Background Taste is an important factor in evaluating the quality of tea. The sweet and umami tastes are usually well-accepted for consumers, whereas the bitter and astringent tastes are usually undesired, but they are important for providing the complex sensory perceptions of Camellia teas. The compounds responsible for the bitterness and astringency in tea leaves can be modified by processing, and subsequently endowed different taste of various tea types. Therefore, the taste of tea is closely related to the chemical constituents, and the taste mechanism is critical for improving tea quality. Scope and approach The aim of this review is to review and discuss the association between chemical composition of Camellia teas and their effects on bitterness, astringency, sweetness aftertaste and umami. Key findings and conclusions In the tea infusion, flavonol-O-glycosides, tannins and galloylated catechins are the main astringent compounds, caffeine and non-galloylated catechins enhance the tea bitterness. Furthermore, l -theanine, succinic acid, gallic acid and theogallin contribute to the umami taste. Sweetness aftertaste is a unique perception of green tea, which is attributed to the hydrolysis of galloylated catechins. T1R2 and T1R3 have been identified as sweet and umami taste receptors, while T2Rs functionas the bitter taste receptor.

Association between chemistry and taste of tea: A review

Astringency and bitterness are organoleptic properties widely linked to tannin compounds. Due to their significance to food chemistry, the food industry, and to human nutrition and health, these tannins' taste properties have been a line of worldwide research. In recent years, significant advances have been made in understanding the molecular perception of astringency pointing to the contribution of different oral key players. Regarding bitterness, several polyphenols have been identified has new agonists of these receptors. This review summarizes the last data about the knowledge of these taste properties perceived by tannins. Ultimately, tannins' astringency and bitterness are hand-in-hand taste properties, and future studies should be adapted to understand how the proper perception of one taste could affect the perception of the other one.

https://www.mdpi.com/1420-3049/25/11/2590/pdf

Tannins in Food: Insights into the Molecular Perception of Astringency and Bitter Taste.

Flavonoids are polyphenolic phytochemicals produced in fruits, nuts and vegetables and dietary consumption of these structurally diverse compounds is associated with multiple health benefits including increased lifespan, decreased cardiovascular problems and low rates of metabolic diseases. Preclinical studies with individual flavonoids demonstrate that these compounds exhibit anti-inflammatory and anticancer activities and they enhance the immune system. Their effectiveness in both chemoprevention and chemotherapy is associated with their targeting of multiple genes/pathways including nuclear receptors, the aryl hydrocarbon receptor (AhR), kinases, receptor tyrosine kinases and G protein-coupled receptors. However, despite the remarkable preclinical activities of flavonoids, their clinical applications have been limited and this is due, in part, to problems in drug delivery and poor bioavailability and these problems are being addressed. Further improvements that will expand clinical applications of flavonoids include mechanism-based precision medicine approaches which will identify critical mechanisms of action of individual flavonoids with optimal activities that can be used in combination therapies.

Flavonoids: structure-function and mechanisms of action and opportunities for drug development.

Taste receptors, first identified on the tongue, are best known for their role in guiding our dietary preferences. The expression of taste receptors for umami, sweet, and bitter have been demonstrated in tissues outside of the oral cavity, including in the airway, brain, gastrointestinal tract, and reproductive organs. The extra-oral taste receptor chemosensory pathways and the endogenous taste receptor ligands are generally unknown, but there is increasing data suggesting that taste receptors are involved in regulating some aspects of innate immunity, and may potentially control the composition of the nasal microbiome in healthy individuals or patients with upper respiratory diseases like chronic rhinosinusitis (CRS). For this reason, taste receptors may serve as potential therapeutic targets, providing alternatives to conventional antibiotics. This review focuses on the physiology of sweet (T1R) and bitter (T2R) taste receptors in the airway and their activation by secreted bacterial products. There is particular focus on T2R38 in sinonasal ciliated cells, as well as the sweet and bitter receptors found on specialized sinonasal solitary chemosensory cells. Additionally, this review explores the impact of genetic variations in these receptors on the differential susceptibility of patients to upper airway infections, such as CRS.

https://www.mdpi.com/2072-6643/11/9/2017/pdf

Taste Receptors in Upper Airway Innate Immunity

A novel strategy for discriminating different cultivation and screening odor and taste flavor compounds in Xinhui tangerine peel using E-nose, E-tongue, and chemometrics.

Hochu-ekki-to (TJ-41, a traditional herbal medicine) is used for preventing airway infections caused by viruss or bacterias in patients with chronic obstructive pulmonary disease (COPD). TJ-41 is known to inhibit viral adhesion and to activate mucosal immune system. However, the effects of TJ-41 on the beating cilia, which sweep away inhaled viruses and bacteria from the airways (mucociliary clearance, a host defense mechanism of lungs), still remain uncertain. TJ-41 may activate the airway ciliary beating, activation of which increases the rate of mucociliary clearance leading to inhibit airway infections. TJ-41 was orally administered to mice for 4–8 weeks (1.7 - 1.8 g/kg/day) and airway ciliary cells were isolated from the lungs. The frequency (CBF) and angle (CBA, an index of amplitude) of ciliary beating were measured using the high-speed video microscopy (HSVM, 500 fps), in relation to intracellular Ca2+ concentration ([Ca2+]i). Differences were considered significant p<0.05. The TJ-41 treatment enhanced the basal CBF and CBA of airway cilia by 10% mediated via an increase in [Ca2+]i. Moreover, ionomycin (IM) at 1 nM increased CBF and CBA by elevating [Ca2+]i in airway ciliary cells from TJ-41 treated mice, but never in those from non-TJ-41 treated mice. These increases in [Ca2+]i were caused by the enhancement of temperature-dependent Ca2+ entry pathways, including transient receptor potential vanilloid 4 (TRPV4). TJ-41 enhanced the basal CBF and CBA of airway cilia via a temperature-dependent [Ca2+]i increase and it also enhanced increases in CBF, CBA and [Ca2+]i stimulated by 1 nM IM, which never increased CBF and CBA in cells from non-treated mice. Thus, TJ-41 enhanced the temperature-dependent Ca2+ entry pathways including TRPV4. The [Ca2+]i increase, a novel action of TJ-41, may induce various pharmacological activities already reported including activation of the airway ciliary beating. These results suggest the potential of therapeutic use for patients suffering from chronic disease to improve various activities in addition to prevention of airway infections. 

Hochu-ekki-to enhanced airway ciliary beating by an [Ca2+]i increase via TRPV4 in mice

We purified several hundred mgs of four major theaflavins (theaflavin, theaflavin-3-O-gallate, theaflavin-3'-O-gallate, and theaflavin-3,3'-O-digallate). Among the 25 hTAS2Rs expressed in HEK293T cells, hTAS2R39 and hTAS2R14 were activated by theaflavins. Both hTAS2R39 and hTAS2R14 responded to theaflavin-3'-O-gallate. In addition, hTAS2R39 was activated by theaflavin and theaflavin-3,3'-O-gallate, but not by theaflavin-3-O-gallate. In contrast, hTAS2R14 responded to theaflavin-3-O-gallate.

The human bitter taste receptor hTAS2R39 is the primary receptor for the bitterness of theaflavins

Tangeretin and nobiletin are polymethoxylated flavonoids in citrus peel. Both tangeretin and nobiletin are bitter; however, their bitterness has not been evaluated using human bitter taste receptors (hTAS2Rs). We screened 25 kinds of hTAS2Rs and found that hTAS2R14 and hTAS2R46 received both compounds.

Activation of human bitter taste receptors by polymethoxylated flavonoids

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Hochu-ekki-to enhanced airway ciliary beating by an [Ca2+]i increase via TRPV4 in mice

The human bitter taste receptor hTAS2R39 is the primary receptor for the bitterness of theaflavins

Activation of human bitter taste receptors by polymethoxylated flavonoids