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Bitter is part of our lives. Whether consciously or unconsciously, we taste bitterness regularly through our diet. The perception of this taste has a major effect on food preferences, as humans often make consumption choices based on satisfaction, frequently avoiding foods that create unpleasant sensations.

Bitterness is considered a modulator that balances the flavor profile of many beverages and foods. Interactions between the basic tastes and between taste and volatile compounds, add complexity, enhancing the acceptability and enjoyment of beverages such as coffee, tea, chocolate, fruit juices, and others.

Since general consumers regularly associate coffee with bitterness, it is crucial for sensory professionals to understand the science behind this taste modality. Bitterness is the most complex taste, and it is associated with a variety of compounds in foods, including phenols and polyphenols, flavonoids and terpenes, amino acids and peptides, esters and lactones, methylxanthines (caffeine) sulfimide (saccharine), and even organic and inorganic salts.

Taste is accomplished through taste buds located on the tongue and back of the oral cavity. They allowhumans to perceive sweetness, saltiness, sourness, bitterness, and umami. The general process of taste perception at the molecular level includes a psychophysiology process of three main successive steps:reception, transduction, and the neural mechanisms of coding electrical impulse information. Bitter as well as sweet and umami initial perception involves a structural lock-and-key conceptual model of binding of a flavor molecule to a specific receptor protein in the membrane of a receptor cell. When the binding of the flavor molecule to the receptor protein occurs, the chemical energy is transduced into electrical energy throughspecific series of biochemical reactions.

If we focus on bitterness, different aspects need to be considered:

The anatomy of perception is complex. The molecular structure–receptor relationship appears to be very close to sweetness perception. Bitterness and sweetness depend on the stereochemistry of stimuli-responsivemolecules, which means the organization and orientation of the molecule can have dramatically different outcomes. In this case, a molecule made up with the same elements oriented differently may result in the generation of both bitter and sweet sensations. The ability to perceive some bitter tastes varies significantly across individuals. In some cases, it can be a genetic/inherited aspect. At a defined concentration, certain substances may be bitter, bitter–sweet, or tasteless depending on the individual. In general, bitter substances have lower taste thresholds than other taste substances, meaning that they are identifiable in small amounts.

Scientists have studied bitterness perception for almost 100 years through phenylthiocarbamide and 6-propylthiouracil (PTC/PROP) tests, using this as a proxy to identify individuals with a high sensitivity to bitter compounds such as caffeine, saccharin, quinine hydrochloride and naringin (bitterness characteristic of citrus). Several studies have reported that human type 2 taste receptor (TAS2R) is associated with differences in the sensitivity to PTC bitterness and PROP. In 1930, it was reported that Caucasian–American population tasted“blind” for PTC and the other 60% of the same population perceived the bitterness taste. Other recent study carried out in China showed that around 21% of population were considered super-taster based on the bitter perception theory, 65% were part of the group that had medium perception, and 14% showed the taste polymorphism condition (blindness) to bitterness. Because of the ability to perceive bitterness of PTC is so clearly genetically controlled, the PTC test has been uses as a marker for exploring behavioral and metabolic differences between bitter tasters and non-tasters.

Today, it is well known that bitter taste perception comprises not only multiple transduction mechanisms, but also a large number of receptors. Each individual is estimated to have between 40 and 80 different bitterness taste receptors, with 25 distinct functional TAS2R genes. These receptors (T2Rs) are arranged in clusters in the genome and are genetically linked to loci that control bitter perception in humans. T2Rs are found in all taste buds of the circumvallate and foliate papillae, as well as in the palate taste buds. However, T2Rs are scarcely expressed in the fungiform papillae. In the few fungiform taste buds that do express T2Rs, a full range of different receptors are present, suggesting that each cell can recognize multiple bitter compounds.

In the food industry, quinine (an alkaloid) is generally accepted as a standard for the bitter sensation. Thedetection threshold for quinine hydrochloride is about 10 ppm (parts per million). Quinine is used in the food industry as an additive in beverages, such as soft drinks that also have sweet/tart-tangy attributes. The bitterness with other tastes solution could produce refreshing gustatory sensations (taste modulation) in these beverages, and that confirms its modulating action.

Tea, coffee, and chocolate are complex flavor mixtures that contain numerous bitter phytochemicals. Theobromine, an alkaloid primarily found in cocoa, contributes to its bitterness. Caffeine, another bitter compound, is moderately bitter at concentrations of 150–200 ppm in water. It is added to cola beverage and other food products up to concentrations of 200 ppm as a flavoring agent, often derived from greed coffee during the decaffeination process. However, the strong taste of bitter in coffee and astringency is not only due to caffeine but also influenced by phenolic acids and is extremely correlated to the roasting profile.

It's important to note that the ability to detect very low concentrations of bitter and salty tastes tends to decline with age. In contrast, perception of sweet and sour flavors may remain relatively stable over time. For sensory evaluators in the coffee industry, maintaining skills requires a disciplined approach and consistent repetition, alongside their knowledge and criteria for evaluating coffee. This ongoing practice ensures that their assessments remain accurate and reliable despite any changes in sensory perception.

References:

The Science and Complexity of Bitter Taste by Adam Drewnowski, Ph.D. Nutrition Reviews, Vol. 59, No. 6 163. June 2001:163-169.

Fennema’s Food Chemisty. Chapter 11: Flavors. Fifth Edition. CRC Press. 2017

Variations in the TAS2R38 gene among college students in Hubei. Xiaojun Wang and others.Hereditas volume 159, Article number: 46. 2022

Analysis of Genetic Polymorphism of Bitter Taste Receptor TAS2R38 and TAS2R46, and Its Relationship with Eating and Drinking Habits in Japanese ToMMo Subjects. J Nutr Sci Vitaminol (Tokyo). 2023; 69(5):347-356.

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