**2. Perception of astringency**

The term astringency derives from the Latin verb, *ad-stringere* that means tightly bind, strongly join. It refers to the propensity of vegetable tannins to complex with macromolecules, such as proteins and polysaccharides, and alkaloids. Bate-Smith [9] first speculated that astringent sensations were caused by the increase in friction between the mucosal surfaces, which resulted from a reduction in lubrication in the oral cavity as astringent compounds bound salivary proteins. The binding between polyphenols/salivary proteins forms soluble complexes and/or precipitates that can cause the rupture of the salivary pellicle [10], interact with oral cells [11], and stimulate and activate mechanoreceptors (MRs) hold in the mouth [12]. MRs are nerve endings that function like those of the skin, except that they have smaller receptive fields and lower activation thresholds [13]. They are selectively sensitive to different stimulus properties, such as particle size and/or mouth movements, and project such information to the central nervous system [14]. Besides, the activation of G-coupled proteins also seems to be involved in the perception of astringency, activating signal transduction pattern as that of taste recognition [15]. Some brain regions (hippocampus and anterior cingulate cortex) that have been shown to respond to basic tastes were activated by the intensity and pleasantness of astringency [16]. In particular, the right ventral anterior insula that responded to astringent stimuli contributed to the ability to recognise the qualitative features of astringency. The activation of the trigeminal nerve, chorda tympani, and brain regions involved in memory and emotions could explain astringency as a multi perceptual phenomenon.

Whilst the chemical definition of astringency is related to the ability of tannins to bind proteins, in sensory terms, it is described as different and concomitant feelings of drying, puckering, and roughing [17, 18]. Astringency can be defined as a tactile sensation, because: (i) it is perceived on non-gustatory surfaces such as on the soft palate, gingiva, lips [12], (ii) does not show adaptation but also (iii) increases upon repeated ingestion [19], leading to carry-over effects during the tasting. However, side tastes as bitterness, sourness, and sweetness can highly modulate the overall

**147**

glycosylation [54].

*Salivary Protein-Tannin Interaction: The Binding behind Astringency*

attribute "mouth-coat" contributed to the quality of the wine.

rich proteins (PRPs), cystatins, histatins and statherins [44].

have been produced during the past years [32–38].

**3. Salivary proteins**

astringency [20]. The sensitivity of MRs to astringents as well as basic tastes may elucidate the complexity of red wine astringency, which has been described by 33 different subqualities [21]. Amongst these "hard," "green," and "rich" have been associated with bitterness, acidity, and high flavour concentration, respectively [22], "harsh," "abrasive," and "drying" have been found to define astringency as a negative sensation, whilst the "complex" and "mouth-coat" subqualities have been associated to a positive impact during tasting [21]. These subqualities were also associated with touch standards when utilised to describe the tactile astringent sensations in the mouth elicited by red wines [23, 24]. The qualitative traits of astringency as "soft", "mouth-coat", and "rich" represented the drivers of liking for Sangiovese wine [25]. Similarly, for Tannat [26], and Côtes du Rhône and Rioja appellations wines [27], the

It is also true that the perception of astringency is mediated by psychological factors [28], but salivary protein composition [29] and tannin's structure and composition [30, 31] represent the principal factors. In this regard, numerous reviews

Saliva is a biological fluid primarily produced by the three pairs of "major" salivary glands (parotid, submandibular, and sublingual glands) in mouth and by the minor ones by 10% [39]. In the whole, saliva are presently more than 2000 different proteins and peptides [40, 41], which are the result of protein post-translational modifications before being secreted in the mouth [42]. Although saliva is predominantly a watery fluid (99.5%) with a complex mixture of proteins (0.3%; 1–2 mg/mL), ions and other organic compounds (0.2%) are also present. The whole saliva continuously baths the oral cavity and having a pH ranging from 6.2 to 7.4 acts as a buffering system. The saliva is continuously secreted (0.3–7 mL/min) and plays a role in protecting the tooth and mucosal integrity, in antibacterial and antiviral activity, digestion of food, speech, lubrication, taste, and represents a biomarker tool for some diseases [41, 43]. The main families of proteins include enzymes (amylase, carbohydrase, lipase), lactoferrin, high (M1), and low (M2) molecularweight glycoproteins (mucins), peptides as agglutinins, immunoglobulins, proline-

There is evidence that saliva may affect the way we perceive the taste and mouthfeel of foods in various ways [45–47]. During the wine tasting, saliva transports and dissolves the stimuli substances [48]. Saliva constituents are of great importance for establishing protein-tannin interactions. In particular, the PRPs, histatins, mucin, amylase are the main salivary proteins involved in the binding with polyphenols eliciting astringency [49]. The differences between the binding of the same polyphenol to different proteins result from differences in the amino acid sequences [50]. The PRPs account for approximately 70% of the total secretory protein and are subdivided into acidic, basic, and glycosylated PRPs. They are characterised by an abundance of proline, glutamic acid/glutamine, and glycine [51]. The presence of these four amino acids, especially proline, which are the so-called alpha-helix breakers, enables the protein to form secondary structures, which assumes a random coils conformation in solution [10, 52]. This feature may allow PRPs to universally bind various types of polyphenols, mainly tannins with different sizes and structures. Some species, such as humans, rats, and mice, produce PRPs containing about 40% proline [53, 54]. However, some species produce salivary proteins, which are rich in proline but do not show a high affinity to tannins due to extensive

*DOI: http://dx.doi.org/10.5772/intechopen.93611*

*Salivary Protein-Tannin Interaction: The Binding behind Astringency DOI: http://dx.doi.org/10.5772/intechopen.93611*

astringency [20]. The sensitivity of MRs to astringents as well as basic tastes may elucidate the complexity of red wine astringency, which has been described by 33 different subqualities [21]. Amongst these "hard," "green," and "rich" have been associated with bitterness, acidity, and high flavour concentration, respectively [22], "harsh," "abrasive," and "drying" have been found to define astringency as a negative sensation, whilst the "complex" and "mouth-coat" subqualities have been associated to a positive impact during tasting [21]. These subqualities were also associated with touch standards when utilised to describe the tactile astringent sensations in the mouth elicited by red wines [23, 24]. The qualitative traits of astringency as "soft", "mouth-coat", and "rich" represented the drivers of liking for Sangiovese wine [25]. Similarly, for Tannat [26], and Côtes du Rhône and Rioja appellations wines [27], the attribute "mouth-coat" contributed to the quality of the wine.

It is also true that the perception of astringency is mediated by psychological factors [28], but salivary protein composition [29] and tannin's structure and composition [30, 31] represent the principal factors. In this regard, numerous reviews have been produced during the past years [32–38].
