In vivo interactions between procyanidins and human saliva proteins: effect of repeated exposures to procyanidins solution

Addressing astringency of grape seed extract by covalent conjugation with lupin protein

Astringency of phenolic-rich foods is a key tactile perception responsible for acceptability/rejection of plant extracts as ingredients in formulations. Covalent conjugation of phenolic extracts with plant proteins might be a promising strategy to control astringency, but suffers from a lack of mechanistic understanding from the lubrication point of view. To shed light on this, this ex vivo study evaluated the effect of conjugation of a phenolic grape seed extract (GSE) with legume protein (lupin, LP) on tribological and surface adsorption performance of GSE in the absence and presence of human saliva (ex vivo). Tribological results confirmed GSE had an inferior lubrication capacity as compared to LP. The lubrication performance of LP-GSE dispersions was comparable to their corresponding LP dispersion (p > 0.05) when covalently conjugated with LP (LP-GSE) with increasing LP:GSE ratio up to 1:0.04 w/w and at a specific degree of conjugation (DC: 2%). Tribological and surface adsorption measurements confirmed the tendency of GSE to interact with human saliva (ex vivo, n = 17 subjects), impairing the lubricity of salivary films. The covalent bonding of LP to GSE hindered GSE's interaction with human saliva, implying the potential influence of covalent conjugation on attenuating astringency. LP appeared to compete with human saliva for surface adsorption and governed the lubrication behaviour in LP-GSE dispersions. Findings from this study provide valuable knowledge to guide the rational design of sustainable, functional foods using conjugation of phenolics with plant proteins to incorporate larger proportions of health-promoting phenolics while controlling astringency, which needs validation by sensory trials.

Arabinogalactan proteins and polysaccharides compete directly with condensed tannins for saliva proteins influencing astringency perception of Cabernet Sauvignon wines

Wine astringency is thought to be due to salivary protein precipitation; however, the actual mechanism is not well-defined. This study aimed understand the relationship between whole polysaccharide extracts, produced with and without enzyme maceration, and the saliva protein-tannin precipitation reaction. Polysaccharides were analyzed in the context of salivary protein-tannin interactions using gel electrophoresis, quantitative 1H proton nuclear magnetic resonance (qHNMR), size separation chromatography, immunochemistry, and sensory analysis. Polysaccharide addition reduced saliva protein concentration in tannin-saliva protein-polysaccharide mixtures, indicating that native-wine polysaccharides compete with condensed tannins for salivary protein as ligand partners. qHNMR showed that tannin levels were increased by adding polysaccharides, suggesting that in these conditions, polysaccharides interact with saliva proteins via competitive protein-polysaccharide complex formation. Polysaccharides from non-enzyme-treated wines had threshold concentration of 121 mg/mL versus 86 mg/ml for enzyme-treated as detected by a sensory panel. Enzyme-treated polysaccharides changed astringency perception at a lower concentration than non-enzyme-treated polysaccharides.

Astringency and its sub-qualities: a review of astringency mechanisms and methods for measuring saliva lubrication

Astringency is an important mouthfeel attribute that influences the sensory experiences of many food and beverage products. While salivary lubricity loss and increased oral friction were previously believed to be the only astringency mechanisms, recent research has demonstrated that nontactile oral receptors can trigger astringency by responding to astringents without mechanical stimulation. Various human factors have also been identified that affect individual responses to astringents. This article presents a critical review of the key research milestones contributing to the current understanding of astringency mechanisms and the instrumental approaches used to quantify perceived astringency intensity. Although various chemical assays or physical measures mimic in-mouth processes involved in astringent mouthfeel, this review highlights how one chemical or physical approach can only provide a single measure of astringency determined by a specific mechanism. Subsequently, using a single measurement to predict astringency perception is overly idealistic. Astringency has not been quantified beyond the loss of saliva lubrication; therefore, nontactile receptor-based responses must also be explored. An important question remains about whether astringency is a single perception or involves distinct sub-qualities such as pucker, drying, and roughness. Although these sub-quality lexicons have been frequently cited, most studies currently view astringency as a single perception rather than dividing it into sub-qualities and investigating the potentially independent mechanisms of each. Addressing these knowledge gaps should be an important priority for future research.

Artificial saliva precipitation index (ASPI): An efficient evaluation method of wine astringency

Astringency is one of the most important organoleptic characteristics of red wines, and its intensity evaluation method has been the focus of research in recent years. An artificial saliva system was developed to establish an accurate and reliable evaluation method for the astringency intensity of dry red wines based on saliva precipitation index (SPI). To achieve this, five key protein families, which presented high reactivities and sensitivities in protein-tannin binding reactions, were selected from human whole saliva. The concentrations of the five proteins (proline-rich protein, α-amylase, lactoferrin, lysozyme, and albumin) and pH were optimized using response surface methodology based on the human salivary conditions to simulate the real salivary environment. The artificial saliva precipitation index method was applied to 60 commercial dry red wines and it exhibited a high correlation (Coef_ASPI = 0.94) with the sensory scores, indicating better performance than the traditional SPI method and other analytical approaches.

Study of Serial Exposures of an Astringent Green Tea Flavonoid Extract with Oral Cell-Based Models

It is well known that repeated exposure to phenolic compounds (PCs) raises astringency perception. However, the link between this increase and the oral cavity's interactions with salivary proteins (SPs) and other oral constituents is unknown. To delve deeper into this connection, a flavonoid-rich green tea extract was tested in a series of exposures to two oral cell-based models using a tongue cell line (HSC3) and a buccal mucosa cell line (TR146). Serial exposures show cumulative PC binding to all oral models at all concentrations of the green tea extract; however, the contribution for the first and second exposures varies. The tongue mucosal pellicle (HSC3-Mu-SP) may contribute more to first-stage astringency (retaining 0.15 ± 0.01 mg mL^-1 PCs at the first exposure), whereas the buccal mucosal pellicle (TR146-Mu-SP) retained significantly less (0.08 ± 0.02 mg mL^-1). Additionally, increased salivary volume (SV+), which simulates the stimulation of salivary flow brought by a food stimulus, significantly enhances PC binding, particularly for TR146 cells: TR46-Mu-SP_SV+ bound significantly higher total PC concentration (0.17 ± 0.02 mg mL^-1) than the model without increased salivary volume TR146-Mu-SP_SV- (0.09 ± 0.03 mg mL^-1). This could be associated with a higher contribution of these oral cells for astringency perception during repeated exposures. Furthermore, PCs adsorbed in the first exposure to cell monolayer models (+TR146 and +HSC3) change the profile of PCs bound to these models in the second exposure. Regarding the structure binding activity, PCs with a total higher number of hydroxyl groups were more bound by the models containing SP. Regarding the SP, basic proline-rich proteins (bPRPs) may be involved in the increased perception of astringency upon repeated exposures. The extent of bPRP precipitation by PCs in mucosal pellicle models for both cell lines (HSC3 and TR146) in the second exposure (76 ± 13 and 83 ± 6%, respectively) was significantly higher than in the first one (25 ± 14 and 5 ± 6%, respectively).

New insights into the oral interactions of different families of phenolic compounds: Deepening the astringency mouthfeels

Astringency is a tactile sensation of puckering, tightening and dryness in the oral cavity, commonly induced by polyphenols. In this study, the interaction of two phenolic compound mixtures, one rich in gallotannins and the other in flavonols, with two oral models (tongue (HSC3) or buccal mucosa (TR146) was evaluated. Results provided evidence that gallotannins and flavonols seem to bind in a different way to the different oral constituents and models used. Gallotannins seems to bind more to the tongue than to the buccal mucosa cell line, but this difference is overcome by the presence of salivary proteins. Conversely, for the flavonol mixture, the presence of salivary proteins seems to restrain the interaction with oral cell lines. Structure-binding activity relationships were evidenced within each mixture: for gallotannins, interactions seem to increase along with the galloylation degree while for flavonol it was observed that increasing numbers of glucose residues decreased the binding activity.

Interaction between salivary proteins and cork phenolic compounds able to migrate to wine model solutions

This work reports the study of the interaction of human salivary proteins (SP) with phenolic compounds that migrate from cork stoppers to wine. This study yields valuable data to understand the influence that these compounds may have on the sensory perception of wine from an astringency perspective. For that, three cork fractions containing the phenolic compounds that migrate in greater amounts from cork to model wine solutions were selected. Fraction M1 contains gallic acid, protocatechuic acid, vanillin and protocatechuic aldehyde; fraction M2 comprises essentially gallic acid and ellagic acid, as well as castalagin and dehydrocastalagin; and fraction M3 contains the two isomeric ellagitannins castalagin and vescalagin. The reactivity of each fraction towards SP was M3 > M2 > M1. Within M3 fraction, castalagin showed a higher ability to precipitate SP (mainly aPRPs, statherin and P-B peptide) comparatively to vescalagin. In M1 fraction, caffeic and sinapic acids were the compounds with the highest interaction with SP, mainly cystatins. In addition, there also seems to be a matrix effect (presence of other compounds) that could be affecting these interactions.

An overview of the perception and mitigation of astringency associated with phenolic compounds

Astringency, as a kind of puckering, drying, or rough sensation, is widely perceived from natural foods, especially plants rich in phenolic compounds. Although the interaction and precipitation of salivary proteins by phenolic compounds was often believed as the major mechanism of astringency, a definitive theory about astringency is still lacking due to the complex oral sensations. The interaction with oral epithelial cells and the activation of trigeminal chemoreceptors and mechanoreceptors also shed light on some of the phenolic astringency mechanisms, which complement the insufficient mechanism of interaction with salivary proteins. Since phenolic compounds with different types and structures show different astringency thresholds in a certain regularity, there might be some relationships between the phenolic structures and perceived astringency. On the other hand, novel approaches to reducing the unfavorable perception of phenolic astringency have been increasingly emerging; however, the according summary is still sparse. Therefore, this review aims to: (a) illustrate the possible mechanisms of astringency elicited by phenolic compounds, (b) reveal the possible relationships between phenolic structures and perception of astringency, and (c) summarize the emerging mitigation approaches to astringency triggered by phenolic compounds. This comprehensive review would be of great value to both the understanding of phenolic astringency and the finding of appropriate mitigation approaches to phenolic astringency in future research.

Polyphenolic Characterization of Nebbiolo Red Wines and Their Interaction with Salivary Proteins

The present study correlates the polyphenolic composition of two different Nebbiolo red wines from the 2015 vintage (M and P), with the salivary proteins’ precipitation process. The work centered on the polyphenolic characterization of Nebbiolo wines and their interaction with different families of salivary proteins. Overall, both wines were found to be very reactive with human saliva which was supposed to contribute to their astringent character. The comparison of both wines showed that the M wine presented higher values of total phenolics, total proanthocyanidins, and tannin specific activity. Moreover, this wine showed a higher interaction with salivary proteins. Altogether, the chemical characterization and reactivity toward human saliva could contribute to the wine astringency.

Interaction of a Procyanidin Mixture with Human Saliva and the Variations of Salivary Protein Profiles over a 1-Year Period

Procyanidins are widely associated with astringency perception and promptly interact/precipitate salivary proteins (SPs). In this work, the SP profile of 17 volunteers was monitored for 1 year, focusing on the SP families most related to astringency [aPRPs (acidic proline-rich proteins), bPRPs (basic proline-rich proteins), gPRPs (glycosylated proline-rich proteins), cystatins, P-B peptide, and statherin]. Although the total SP content remained constant, bPRPs showed high variability. Saliva from 5 volunteers was selected, each individual's saliva presenting a prominence in one of the referred SP families; each was used to interact with grape seed procyanidin oligomeric fraction. Independent of the prominences, a total depletion in statherin and P-B peptide was observed. These subjects performed a sensory assay and the limit of detection for astringency was determined. Overall, the specificity of SP toward procyanidins seemed to be more important in the interactions than the total SP content. The highest reactivity toward SPs was observed for epicatechin gallate, procyanidin dimers B7, B2g, and trimer C1.

Interactions between polyphenol oxidation products and salivary proteins: Specific affinity of CQA dehydrodimers with cystatins and P-B peptide

Throughout the apple juice and cider making process, polyphenols undergo enzymatic oxidation which generates a great variety of polyphenol oxidation products. Since 5'-O-Caffeoylquinic acid (CQA) is one of the major phenolic compounds and the preferential substrate for polyphenoloxidase in apple juice, its oxidation leads to the formation of newly formed molecules by which dehydrodimers (MW 706 Da) are included. Interactions of salivary proteins (SP) with native polyphenols is a well-known phenomenon, but their interactions with polyphenol oxidation products has not been studied yet. In this work, we decided to decipher the interactions between CQA dehydrodimers and SP (gPRPs, aPRPs, statherins/P-B peptide, and cystatins) using HPLC-UV and fluorescence. These results showed that contrary to what was expected, CQA dehydrodimers presented a low interaction with PRPs, but revealed a specific interaction with statherins/P-B peptide and cystatins. This work settles for the first time the interactions between SP and polyphenol oxidation products.

Time Course of Salivary Protein Responses to Cranberry-Derived Polyphenol Exposure as a Function of PROP Taster Status

Astringency is a complex oral sensation, commonly experienced when dietary polyphenols interact with salivary proteins. Most astringent stimuli alter protein levels, which then require time to be replenished. Although it is standard practice in astringency research to provide breaks in between stimuli, there is limited consensus over the amount of time needed to restore the oral environment to baseline levels. Here we examined salivary protein levels after exposure to 20 mL of a model stimulus (cranberry polyphenol extract, 0.75 g/L CPE) or unsweetened cranberry juice (CJ), over a 10 min period. Whole saliva from healthy subjects (n = 60) was collected at baseline and after 5 and 10 min following either stimulus. Five families of proteins: basic proline-rich proteins (bPRPs); acidic proline-rich proteins (aPRPs); histatins; statherin; and S-type cystatins, were analyzed in whole saliva via HPLC-low resolution-ESI-IT-MS, using the area of the extracted ion current (XIC) peaks. Amylase was quantified via immunoblotting. In comparison to baseline (resting), both stimuli led to a rise in levels of aPRPs (p < 0.000) at 5 min which remained elevated at 10 min after stimulation. Additionally, an interaction of PROP taster status and time was observed, wherein super-tasters had higher levels of amylase in comparison to non-tasters after stimulation with CJ at both timepoints (p = 0.014–0.000). Further, male super-tasters had higher levels of bPRPs at 5 min after stimulation with both CJ and CPE (p = 0.015–0.007) in comparison to baseline. These data provide novel findings of interindividual differences in the salivary proteome that may influence the development of astringency and that help inform the design of sensory experiments of astringency.

Tannins in Food: Insights into the Molecular Perception of Astringency and Bitter Taste

Astringency and bitterness are organoleptic properties widely linked to tannin compounds. Due to their significance to food chemistry, the food industry, and to human nutrition and health, these tannins' taste properties have been a line of worldwide research. In recent years, significant advances have been made in understanding the molecular perception of astringency pointing to the contribution of different oral key players. Regarding bitterness, several polyphenols have been identified has new agonists of these receptors. This review summarizes the last data about the knowledge of these taste properties perceived by tannins. Ultimately, tannins' astringency and bitterness are hand-in-hand taste properties, and future studies should be adapted to understand how the proper perception of one taste could affect the perception of the other one.

Salivary Cystatin SN Binds to Phytic Acid In Vitro and Is a Predictor of Nonheme Iron Bioavailability with Phytic Acid Supplementation in a Proof of Concept Pilot Study

BACKGROUND

Acute phytic acid intake has been found to decrease iron bioavailability; however, repeated phytic acid consumption leads to iron absorption adaptation. Salivary proline-rich proteins (PRPs) have been shown to inhibit iron chelation to tannins and may mediate similar iron absorption adaptation with phytic acid intake.

OBJECTIVES

The objectives of this study were to determine whether salivary proteins bind to phytic acid in vitro, and to explore a proof of concept in a pilot study that examined the impact of 4-wk, daily phytic acid supplementation on individuals' iron status, bioavailability, and salivary PRP concentrations.

METHODS

High-performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization-time of flight were used to characterize in vitro salivary protein-phytic acid interactions. Nonanemic women (n = 7) consumed 350 mg phytic acid supplements 3 times daily for 4 wk, and meal challenges were employed to determine iron bioavailability, iron status, and salivary protein concentrations before and after supplementation periods. Enzyme-linked immunosorbent assay (ELISA) analysis of purified protein fractions and participant saliva identified proteins bound to phytic acid.

RESULTS

In vitro salivary protein-phytic acid interaction identified cystatin SN, a non-proline rich salivary protein, as the specific bound protein to phytic acid. Iron bioavailability (P = 0.32), hemoglobin (P = 0.72), and serum ferritin (P = 0.08) concentrations were not reduced from week 0 to week 4 after phytic acid supplementation. Basic PRPs and cystatin SN concentrations were positively correlated with iron bioavailability at week 4.

CONCLUSIONS

Overall, results suggest that phytic acid binds to the non-PRP cystatin SN and that salivary protein production may improve iron bioavailability with phytic acid consumption.

Effect of the harvest season on phenolic composition and oenological parameters of grapes and wines cv. ‘Touriga Nacional’ (Vitis vinifera L.) produced under tropical semi-arid climate, in the state of Pernambuco, Brazil

The northeastern region of São Francisco Valley is the third largest wine producer in Brazil, differentiated by the production of at least two harvests per year, generally in the first and second semesters, respectively. The productive cycle of the vine in the first semester is higher than in the second semester, mainly due to differences in temperature, with maximum and average temperatures of approximately 30 ºC and 26 ºC, respectively. Second semester is characterized by higher temperatures (summer season) and that promote a rapid maturation of the grapes. The objective of this work was to evaluate the phenolic composition, other oenological parameters and the sensorial profile of grapes and wines of cv. 'Touriga Nacional' (Vitis vinifera L.), characterizing grapes and wines from four harvest seasons, two of them referring to the first half of the year and two referring to the second half of the year, using a broad analytical approach. The experimental design consisted of three randomized blocks, with sixty plants marked in different lines and positions (twenty plants in each block at the beginning, middle and end of the vineyard). The results showed that there was a harvesting effect for some of the grape chemical compounds (monomeric anthocyanins, organic acids, condensed tannins and monomeric flavanols, for example) as well as in wines (calcium, monomeric anthocyanins, condensed tannins and monomeric flavanols). Regarding the sensorial profile of the wines, it was possible to observe that higher scores were attributed to floral aroma in wines from the first harvest (first half of the year). Fruity, spice and empyreumatic aromas were higher in wines from second harvest season (second half of the year). For other attributes, there was a tendency for higher variations between the period of study (2014 – 2017) than between the harvest seasons.

Saliva and Flavor Perception: Perspectives

This paper reports the main trends and perspectives related to the current understanding of the relationships between saliva and flavor perception. Saliva is a key factor in flavor perception and controls the transport of flavor molecules to their receptors, their adsorption onto the mouth surfaces (i.e., oral mucosa), their metabolism by enzymatic modification, and the friction force in the oral cavity. The proteins in free saliva or in the mucosal pellicle contribute to flavor perception by interacting with or metabolizing flavor compounds. Most of these reactions were observed when using fresh whole saliva; however, they were absent or less frequently observed when using artificial saliva or depleted/frozen whole saliva. There is a need to better understand the role of protein aggregates in flavor perception. Within humans, there is great interindividual variation in salivary composition, which has been related to differences in flavor perception. However, the relative role of salivary proteins and the microbiota should be deeply investigated together with the impact of their composition on individual perception during life. Finally, future results must also consider cross-modal interactions at the brain level.

Interactions between wine phenolic compounds and human saliva in astringency perception

Astringency is a complex perceptual phenomenon involving several sensations that are perceived simultaneously. The mechanism leading to these sensations has been thoroughly and controversially discussed in the literature and it is still not well understood since there are many contributing factors. Although we are still far from elucidating the mechanisms whereby astringency develops, the interaction between phenolic compounds and proteins (from saliva, oral mucosa or cells) seems to be most important. This review summarizes the recent trends in the protein-phenol interaction, focusing on the effect of the structure of the phenolic compound on the interaction with salivary proteins and on methodologies based on these interactions to determine astringency.

Long-Term Dose-Response Condensed Tannin Supplementation Does Not Affect Iron Status or Bioavailability

Background: Repeated phytic acid consumption leads to iron absorption adaptation but, to the best of our knowledge, the impact of repeated tannin consumption has not yet been established. Salivary proline-rich proteins (PRPs) may improve iron absorption by precipitating tannins. Objectives: This study aimed to determine the effect of long-term, dose-response condensed tannin supplementation on iron bioavailability and status and to assess the effect of salivary proteins on iron bioavailability during prolonged condensed tannin consumption. A secondary objective was to assess astringency as a potential marker for adaptation to tannins and iron bioavailability. Methods: Eleven nonanemic women were enrolled in a double-blind 3-dose crossover trial. Three (1.5, 0.25, or 0.03 g) condensed tannin supplements were consumed 3 times/d for 4 wk in random order, with 2-wk washouts in between. Meal challenges were employed before and after supplementation to assess iron bioavailability, iron status, salivary PRP changes, and astringency. Results: Tannin supplementation in any dose did not change iron bioavailability at any dose (P > 0.82) from weeks 0 to 4. Hemoglobin (P = 0.126) and serum ferritin (P = 0.83) were unchanged by tannin dose from weeks 0 to 4. There were significant correlations among tannin supplementation and iron bioavailability, basic proline-rich proteins (bPRPs) (r = 0.366, P = 0.003), and cystatin production (r = 0.27, P = 0.03). Astringency ratings did not change significantly within or between tannin doses (P > 0.126), but there were negative relations among bPRP (r < -0.32, P < 0.21), cystatin production (r < -0.2, P < 0.28), and astringency ratings. Conclusions: Condensed tannin consumption did not affect iron bioavailability or status regardless of the supplementation period in premenopausal nonanemic women. Correlation analyses suggest that bPRPs and cystatins are associated with improved iron bioavailability and that lower ratings of astringency may predict improved iron absorption with repeated tannin consumption.

Molecular Interaction Between Salivary Proteins and Food Tannins

Polyphenols interaction with salivary proteins (SP) has been related with organoleptic features such as astringency. The aim of this work was to study the interaction between some human SP and tannins through two spectroscopic techniques, fluorescence quenching, and saturation transfer difference-nuclear magnetic resonance (STD-NMR). Generally, the results showed a significant interaction between SP and both condensed tannins and ellagitannins. Herein, STD-NMR proved to be a useful tool to map tannins' epitopes of binding, while fluorescence quenching allowed one to discriminate binding affinities. Ellagitannins showed the greatest binding constants values (K_SV from 20.1 to 94.1 mM^-1; K_A from 0.7 to 8.3 mM^-1) in comparison with procyanidins (K_SV from 5.4 to 40.0 mM^-1; K_A from 1.1 to 2.7 mM^-1). In fact, punicalagin was the tannin that demonstrated the highest affinity for all three SP. Regarding SP, P-B peptide was the one with higher affinity for ellagitannins. On the other hand, cystatins showed in general the lower K_SV and K_A values. In the case of condensed tannins, statherin was the SP with the highest affinity, contrasting with the other two SP. Altogether, these results are evidence that the distinct SP present in the oral cavity have different abilities to interact with food tannins class.

Proanthocyanidins and hydrolysable tannins: occurrence, dietary intake and pharmacological effects

Tannins are a heterogeneous group of high MW, water-soluble, polyphenolic compounds, naturally present in cereals, leguminous seeds and, predominantly, in many fruits and vegetables, where they provide protection against a wide range of biotic and abiotic stressors. Tannins exert several pharmacological effects, including antioxidant and free radical scavenging activity as well as antimicrobial, anti-cancer, anti-nutritional and cardio-protective properties. They also seem to exert beneficial effects on metabolic disorders and prevent the onset of several oxidative stress-related diseases. Although the bioavailability and pharmacokinetic data for these phytochemicals are still sparse, gut absorption of these compounds seems to be inversely correlated with the degree of polymerization. Further studies are mandatory to better clarify how these molecules and their metabolites are able to cross the intestinal barrier in order to exert their biological properties. This review summarizes the current literature on tannins, focusing on the main, recently proposed mechanisms of action that underlie their pharmacological and disease-prevention properties, as well as their bioavailability, safety and toxicology.

LINKED ARTICLES

This article is part of a themed section on Principles of Pharmacological Research of Nutraceuticals. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.11/issuetoc.

Effect of Astringent Stimuli on Salivary Protein Interactions Elucidated by Complementary Proteomics Approaches

The interaction of astringent substances with salivary proteins, which results in protein precipitation, is considered a key event in the molecular mechanism underlying the oral sensation of puckering astringency. As the chemical nature of orally active astringents is diverse and the knowledge of their interactions with salivary proteins rather fragmentary, human whole saliva samples were incubated with suprathreshold and isointensity solutions of the astringent polyphenol (-)-epigallocatechin gallate, the multivalent metal salt iron(III) sulfate, the amino-functionalized polysaccharide chitosan, and the basic protein lysozyme. After separation of the precipitated proteins, the proteins affected by the astringents were identified and relatively quantified for the first time by complementary bottom-up and top-down mass spectrometry-based proteomics approaches. Major salivary target proteins, which may be involved in astringency perception, are reported here for each astringent stimulus.

Sensory perception of and salivary protein response to astringency as a function of the 6-n-propylthioural (PROP) bitter-taste phenotype

Individual differences in astringency perception are poorly understood. Astringency from tannins stimulates the release of specific classes of salivary proteins. These proteins form complexes with tannins, altering their perceived astringency and reducing their bioavailability. We studied the bitter compound, 6-n-propylthioural (PROP), as a phenotypic marker for variation in astringency perception and salivary protein responses. Seventy-nine subjects classified by PROP taster status rated cranberry juice cocktail (CJC; with added sugar) supplemented with 0, 1.5 or 2.0g/L tannic acid (TA). Saliva for protein analyses was collected at rest, or after stimulation with TA or cranberry juice (CJ; without added sugar). CJC with 1.5g/L tannic acid was found to be less astringent, and was liked more by PROP non-taster males than PROP taster males, consistent with the expectation that non-tasters are less sensitive to astringency. Levels of acidic Proline Rich Proteins (aPRPs) and basic Proline Rich Proteins (bPRPs) decreased after TA, while levels of aPRPs, bPRPs and Cystatins unexpectedly rose after CJ. Increases in bPRPs and Cystatins were only observed in PROP tasters. The PROP phenotype plays a gender-specific, but somewhat limited role in the perceived astringency of tannic-acid supplemented, cranberry juice cocktail. The PROP phenotype (regardless of gender) may also be involved in the release of salivary proteins previously implicated in oral health.

Contribution of Human Oral Cells to Astringency by Binding Salivary Protein/Tannin Complexes

The most widely accepted mechanism to explain astringency is the interaction and precipitation of salivary proteins by food tannins, in particular proline-rich proteins. However, other mechanisms have been arising to explain astringency, such as binding of tannins to oral cells. In this work, an experimental method was adapted to study the possible contribution of both salivary proteins and oral cells to astringency induced by grape seed procyanidin fractions. Overall, in the absence of salivary proteins, the extent of procyanidin complexation with oral cells increased with increasing procyanidin degree of polymerization (mDP). Procyanidin fractions rich in monomers were the ones with the lowest ability to bind to oral cells. In the presence of salivary proteins and for procyanidins with mDP 2 the highest concentrations (1.5 and 2.0 mM) resulted in an increased binding of procyanidins to oral cells. This was even more evident for fractions III and IV at 1.0 mM and upper concentrations. Regarding the salivary proteins affected, it was possible to observe a decrease of P-B peptide and aPRP proteins for fractions II and III. This decrease is greater as the procyanidins' mDP increases. In fact, for fraction IV an almost total depletion of all salivary proteins was observed. This decrease is due to the formation of insoluble salivary protein/procyanidin complexes. Altogether, these data suggest that some procyanidins are able to bind to oral cells and that the salivary proteins interact with procyanidins forming salivary protein/procyanidin complexes that are also able to link to oral cells. The procyanidins that remain unbound to oral cells are able to bind to salivary proteins forming a large network of salivary protein/procyanidin complexes. Overall, the results presented herein provide one more step to understand food oral astringency onset.