Given the wide distribution of plant-based bitter toxins, past efforts to develop less bitter cultivars of common plant foods may have been driven not so much by taste as by safety concerns Detection thresholds for bitter taste are extremely low 25 , Bitter compounds, including extremely toxic bitter poisons, are detected by humans in micromolar amounts.
The bitter taste sensation was also more prolonged than were sweet, salty, or sour sensations The biology of bitter-taste perception is poorly understood. The long-term challenge has been to explain how so many structurally unrelated compounds can give rise to a uniform bitter taste. Among bitter-tasting compounds are amino acids and peptides, sulfimides saccharin , ureas and thioureas [6- n -propylthiouracil PROP and phenylthiocarbamide PTC ], esters and lactones, terpenoids, and phenols and polyphenols The diverse chemical structures of these compounds has long suggested the existence of multiple bitter-taste receptors.
Other studies 40 , 41 suggested a common mechanism in the perception of sweet and bitter tastes, possibly linked to G proteins, given that small changes in chemical structure can alter the taste of a given substance from bitter to very sweet. A very recent discovery of a novel family of bitter-taste receptors placed the number of gustducin-linked bitter taste receptors in humans as high as 40—80 42 , These candidate taste receptors T2Rs are organized in the genome in clusters and are genetically linked to loci that influence bitter perception in humans and mice T2Rs were expressed in all taste buds of circumvallate and foliate papillae and in the palate Although T2Rs were rarely expressed in fungiform papillae, those fungiform taste buds that did express T2Rs usually had a full repertoire of different receptors, suggesting that each cell may be capable of recognizing multiple bitter tastants.
This is consistent with the observation that humans are capable of recognizing diverse bitter substances but not always distinguishing among them A complementary study 43 showed that a human bitter-taste receptor hT 2 R-4 responded only to denatonium and PROP, whereas a mouse receptor MT2R-5 responded only to bitter cycloheximide. Genetic linkage studies in humans linked the ability to taste PROP with a locus at 5p15 On average, supertaster women had more fungiform papillae and a higher density of taste buds per papilla than did either medium tasters or nontasters of PROP Anthropologists have long thought that the protective value of this genetic polymorphism was to identify and reject bitter poisons PROP tasters tended to dislike bitter caffeine and naringin solutions and bitter infusions of Japanese green tea 49 — PROP tasters also gave lower acceptance ratings to coffee and grapefruit juice, cruciferous vegetables, and some salad greens As a general rule, heightened perception of bitterness was the principal reason for food rejection.
Phenolic compounds are responsible for the bitterness and astringency of many foods and beverages 17 , The flavonoid group includes flavanones, flavonols, flavones, isoflavones, flavans catechins , and anthocyanins. Whereas lower-molecular-weight phenolic compounds tend to be bitter, higher-molecular-weight polymers are more likely to be astringent Astringency, defined as a drying or puckering mouth feel detectable throughout the oral cavity, may be due to a complexing reaction between dietary polyphenols and proteins of the mouth and saliva 53 , Some bitter tasting phenolic compounds are shown in Table 1.
Phenolic compounds act as natural pesticides 76 , providing plants with resistance to pathogens, parasites, and predators 14 , Amounts of phenolic compounds in plant foods and the level of bitterness are influenced by genetic factors and by environmental conditions 8.
The type of cultivar, germination, degree of ripeness, and processing and storage conditions can all influence the content of plant phenolics 17 , Bitter phenolics, such as quercetin, are the most common bitter compounds in immature apples and other fruit Generally, higher concentrations of phenolic compounds are found in sprouts and seedlings than in the mature plant, consistent with the notion that plant phenolics provide a degree of protection against predation Some bitter phytonutrients in common plant foods 1.
High-molecular-weight polyphenols or tannins have long been regarded as antinutrients because they interfere with protein absorption or reduce iron availability Tannins are widely distributed in grains such as sorghum, millet and barley, peas, carobs, dry beans and legumes, fruit, tea, wine, and a variety of forage plants 8. Tannins complex with proteins, starches, and digestive enzymes and are thought to reduce the nutritional value of foods 8. Flavonoids in citrus fruit include flavanones naringin , flavones nobiletin , and flavonols quercetin.
Sweet and bitter taste in the brain of awake behaving animals
Polymethoxylated flavones tangeretin and nobiletin are concentrated in the skin of unripe fruit and are the constituents of bitter citrus oils Bitter flavonoids may act through bactericidal activity or by making the plant unpalatable Some flavonoids are very bitter whereas others are not, depending on the type of the glycoside chain. Naringin, a flavanone neohesperidoside, and neohesperidin are very bitter, whereas hesperidin is tasteless.
On the other hand, neohesperidin dihydrochalcone is intensely sweet. Naringin concentrations are highest in young leaves and in the pulp albedo of immature fruit Blending of juices means that bitterness affects even more of the crop. Limonin, a triterpene, is responsible for the so-called delayed bitterness of citrus juices. A tasteless limonin precursor, released when fruit tissue is damaged, is gradually converted to limonin, resulting in bitterness.
Bitterness due to flavonoids and limonoids poses a major problem for the citrus industry. As described below, a wide variety of patented techniques have been developed to remove or adsorb excess naringin and limonin from citrus juices 56 , Phenolic compounds, including catechin and epicatechin pentahydroxylated flavans , also occur in tea 65 , A typical green tea polyphenol is epigallocatechin gallate Epicatechin is generally more bitter than is catechin Japanese green tea has higher concentrations of catechins and epigallocatechin than do either fermented black or semifermented oolong tea and is also the most bitter.
Bitterness of tea is generally ascribed to the combination of catechins, saponin, caffeine, and amino acids Depending on molecular weight, catechins can be bitter or astringent, whereas saponins are often described as acrid Recent studies suggested that the bitter taste of chocolate may also be due to catechins, present in higher amounts in bitter than in milk chocolate Fermented cocoa contains epicatechin, polyphenols, and anthocyanins Anthocyanins are glycoside forms of anthocyanidins, with a sugar moiety attached.
Catechins in cocoa have been described variously as bitter with a sweet aftertaste or as bitter and astringent Fermentation of cocoa beans leads to polymerization of catechins and to complexing with proteins. The presence of catechins in chocolate advances our understanding of its bitter taste, which was thought previously to be caused by caffeine, theobromine, and the interaction of theobromine a methylxanthine and diketopiperazines during roasting Whereas the taste of theobromine is described as bitter and metallic, diketopiperazines are associated with the flavor of roasted malt.
Phenolic compounds in wine range from low-molecular-weight catechins to high-molecular-weight tannins 86 — As shown in Figure 1 , perceived bitterness and astringency increased as a linear function of concentration for catechin and for grape-seed tannin Flavonoid monomers such as catechin and epicatechin were rated as more bitter than astringent At higher molecular weights, catechin polymers became progessively more astringent.
As is often the case, sensory studies used 5-point category scales and were based on a limited number of respondents Adapted from reference 89, with permission. A sensory study of catechins in red wine and in a model system similar in composition to a dry table wine 66 showed that epicatechin was significantly more bitter and astringent than was catechin. Ratings of bitterness and astringency were associated with perceived mouth drying and with mouth roughening, especially at higher concentrations The concentrations of catechins used were within the range found in wine.
Phenolic content of red wines can thus reach —3.
The bitterness of phenolics is reduced by sucrose and is substantially enhanced by ethanol 53 , Genistin, a bitter and astringent isoflavone glucoside 92 , is thought to be responsible for the objectionable taste of soy protein 70 , Isoflavones are associated with the protein fraction in soybeans 69 , soy isolates, and texturized soy protein 94 , Enzyme or acid-based hydrolysis of soy proteins produces additional bitter soy peptides and bitter hydroxy fatty acids. Soy flours are reported to have an astringent aftertaste and a chalky mouth feel Bitter isoflavone glucosides, genistin and daidzin, are hydrolyzed during fermentation to bitter isoflavone aglycones, genistein and daidzein.
Genistein and daidzein are said to be responsible for the objectionable taste of soy milk. Their concentrations increase during soaking of soybeans, the first step of soy milk manufacturing. They also impart the characteristic taste to the secondary products miso, soybean paste, and soy sauce As shown in Figure 2 , the objectionable aftertaste of soy milk was linked to its genistein and dadzein contents Data from reference Organosulfur compounds are another plant defense against predation. Cruciferous vegetables broccoli, cauliflower, kale, turnips, collards, Brussels sprouts, cabbage, kohlrabi, rutabaga, Chinese cabbage, and bok choy contain stable glucosinolates in the amounts of 0.
Glucosinolates are natural pesticides, being toxic to insects 27 , The major glucosinolates in cabbage and Brussels sprouts—sinigrin, progoitrin, and glucobrassicin—are toxic to rats Their goitrogenic activity and instances of kale poisoning in cattle are well described in the literature Excessive glucosinolate concentrations in animal feed have been associated with signs of disease in dairy cattle 98 and with thyroid, liver, and kidney toxicity in animal models Crambe meal in broiler chick diets has toxic effects and is not recommended for long-term feeding As with citrus flavonoids, concentrations of bioactive compounds are generally higher in young sprouts than in mature plants Three-day-old broccoli sprouts contained higher concentrations of sulforaphane than did the mature plant Brassica glucosinolates, otherwise known as mustard oil glycosides, tend to be bitter.
Studies showed that these compounds are responsible for the unpleasant taste of cruciferous vegetables, raw or cooked. Because the enzyme myrosinase is inactivated by cooking, food scientists took the position that sinigrin, as opposed to its metabolites, was responsible for the bitter taste of Brussels sprouts There was a 0. The correlation between the sinigrin content of fresh Brussels sprouts and the resulting bitterness of the cooked product was 0.
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Again, taste panelists were extremely sensitive to very low concentrations of Brassica glucosinolates. These data are shown in Figure 3. These data strengthen earlier reports that glucosinolate concentrations in cruciferous vegetables are the principal barrier against consumer acceptance Isothiocyanates, otherwise known as mustard oils, are the key phase 2 enzyme inducers.
Although inducer potency can vary by cultivar, maturation, handling, and storage, it is directly linked to the glucosinolate content of the original plant. Volatile isothiocyanates, or mustard oils, have been described as pungent, acrid, corrosive, and lachrymatory or tear-inducing.
Some are described as garlic-like or watercress-like and are said to provoke a tingling sensation The aversive pungent odor of phenylethyl isothiocyanate can be detected at 6 ppb, whereas the cabbage-like smell of 2-propenyl isothiocyanate is used in repellant aerosols Bitter goitrin, 5-vinyloxazolidinethione, is the chief breakdown product of progoitrin Taste responsiveness to 5-vinyloxazolidinethione, a breakdown product of progoitrin, may be genetically mediated because it shows the same bimodal characteristics as do PTC and PROP 18 , Whatever their putative health effects, these compounds contribute little to eating pleasure.
Some dietary phytochemicals are bitter, toxic, and lethal. Plants in the cucurbitaceae family can be so bitter as to be inedible. Cultivated species, selected for low bitterness and low toxicity, include cucumbers, gherkins, zucchini, squash, pumpkins, and melons. Cucurbitacins, or oxygenated tetracyclic triterpenes, are toxic and their consumption has been linked to illness and death. Bitter zucchini, containing 50— ppm cucurbitacins, has toxic effects in humans in doses of 3 g Cucumbers are often rejected by the consumer because of excessive bitterness.
Kidney, haricot and navy beans, black-eyed peas, and lima beans contain cyanogenetic glycosides. Bitter cyanogentic glycosides are contained in kernels of almonds, lemons, limes, apples, pears, cherries, apricots, prunes, and plums. Some of these compounds are purported to have a use in chemoprevention. Bitterness in plant foods has been described as a sensory defect with a major economic effect 20 , The degree of bitterness depends on the cultivar, strain, ripening, and storage conditions.
Industry efforts have focused on the formation of bitter compounds, their role during the transition from raw to ripened product, and their breakdown in food or juice. Responding to taste-driven consumer demand, the food industry generally removes phenolic compounds, flavonoids, isoflavones, terpenes, and tannins from foods destined for human consumption. Because of such efforts, the current food supply is less bitter than it might otherwise be Potential approaches to removing bitter phytochemicals include selective breeding of new and less bitter cultivars 18 , 19 , 73 , , For example, high concentrations of sinigrin and progoitrin in Brussels sprouts are generally regarded not as a health benefit but as a major sensory defect Apart from unpleasant taste, high concentrations of progoitrin can cause contact dermatitis.
Food scientists have argued that progoitrin should be bred out of Brassica crops that are intended for human consumption The undesirable characteristics of glucosinolates in Brassica vegetables have long been known to the food industry. Removal of progoitrin and reduction in sinigrin was thought to reduce bitterness and increase consumer acceptance In vitro assays of bitter compounds were used to detect, screen, and remove the most bitter varieties.
Bitter taste had been identified as the main reason for avoidance of vegetables and was reported as being the least well tolerated for Brassica vegetables, spinach, squash, and onions 30 , 73 , The current health-oriented push toward selective breeding of phytonutrient-rich and therefore more bitter varieties runs counter to the published studies on vegetables and consumer acceptance 30 , 73 , Creation of new and less bitter cultivars has also occurred with other crops.
A wholesale reduction of glucosinolates in such oilseed crops as rapeseed and Crambe abyssinica was carried out successfully worldwide The development of a transgenic citrus fruit, free of limonin, was also described in the literature Some scientists have argued that these modifications may have had unintended consequences.
Many bitter phytochemicals are associated with resistance of the developing and mature plant to microbial, insect, or pest attack. It has been suggested 79 that plant varieties that have been selected for palatability by humans are usually more susceptible to disease, leading to increased reliance on synthetic pesticides.
Bitter compounds are also removed from processed foods. Some of the common debittering processes are summarized in Table 2. In many cases, these methods are patented and their details are guarded commercial secrets of a particular food company Beany flavor, bitter taste, and flatus factors pose a major barrier to the inclusion of more soy products in the Western diet Solvents, precipitation, filters, and microorganisms have all been used to produce nonbitter and bland soy. Most debittering processes were designed to remove oxidized soy lipids and bitter peptides However, isoflavones are probably removed as well.
Early attempts to supplement foods with soy protein met with limited success, again because of taste factors Although hydrolyzed soy proteins are added to soups to enhance flavor, their bitterness is often minimized by the addition of gelatin or maltodextrins Bitter phenolic compounds are routinely adsorbed to resins, trapped on polymers, precipitated, extracted with solvents, or converted to nonbitter compounds. Phenolic compounds and tannins are removed from wine by a variety of procedures Wine tannins are also adsorbed with the use of polyvinyl polypyrrolidone matrixes Aging of wine reduces both bitterness and astringency because phenols continue to polymerize and eventually precipitate.
Sugar has also been added to wines to reduce bitter taste Bitterness can be masked. Cyclodextrin, a common commercial product, dissolves flavonoids and masks the bitter taste of citrus juice. Because flavonoids are still present in the juice, their bioactive potential is unchanged.
Sweet Acceptance Versus Bitter Resistance (2nd ed.)
Other effective ways of reducing bitterness of plant foods involve cooking or the addition of fat, sugar, or salt. Diets high in vegetables and fruit have been associated with reduced cancer risk 5 ; many studies focused on the chemoprotective role of phytochemicals. Chemoprotective agents generally belong in 1 of 3 categories: those that block metabolic activation of carcinogens, those that prevent the formation of carcinogens from precursors, and those that suppress neoplasia in cells previously exposed to carcinogens , Phytonutrients and their metabolites elicit a variety of biological activities, acting as antioxidants, phytoestrogens, or enzyme inducers Among the most promising compounds under study are bitter phenols and polyphenols, flavonoids, isoflavones, and glucosinolates.
Phenolic acids, catechins, flavonols, and polymeric anthocyanins in wine were reported to have antioxidant activity , However, even such low amounts may have a potent antioxidant effect in vivo Studies of their anticarcinogenic action focused on the activation of enzymes involved in the metabolism of xenobiotics phase 2 enzymes.
Naringin, the bitter flavonoid component of grapefruit juice, or its metabolites, are reported to inhibit the activity of cytochrome P Glucosinolates and isothiocyanates are also regarded as dietary protectors against cancer 7. Isothiocyanates inhibit the activation of carcinogens by cytochrome P phase 1 enzymes and promote detoxification of activator carcinogens by inducing phase 2 enzymes. Phase 2 enzymes inactivate carcinogens by neutralizing their toxic properties and speeding their elimination from the body Some phase 2 enzymes function as inhibitors of cytochrome P 7.
The results of early studies 18 , 19 and more recent data suggest that one of the most potent inducers is sulforaphane, or 4-methylsulfinylbutyl isothiocyanate 33 , which is derived from glucoraphanin. Inducer potency has been linked to concentrations of bitter sinigrin, progoitrin, glucobrassicin, gluconapin, and glucoraphanin. As a result, several clinical studies attempted to feed patients large amounts of Brassica vegetables daily.
Epidemiologic outcomes of diets high in vegetables and fruit were summarized in a recent report 4. Research has focused on vegetables rather than fruit, with special emphasis on green leafy and Brassica vegetables, citrus fruit, soybeans, and red wine. Population-based studies 9 , showed that the risk of coronary heart disease was reduced at higher estimated intakes of flavonoids apigenin, kaempferol, luteolin, myricetin, and quercetin. Data on diet and colon cancer 64 also show a significant decline in risk with higher consumption of vegetables, green leafy vegetables, and cabbage.
There is a general consensus that a diet higher in plant foods than is the current norm is associated with improved health and reduced disease risk. However, the most potent plant products are also likely to be the most bitter and therefore the most aversive to the consumer. The competing demands of taste and health pose a dilemma for the food industry. The major determinant of food selection is taste.
Foods that are bitter, acrid, or astringent tend to be rejected by the consumer—and generally for the right reasons. The instinctive rejection of bitter taste may not be modifiable because it is a key mechanism for survival. Bitter taste is the main reason for the rejection of diverse food products 23 , Very highly rated and accepted by individuals going through various 12 step programs, individual counseling as well as people with no addictions support.
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