Acidity vs sourness, bitterness vs astringency, and the tongue taste map: Some sensory misapprehensions

The overall taste of a wine is a combination of its odor, taste, and tactile perceptions as generated by specialized nerve cells resident in the nasal and oral cavities. Understanding the stimuli to which each of these specialized cells react is important to effective wine tasting and description. This brief post attempts to shed some light on a few oral sensory misapprehensions

Acidity versus Sourness
Acids play an important role in the cellular and metabolic functions of the grape berry and in the color and texture of the fermented wine. The primary acids found in grapes and fermented wine are tartaric, malic, and citric acids, as well as the tartaric and malic derivatives potassium hydrogen tartrate (cream of tartar) and potassium hydrogen malate. Acidity (as it relates to wine) is the measure of its acid content.

Total acidity is the sum of the hydrogen ions of both fixed and volatile acids that are present in the wine and, as such, is the most accurate representation of acid concentration.  Total acidity is difficult to measure accurately, however, and so the more easily measurable titratable acidity (TA) is used as its proxy. Red table wines generally range between 0.6% and 0.8% TA as levels below 0.4% render the wine susceptible to infection and spoilage.

A second method for measuring the acidity of a wine is through observation of its pH (potential of hydrogen) level.  The higher the number of hydrogen ions (H+) in a liquid, the more acidic it is while the higher the number of hydroxide ions (formed when an oxygen ion bonds to a hydrogen ion and represented as OH-) in the liquid, the higher its alkalinity. The pH scale runs from 0-14 with acidic solutions falling below 7.

Sour is one of the five recognized taste sensations (the others being sweet, bitter, salt, and umami) and is the human mechanism for recognizing the presence of acidity. According to Jackson (Wine Tasting, p.204), “sourness is a complex function of the acid, its dissociation constant and wine pH.” 

To summarize then, acidity (as it relates to wine) is a measure of acid concentration in the wine while sourness is the way that that acidity manifests itself as a human taste sensation. 

Bitterness versus astringency
As indicated above, bitter is one of the five taste sensations. Sources of the bitter sensations in wine are flavonoid phenolics (primary), with several glycosoids, terpenes, and alkaloids contributing from time to time (Jackson, Wine Tasting). Bitterness and astringency are sometimes confused with each other because (i) they are both induced by the same compound; (ii) they are late-arriving (sweet and sour are the first taste sensations to make their presence felt); and (iii) they are slow to depart. The perception of bitterness is reduced by sugar, enhanced by alcohol, and hidden by astringency.

According to sensorysociety.org (and Richard Gawel, Secret of the Spit Bucket Revealed, aromadictionary.com), astringency is a tactile sensation, rather than a taste, and is primarily caused by polyphenolic compounds contained in certain foods (including wine) but can also be caused by acids, metal salts (such as alum), and alcohols. A key characteristic of astringency is the fact that it is difficult to clear from the mouth and, as such, builds in intensity on repeated exposure to the source. The source of astringency in wines is tannins, "a heterogeneous group of phenolic compounds" with properties to include: astringency (caused when the tannin binds with protein in saliva; evidenced by mouth pucker and a bitter aftertaste); bitterness; the ability to react with ferric chloride; and the ability to bind with proteins (Dr. Bruce Zoecklin, Enology Note #16). 

There are two theories as to how astringency presents in the mouth (sensorysociety.org): (i) Polyphenols bind with the proteins in saliva and the resultant proline-rich proteins precipitate out (Gawel sees these precipitates as being reflected in the "stringy" salivary material that we spit into the buckets at wine tastings), thus reducing the ability of saliva to lubricate the mouth (this loss of "lubricity" is perceived as an increase in oral friction). (ii) The astringents directly effect the oral epithelium with a sensation of harshness presenting when the gums brush against the insides of the mouth. 

According to both Gawel and sensorysociety.org, individuals with high saliva secretion rates will experience lower levels of astringency. Gawel alludes to three types of astringency:

1. The feeling of having fine particles on the surface of the mouth; referred to by terms such as Powdery, Chalky, and Grainy
2. Roughness of feeling inside the mouth; referred to by words such as Silky, Emery, Velvety, and Furry
3. Causing the mouth to move; referred to by words such as Pucker, Chewy, Grippy, and Adhesive.

In summary, then, while sharing a few characteristics, bitterness and astringency differ in that the former is a taste sensation while the latter is a tactile sensation.

The Tongue Taste Map
The tongue taste map is truly a myth. A turn-of-the-century research paper indicated that certain areas of the tongue had been shown to be sensitive to specific tastes. Subsequent research had found these findings to be inaccurate because, in fact, most taste buds have receptors that can sense and report on all of the taste sensations. And though two-thirds of the taste buds are located on the tongue, they are also present in the soft palate and epiglottis and a few may even be located on the pharynx, larynx, and upper portion of the esophagus (Ronald S. Jackson, Wine Tasting: A Professional Handbook, Elsevier, 2002). Thenibble.com identified that early researcher as D.P Hanig, a German scientist, and also revealed that the findings were based on “subjective responses of volunteers.” This initial qualitative research was “coated” with numbers in 1942 by Edwards Boring of Harvard University (thenibble.com). Work in 1974 by Dr. Virginia Collins, and additional subsequent research, has shown that all tastes can be detected wherever there are taste buds yet the tongue taste map retains its Dracula persona.

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