC and water-soluble pectin. As for water, caffeine, EGCG and water-soluble pectin, creaming layers were also observed between the oil and emulsion layers. However, the oil layer of tea-leaf saponins occurred in very small amounts. This means that the emulsion of tea-leaf saponins is stable after 24 h. Thus, it is appears that after tea-leaf saponins emulsify with oil, saponins hold oil within a micelle state over a long period.
Visual observation of the detachment process on the emulsion of tea components. Solid line indicate the boundary between oil and emulsion area.
Effects of tea-leaf saponins on attenuation of fat sensation Given the results of the emulsifying properties and emulsion stability, tea-leaf saponins may play a role in removing fat from the oral cavity. We therefore evaluated the effects of tea-leaf saponins as an emulsifier on the reduction of fattiness by sensory evaluation.
However, it was difficult to evaluate oral sensation using a saponin solution rinse or saponin-enhanced oolong tea, because of its soap-like taste. In addition, it is necessary to use beverages that have similar taste qualities as oolong tea, which was the control beverage, in order to negate the effects of taste differences. Therefore, we searched for a suitable test beverage for sensory evaluation that showed significantly lower interfacial tension and contained more saponins than oolong tea, while possessing a similar taste as oolong tea. Among commercial oolong teas, polyphenol-enhanced oolong tea contained 69.1 ppm tea-leaf saponins. Nevertheless, the profile of major contents was almost the same as oolong tea, and the interfacial tension of polyphenol enhanced oolong tea was lower than that of oolong tea (Table. 1). Although it is possible that polymerized polyphenols and other compounds related to lowered interfacial tension in polyphenols enhanced oolong tea, we evaluated the effects of tea-leaf saponins on the reduction of fattiness by comparing oolong tea and polyphenol-enhanced oolong tea.
| Components of oolong tea analyzed by HPLC | ||
|---|---|---|
| (ppm) | Oolong tea | Polyphenol enhanced oolong tea |
| Gallocatechin | 40 | 46 |
| Epigallocatechin | 37 | 33 |
| Catechin | 11 | 10 |
| Epicatechin | 7 | 6 |
| Epigallocatechin gallate | 41 | 48 |
| Gallocatechin gallate | 30 | 38 |
| Epigallocatechin gallate | 10 | 11 |
| Catechin gallate | 6 | 7 |
| Caffeine | 159 | 141 |
| Interfacial tension analyzed by pendant-drop method | ||
| Dyne (mN/m) | 20.3 ± 0.18 | 17.0 ± 1.23 * |
The Value of interfacial tension represent mean ± SD (n = 3). Asterisk: p < 0.05, versus oolong tea (ANOVA Bonferroni adjustment).
Therefore, polyphenol-enhanced oolong tea was used as a test beverage in sensory evaluation. Oolong tea was used as a control beverage. The protocol used a single sip of beverage to exclude the effects of accumulation of astringency (Fig. 1B). The results are shown in Figure 6. The interaction between time and beverage on fattiness reached significance over the course of the test (Fig. 6A). The reduction induced by polyphenol-enhanced oolong tea on fattiness over the test was 1.4 times higher than that of oolong tea.
(A) Perceived fattiness ratings over single-sip condition. (B) Perceived astringency ratings over single-sip. The white square indicate oolong tea drinking. The solid square indicate polyphenol enhanced oolong tea drinking. The rating after ingesting fatty meal was shown on F1. The rating after drinking beverages was shown on B1 and B2. Value represent mean ± SEM (n = 9). Asterisk: p < 0.05, oolong tea versus polyphenol enhanced oolong tea (paired two-way ANOVA).
On the other hand, the interaction between time and beverage on astringency did not reach significance over the test (p = 0.05) (Fig. 6B). These results indicated that the increase of emulsifying ability may support a decrease in fat sensation under the same astringency. In addition, tea-leaf saponins may play a role on the attenuation of oral fattiness during meals.
Tea-leaf saponins are only present at 0.04 – 0.3% in tea leaves (Muramatsu and Kokuni et al., 2002). Using more tea leaves and extracting at higher temperatures may yield more tea-leaf saponins in tea. However, these conditions impair tea flavor because of the increase of other taste compounds, which have bitter, astringent or unpleasant flavors. The amount of bitter catechins, such as EC and EGCG, in oolong tea is lower than in green tea (Balentine and Wiseman et al., 1997), but it contains numerous polymerized catechins with astringent properties (Peleg and Gacon et al., 1999), and polysaccharides such as saponins are formed during the fermentation process.
In this study, the psychophysical factors and physical properties that underlined the combination of oolong tea and fatty food were analyzed. As a result, two factors were identified. The first factor is the perceptual cleaning effect that astringent stimuli attenuate fat texture based on tribology in the mouth. The second factor is the physical cleaning effect, in which oolong tea helps to remove fat from the mouth due to its high emulsifying ability. In addition, it appears that tea-leaf saponins play a role in emulsifying ability and stability.
These background factors may affect the pairing of oolong tea and fatty food during meals. By means of both perceptual and physical elimination of excess fat sensation throughout the multiple exposure of oolong tea during meals, oolong tea may give a “refreshing effect” when eating fatty foods.
Acknowledgements We would like to thank Dr. Paul Breslin and Dr. Catherin Peyrot des Gachons of Monell Chemical Senses Center for advising on the methods for sensory evaluation.
