苦味

Bitterness — Grokipedia Fact-checked by Grok 4 months ago Bitterness Ara Eve Leo Sal 1x Bitterness is one of the five basic taste qualities recognized in humans and other vertebrates, functioning primarily as an evolutionary adaptation to detect and avoid potentially toxic compounds in food, such as alkaloids, cyanogenic glucosides, and glucosinolates found in plants like cassava and brassicas. [1] This sensory mechanism alerts individuals to harmful substances before ingestion, reducing the risk of poisoning from natural defenses in vegetation, though some bitter foods offer nutritional or medicinal benefits when properly prepared. [1] The perception of bitterness is mediated by the TAS2R family of G protein-coupled receptors, with humans possessing approximately 25 functional genes clustered on chromosomes 5, 7, and 12, expressed in taste bud cells on the tongue, soft palate, and pharynx. [1] These receptors detect a wide array of bitter ligands, including quinine, phenylthiocarbamide (PTC), and plant toxins, triggering a signaling cascade involving gustducin and PLCβ2 that leads to neural depolarization and transmission to the brain via the gustatory pathway. [1] Beyond the oral cavity, TAS2Rs are expressed in extraoral tissues like the gut and airways, where they contribute to nutrient sensing, immune responses, and pathogen detection. [1] Evolutionarily, bitter taste perception emerged around 430 million years ago in early bony fishes, paralleling the radiation of vascular plants and their chemical defenses, with gene family expansions in herbivores and contractions in carnivores reflecting dietary pressures. [1] Individual sensitivity to bitterness varies significantly due to genetic polymorphisms, such as in the TAS2R38 gene, which accounts for 50-80% of variation in PTC and PROP tasting ability, influencing food preferences, alcohol and tobacco consumption, and even health outcomes like vegetable intake and chronic disease risk. [1] While innate aversion protects against toxins, cultural and learned factors can make bitter flavors appealing, as seen in beverages like coffee and beer. [1] In a psychological context, bitterness also denotes an emotional state characterized by persistent resentment, anger, or embitterment arising from perceived injustices or losses, often linked to feelings of helplessness and interpersonal conflict. [2] This affective response, distinct from basic taste, can manifest as a mood disorder when chronic, influencing mental health and social behaviors, though it shares metaphorical associations with bitter taste in language and cognition. [3] Taste Sensation Biological Basis Bitterness is recognized as one of the five basic tastes in humans, alongside sweet, sour, salty, and umami, and is primarily detected by specialized sensory structures known as taste buds located on the tongue, soft palate, and other regions of the oral cavity. [4] These taste buds house clusters of gustatory cells, including taste receptor cells that are particularly responsive to bitter stimuli, initiating the sensory process when exposed to bitter compounds. [4] From an evolutionary standpoint, the perception of bitterness serves as a critical survival mechanism, signaling the potential presence of toxic or harmful substances, such as plant alkaloids and other secondary metabolites, thereby prompting avoidance behaviors to prevent ingestion of poisonous materials. [1] This protective role likely emerged around 430 million years ago in early vertebrates, coinciding with the diversification of vascular plants that produce defensive toxins, and it remains prominent in species with plant-based diets while being reduced or absent in strict carnivores. [1] The neural pathway for bitter taste transmission begins in the taste receptor cells, where activation leads to depolarization and release of neurotransmitters that synapse with afferent nerve fibers. [4] These signals are carried via the facial nerve (cranial nerve VII) from the anterior two-thirds of the tongue, the glossopharyngeal nerve (cranial nerve IX) from the posterior third, and the vagus nerve (cranial nerve X) from the throat and epiglottis, converging in the nucleus tractus solitarius in the brainstem before relaying to the thalamus and ultimately the gustatory cortex in the insula for conscious perception. [4] This pathway enables rapid processing of bitter sensations, integrating with emotional and autonomic responses to reinforce aversion. [4] Bitter taste is mediated by G protein-coupled receptors of the TAS2R family expressed in these gustatory cells. [4] Detection Mechanisms The detection of bitterness in humans is mediated by the TAS2R gene family, which consists of 25 functional genes encoding G-protein-coupled receptors (GPCRs) expressed primarily on Type II taste cells in the oral cavity. [1] These receptors, clustered on chromosomes 5, 7, and 12, enable the binding of a diverse array of bitter compounds, such as alkaloids and glucosinolates, allowing for broad surveillance against potential toxins. [1] Each TAS2R receptor features a single exon of approximately 1 kb, and multiple receptors (typically 5–10) are co-expressed per taste cell, contributing to variable sensitivity across individuals and compounds. [1] Upon binding bitter ligands, TAS2R receptors on Type II taste cells initiate a signaling cascade involving the G-protein gustducin. [5] The β-γ subunits of gustducin activate phospholipase Cβ2 (PLCβ2), which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂) into inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). [5] IP₃ then binds to receptors on the endoplasmic reticulum, releasing intracellular calcium (Ca²⁺) stores into the cytosol; this Ca²⁺ elevation activates the transient receptor potential channel TRPM5, permitting Na⁺ influx and subsequent membrane depolarization of the taste cell. [5] This depolarization triggers neurotransmitter release, such as ATP, to convey the bitter signal to afferent nerves. [5] Genetic variations within the TAS2R family significantly influence bitter sensitivity, with polymorphisms in TAS2R38 serving as a well-studied example. [6] TAS2R38, located on chromosome 7, exhibits three key single nucleotide polymorphisms (SNPs) at amino acid positions 49, 262, and 296 (rs713598, rs1726866, rs10246939), defining major haplotypes: the functional PAV (proline-alanine-valine) associated with bitter perception and the non-functional AVI (alanine-valine-isoleucine) linked to insensitivity. [6] These variants explain about 75% of the phenotypic variance in tasting phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP), compounds that taste intensely bitter to carriers of at least one PAV allele but are virtually tasteless to AVI/AVI homozygotes. [6] Individual differences in bitter perception, including the categories of supertasters and non-tasters, arise from TAS2R polymorphisms combined with variations in taste bud density. [7] Non-tasters, comprising approximately 25% of the population, inherit two AVI alleles in TAS2R38 and show reduced sensitivity to PTC/PROP, following a recessive Mendelian pattern. [6] [7] In contrast, supertasters—about 25% of individuals, often PAV/PAV homozygotes with high fungiform papillae density—exhibit heightened bitterness perception, experiencing compounds like PROP much more intensely than medium tasters (the remaining 50%, typically heterozygotes). [7] This bimodal to trimodal distribution of sensitivity underscores the genetic basis for diverse dietary responses to bitter foods. [6] Common Sources Bitterness is elicited by a variety of natural and synthetic substances, primarily through activation of TAS2R bitter taste receptors on the tongue. [8] Plant-derived bitterants are among the most common sources, particularly alkaloids and polyphenols such as tannins. Alkaloids like quinine, extracted from the bark of the cinchona tree ( Cinchona spp.), impart a characteristic bitter flavor detectable at low concentrations, with a typical human detection threshold for quinine hydrochloride around 0.0083 mM. [9] [8] Caffeine, another alkaloid found in coffee beans ( Coffea spp.) and tea leaves ( Camellia sinensis ), has a detection threshold of approximately 1.2 mM and contributes to the bitterness in these beverages. [9] [10] Tannins, polyphenolic compounds abundant in tea leaves and grape skins, add both bitterness and astringency to tea and red wine, while their astringency arises from interactions with salivary proteins, and bitterness from activation of TAS2R receptors. [11] [12] Animal-derived sources include bitter peptides generated during protein breakdown in fermented products like cheese. These hydrophobic peptides, such as those from casein hydrolysis in aged cheeses (e.g., cheddar), develop during ripening and can intensify bitterness if proteolysis is excessive. [13] Synthetic bitterants, like denatonium benzoate, are widely used as non-toxic additives in pharmaceuticals, household products, and anti-smoke formulations to deter ingestion, activating multiple TAS2R receptors at very low thresholds. [14] Common foods high in bitterness feature these compounds prominently, providing dietary exposure. Bitter greens such as endive ( Cichorium endivia ) and arugula ( Eruca vesicaria ) contain glucosinolates and other plant alkaloids that yield a sharp, pungent bite. Dark chocolate derives its intensity from theobromine and polyphenols in cocoa beans ( Theobroma cacao ), while grapefruit ( Citrus paradisi ) owes its profile to naringin, a flavanone glycoside. Hops ( Humulus lupulus ) in beer introduce iso-alpha acids, enhancing the beer's crisp, lingering bitterness balanced by malts. [10] [8] Emotional State Psychological Definition In psychology, bitterness is defined as a complex negative emotion characterized by persistent feelings of resentment, cynicism, and prolonged disappointment arising from perceived injustice, betrayal, or personal loss.