The tongue distinguishes five tastes, not four: sweet, salty, sour, bitter, and umami. The fifth took nearly a century to enter the textbooks. Kikunae Ikeda described it in 1908, from the glutamate of seaweed broth. Physiology only admitted it in 2002, when its T1R1/T1R3 receptors were finally identified: as long as the organ had not been found, the taste remained an impression.

These five tastes are not detected by the same mechanism. Sweet and umami share a family of receptors, the T1R, which makes them close relatives. Bitter alone mobilizes about thirty, the T2R family, by far the largest. Sour and salty do not go through receptors of this type: they use ion channels.

This date of 2002 explains a gap the reader will often run into. The training documents that still teach “four nuances” are not wrong through carelessness; they reproduce the state of physiology from before the 2000s.

In coffee, these tastes do not weigh equally. Sweetness is weak and comes mainly from caramelization products. Sourness comes from organic acids and CGAs. Bitterness comes from lactones, phenylindanes and caffeine.

That leaves salty and umami, marginal but measurable. Enough, in any case, to be named: the World Coffee Research lexicon, for that matter, keeps salty among its basic attributes.

Ikeda, J. Chem. Soc. Tokyo 30 (1908); Nelson et al., Nature 416 (2002); Chandrashekar, Hoon, Ryba, Zuker, Nature 444 (2006), p. 288-294; World Coffee Research, Sensory Lexicon 2.0 (2017)

The rest of what we call taste is tactile, not gustatory. Body, for example, is a sensation of weight and coating. It is carried by lipids, melanoidins and suspended solids, that is, by matter that stays in the liquid and settles on the tongue.

The training documents separate two things that the word body confuses. “Matter” is consistency. “Texture” is the quality of the contact, velvety or rough. They also point out that a coffee described as full-bodied is simply a coffee with presence in the mouth.

This presence does not come from the bean alone. It depends just as much on the preparation: the grind, the dose, the temperature, the time. The same coffee can therefore be made full-bodied or thin by the person who prepares it.

Astringency, that tightening of the mucous membranes, is classically explained by the precipitation of salivary proteins. Polyphenols bind to them, the precipitated saliva no longer lubricates, and the mouth feels dry.

A team at The Ohio State University (Simons, Peterson, Linne) wanted to know what the word body really covers. In 2021 it broke it down into four sub-attributes: coating, astringency, chalkiness and thickness. It then linked astringency to melanoidins, and coating to 3- and 4-caffeoylquinic acids.

One of its results runs against intuition, and the team flags it as unexpected: the coating sensation decreases as the concentration of these acids increases. More of the molecule associated with coating, therefore less perceived coating. A reason to be wary of shortcuts that pin a sensation on one compound.

professional training document, tasting guide, p. 6; Linne, Simons, Peterson, American Chemical Society Fall Meeting, August 22-26, 2021, then Food Research International (2023); level of evidence: conference paper, then peer-reviewed article