Alternative Mechanisms of Astringency – What is the Role of Saliva?

Plant polyphenols, terpenes, and terpenoids in oral health

Introduction

Herbal products are increasingly being used for their potential to promote oral health. Phytochemicals can act as scavengers of reactive or toxic chemicals, selective growth factors for beneficial oral bacteria, fermentation substrates for beneficial oral bacteria, and selective inhibitors of harmful oral microflora. Plant-derived secondary metabolites encompass a variety of natural products, including alkaloids, polyphenols (including flavonoids and tannins), terpenes, terpenoids, steroids, saponins, quinones, coumarins, glycosides, and carotenoids.

Objectives

This review explores medicinal plant-based products for preventing and treating oral diseases, offering updated insights into the scientific basis for using herbs as active ingredients in oral health care. Hence, we focused on: (1) phenolic compounds, the most abundant and common phytochemicals and (2) terpenes and terpenoids, the most diverse and widely distributed group in the plant kingdom.

Methods

Several databases were queried to acquire peer-reviewed studies focusing on the major phytochemical compositions - phenolic compounds, terpenes, and terpenoids - and their oral health benefits.

Results

The review lists numerous medicinal herbs in various forms and applications for treating dental caries, gingivitis, and periodontitis.

Conclusion

This review explores the evidence supporting the oral health-promoting effects of specific constituents in herbs with the potential for incorporation into pharmaceutical formulations.

Chemical characterization and sensory evaluation of a phenolic-rich melanoidin isolate contributing to coffee astringency

The tactile flavor sensations of coffee are an important indicator of product quality, yet the stimuli contributing to these percepts remain insufficiently defined. In the present work, compounds that contribute to astringency in coffee were investigated. A multi-dimensional sensory-guided fractionation method was employed whereby preparative-scale liquid chromatography was leveraged to separate the coffee brew into subfractions, and then followed by sensory evaluation to identify fractions with perceptual impact. This process, paired with high-resolution chemical characterization via Fourier-transform ion cyclotron resonance/mass spectrometry, revealed a complex, phenolic-dominant, melanoidin fraction. Further sensory recombination testing confirmed this fraction imparted a perceptible astringent sensation at coffee-relevant concentrations in both water and coffee matrices. We hereby provide the first evidence of melanoidins contributing to the complex tactile flavor profile of coffee.

Overview of mouthfeel from the perspective of sensory scientists in industry

Mouthfeel is vital in consumer acceptance of foods and beverages. Despite the critical role mouthfeel plays in product development, the concept is often poorly understood and subject to various interpretations. Within this review, five topics of interest are discussed to provide a better understanding of the mouthfeel attribute-the definition, the perception, the importance, the influencing factors, and the methods of measurement of mouthfeel. Mouthfeel encompasses multiple attributes. Although mouthfeel attributes perceived through physical and chemical perceptions, such as cooling and heating, are understood as mouthfeel, attributes perceived through mechanoreceptors, such as creaminess and thickness, are more challenging due to varying opinions on texture and mouthfeel. Other factors, like food composition and temperature, are also vital in understanding the overall effect of mouthfeel in food and beverages. Including all perceived attributes and factors is important for consumer acceptance of products and for developing consistent evaluation of products. Other topics to consider include the dynamic aspects of oral processing and cultural backgrounds as these topics create additional intricacies in defining and understanding mouthfeel perception. Despite the lack of instrumental methodology available to measure mouthfeel attributes, trained panelists can be used to predict facets of mouthfeel in the eating experience. When designing sensory testing for trained panelists, experimental conditions and a product-specific lexicon are important factors to consider. Overall, understanding of the mouthfeel attributes and their role in consumer preference for different types of food and beverages continues to evolve.

Impact of food additives (sodium carbonate and ammonium bicarbonate) on interactions between astringent compounds and an oral cell model

Many foods contain additives like sodium carbonate and ammonium bicarbonate to ensure safety, preserve quality, or extend shelf life. Recent studies suggest these additives may influence interactions between astringent compounds and oral cells. Using a tongue epithelium cell model, we investigated how these salts affect interactions between astringent compounds (phenolic and non-phenolic) and oral constituents. The salts reduced the interaction of green tea flavanol-rich extract (GTE) with salivary proteins and altered interactions with tannic acid and alum. The effects varied depending on the specific astringent compound, salt, and oral constituents, with ionic strength playing a role. The reduced adsorption of flavanols from GTE may be due to oxidation induced by these additives, raising concerns about their use. However, sodium carbonate and ammonium bicarbonate can modify astringent molecule interactions, potentially influencing the astringency of certain products.

Mucoadhesion and Mucins in Oral Processing: Their Role in Food Interaction, Texture, and Sensory Perception

This is a review of mucus, and its principal component, mucins, in oral processing; it examines oral processing from the viewpoint of mucins being integral functional constituents of the food after the latter's insertion into the mouth. Under this light, mucins are treated as an omni-present functional ingredient. The chemical physics of the bolus formation is examined, focused on the role of mucins in the process. The colloidal and rheological aspects of hydrocolloids-mucin systems are subsequently examined, highlighting the role of the oral glycoproteins in complex food models and complex foods. Following the physicochemical and mechanical description of the topic, mucus is examined as a determinant of a food's sensory attributes. Its role in oral sensations such as astringency is reviewed, with a special focus on phenol-mucin interactions. The effect of mucus on the perception of saltiness is then reviewed, and the ensuing strategies for structurally-based reduction of salt are considered. The review critically discusses the challenges and opportunities that emerge from the above, highlighting the role of mucins and their effect on food function.

Caffeine weakens the astringency of epigallocatechin gallate by inhibiting its interaction with salivary proteins

The astringency of green tea is an integrated result of the synergic and antagonistic effects of individual tea components, whose mechanism is highly complex and not completely understood. Herein, we used an epigallocatechin gallate (EGCG)/caffeine (CAF)/saliva model to simulate the oral conditions during tea drinking. The effect of CAF on the interaction between EGCG and salivary proteins was first investigated using molecular docking and isothermal titration calorimetry (ITC). Then, the rheological properties and the micro-network structure of saliva were studied to relate the molecular interactions and perceived astringency. The results revealed that CAF partially occupied the binding sites of EGCG to salivary proteins, inhibiting their interaction and causing changes in the elastic network structure of the salivary film, thereby reducing astringency.

The insights into sour flavor and organic acids in alcoholic beverages

Alcoholic beverages have developed unique flavors over millennia, with sourness playing a vital role in their sensory perception and quality. Organic acids, as crucial flavor compounds, significantly impact flavor. This paper reviews the sensory attribute of sour flavor and key organic acids in alcoholic beverages. Regarding sour flavor, research methods include both static and dynamic sensory approaches and summarize the interaction of sour flavor with aroma, taste, and mouthfeel. In addition, this review focuses on identifying key organic acids, including sample extraction, chromatography, olfactometry/taste, and mass spectrometry. The key organic acids in alcoholic beverages, such as wine, Baijiu, beer, and Huangjiu, and their primary regulatory methods are discussed. Finally, future avenues for the exploration of sour flavor and organic acids by coupling machine learning, database, sensory interactions and electroencephalography are suggested. This systematic review aims to enhance understanding and serve as a reference for further in-depth studies on alcoholic beverages.

A look upon the adsorption of different astringent agents to oral models: Understanding the contribution of alternative mechanisms in astringency

Salivary proteins precipitation by interaction with polyphenols is the major mechanism for astringency. However, alternative mechanisms seem involved in the perception of different subqualities of astringency. In this study, adsorption of four astringent agents to in vitro oral models and their sensory properties were assessed. Overall, green tea infusion and tannic acid have shown a higher adsorption potential for models with oral cells and absence of saliva. Alum and grape seed extract presented higher adsorption in models with presence of oral cells and saliva. Multiple factor analysis suggested that adsorption may represent important mechanisms to elicit the astringency of alum. Models including saliva, were closely associated with overall astringency and aggressive subquality. Models with cells and absent saliva were closely associated with greenness, suggesting a taste receptor mechanism involvement in the perception. For the first time a correlation between an oral-cell based assay and astringency sensory perception was shown.

Astringency sensitivity to tannic acid: Effect of ageing and salivary proline-rich protein levels

The link between salivary composition and sensitivity to astringency as a function of age has still not been established. In this work, we propose the hypothesis that ageing leads to changes in the concentration of salivary proline-rich proteins (PRPs), which alters the astringency perception threshold with age. To test this hypothesis, astringency sensitivity to tannic acid and saliva was assessed in 30 elderly people and 24 young people. Basic PRPs (bPRPs) and glycosylated PRPs (gPRPs) were quantified immunochemically via western blot analysis. The results showed that the amounts of bPRPs and gPRPs were similar between the young and elderly groups. However, a positive correlation between the gPRP amount and astringency threshold was observed only in the young group, while a negative correlation between the bPRP amount and astringency threshold was observed only in the elderly group. This finding suggests differences in the contribution of PRP type to astringency perception as a function of age.

Structure Response of Preadsorbed Saliva Pellicle to the Interaction between Dairy and Saliva Protein

Using the surface characterization techniques of quartz crystal microbalance with dissipation, atomic force microscopy, and scanning electron microscopy, the structure of the salivary pellicle was investigated before and after it was exposed to dairy proteins, including micellar casein, skim milk, whey protein isolate (WPI), and a mixture of skim milk and WPI. We have shown that the hydration, viscoelasticity, and adsorbed proteinaceous mass of a preadsorbed salivary pellicle on a PDMS surface are greatly affected by the type of dairy protein. After interaction with whey protein, the preadsorbed saliva pellicle becomes softer. However, exposure of the saliva pellicle to micellar casein causes the pellicle to partially collapse, which results in a thinner and more rigid surface layer. This structure change correlates with the measured lubrication behavior when the saliva pellicle is exposed to dairy proteins. While previous studies suggest that whey protein is the main component in milk to interact with salivary proteins, our study indicates interactions with casein are more important. The knowledge gained here provides insights into the mechanisms by which different components of dairy foods and beverages contribute to mouthfeel and texture perception, as well as influence oral hygiene.

Oral Astringency in Plant Proteins: An Underestimated Issue in Formulating Next-Generation Plant-Based Foods

Ensuring the supply of affordable, palatable, healthy, and sustainable nutrients to feed the growing population without transgressing the planetary boundaries remains a key challenge in the food science community. A dietary transition toward low-emission, plant-based foods, with less reliance on animal agriculture, is advocated for sustainability, health, and ethical reasons. A major hurdle for mainstream adoption of plant-based foods is their poor sensorial performance, such as nonjuicy and astringent textures as well as various off-flavors. This review presents the current understanding of astringency and oral friction of plant-based foods. It focuses on plant proteins and their application in plant-based meat and dairy analogs. In addition, the latest advances in the quantitative characterization of astringency using tribology, electrochemistry, and cellular tools are covered. Finally, we examine factors influencing astringency and propose easy-to-implement colloidal strategies that may mitigate astringency issues, thereby underpinning the design of the next generation of sustainable and pleasurable plant-based foods.

Perspective on oral processing of plant-based beverages

Around the world, the market for plant-derived beverages is one of the fastest-expanding segments in the functional and specialty beverage areas of newer food product development. Consumers are increasingly likely to choose alternatives to bovine beverages due to factors including lactose intolerance, hypercholesterolemia prevalence, allergies to bovine beverages, and preference for vegan diets that contain functionally active ingredients with health-promoting characteristics. Due to health, ecological, and ethical concerns, many customers are interested in reducing their usage of animal products like bovine milk. A variety of plant-based beverage substitutes are being created by the food sector as a result. To create viable alternatives, it is first necessary to provide an overview of the chemical composition, structure, features, and nutritional attributes of ordinary bovine milk. Sensory acceptability in the case of substitutes for beverages made from legumes is a significant barrier to their widespread acceptance, and thus saliva acts as a sophisticated fluid that serves a variety of purposes in the cavity of the mouth. Designing and producing next-generation plant-based beverages that mimic the physicochemical and functional qualities of conventional bovine-based beverages is gaining popularity, and many of these products can be thought of as colloidal materials that contain the particles or polymers that give them their unique qualities NG-PB foods can have a wide range of rheological qualities, such as fluids with low viscosity (such as plant-based beverages), high-viscosity liquids (like creams), soft liquids (like yogurt), as well as hard solids (such as some cheeses).

Analysis of the association between salivary proteins and oral mucositis in patients with head and neck cancer undergoing IMRT: a longitudinal study

INTRODUCTION

This longitudinal study assessed the association between salivary protein composition and the clinical onset/severity of oral mucositis (OM) in patients with head and neck tumours treated with intensity-modulated-radiotherapy (IMRT).

METHODS

Saliva samples/clinical data were obtained from 40 head and neck cancer patients treated at Guy's Hospital before -IMRT(T0) and after-IMRT (T1 = 6 m, T2 = 12 m) (ethics approval/consent). Salivary flow rate, total protein concentration, and secretion rate were determined from saliva samples and compared with pre-treatment values. OM was assessed, total/specific salivary proteins, including mucin 5B and 7, IgA, cystatin-S, albumin, and α-amylase, were quantified.

RESULTS

95% patients experienced OM during IMRT, with 33 subjects reaching grade 2&3. At T1, there was a significant reduction in salivary flow rate, total protein secretion rate, α-amylase and cystatin-S compared to baseline. Remarkably IMRT did not significantly alter mucin 5B and 7, or the IgA secretion rate at any time point. At T1, all the analyzed proteins were associated with the OM outcomes. In addition, there was a significant inverse correlation between IgA concentration at T0 and the severity of OM during IMRT.

CONCLUSION

This study revealed significant associations between several salivary proteins and OM in patients with head and neck cancer undergoing IMRT. Further longitudinal studies are needed to confirm these results.

CLINICAL SIGNIFICANCE

The study contributes to the understanding of certain salivary proteins association with OM. This could be the first step towards identifying potential salivary markers that could offer perspectives for personalized medicine approaches to improve their quality of life (QoL).

RESEARCH QUESTION

What is the association between salivary proteins and the occurrence and severity of OM in head and neck cancer patients?

AIM

To assess the association between salivary protein composition with the clinical onset/severity of oral mucositis (OM) in head and neck cancer patients treated with intensity modulated radiotherapy.

NULL HYPOTHESIS

There is no association between salivary proteins and onset/severity of OM in HNC patients.

Distinct Changes in Metabolic Profile and Sensory Quality with Different Varieties of Chrysanthemum (Juhua) Tea Measured by LC-MS-Based Untargeted Metabolomics and Electronic Tongue

Chrysanthemum tea, a typical health tea with the same origin as medicine and food, is famous for its unique health benefits and flavor. The taste and sensory quality of chrysanthemum (Juhua) tea are mainly determined by secondary metabolites. Therefore, the present research adopted untargeted metabolomics combined with an electronic tongue system to analyze the correlation between the metabolite profiles and taste characteristics of different varieties of chrysanthemum tea. The results of sensory evaluation showed that there were significant differences in the sensory qualities of five different varieties of chrysanthemum tea, especially bitterness and astringency. The results of principal component analysis (PCA) indicated that there were significant metabolic differences among the five chrysanthemum teas. A total of 1775 metabolites were identified by using untargeted metabolomics based on UPLC-Q-TOF/MS analysis. According to the variable importance in projection (VIP) values of the orthogonal projections to latent structures discriminant analysis (OPLS-DA), 143 VIP metabolites were found to be responsible for metabolic changes between Huangju and Jinsi Huangju tea; among them, 13 metabolites were identified as the key metabolites of the differences in sensory quality between them. Kaempferol, luteolin, genistein, and some quinic acid derivatives were correlated with the "astringency" attributes. In contrast, l-(-)-3 phenyllactic acid and L-malic acid were found to be responsible for the "bitterness" and "umami" attributes in chrysanthemum tea. Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis showed that the flavonoid and flavonol biosynthesis pathways had important effects on the sensory quality of chrysanthemum tea. These findings provide the theoretical basis for understanding the characteristic metabolites that contribute to the distinctive sensory qualities of chrysanthemum tea.

The role of TRPA1 and TRPV1 in the perception of astringency

Astringency, commonly described as a drying, roughening, and/or puckering sensation associated with polyphenol-rich foods affects their palatability. While the compounds eliciting astringency are known, its mechanism of action is debated. This study investigated the role of transient receptor potential (TRP) channels A1 and V1 in astringency perception. If TRP A1 or V1 have a functional role in astringency perception, then desensitizing these receptors should decrease perceived astringency. Thirty-seven panelists underwent unilateral lingual desensitization of TRP A1 and V1 channels using mustard oil and capsaicin, respectively. Panelists then evaluated four astringent stimuli: epicatechin (EC), epigallocatechin gallate (EGCG), tannic acid (TA), and potassium alum (Alum), via 2-AFC and intensity ratings. When TRPA1 receptors were desensitized on one half of the tongue via mustard oil, no significant differences were observed between the treated and untreated sides for both 2-AFC and intensity ratings. Similarly, when TRPV1 receptors were desensitized on one half of the tongue via capsaicin, no significant differences were observed between the treated and untreated sides for both 2-AFC and intensity ratings. These findings challenge the notion that TRP channels play a pivotal role in astringency perception.

The effect of cations and epigallocatechin gallate on in vitro salivary lubrication

Ionic valency influences oral processing by changing salivary behavior and merits more attention since little is known. In this study, the influence of three ionic valences (monovalent, divalent and trivalent), ionic strength and epigallocatechin gallate (EGCG) on lubricating properties of saliva were investigated. Tribological measurements were used to characterize the lubrication response of KCl, MgCl2, FeCl3, and AlCl3 in combination with EGCG to the ex vivo salivary pellicle. KCl at 150 mM ionic strength provided extra lubrication via hydration lubrication. Contrarily, trivalent salts aggregated together with the salivary mucins via ionic cross-link interactions, which led to a decrease in salivary lubrication. FeCl3 and AlCl3 affected the salivary lubrication differently, which was attributed to changes in the pH. Finally, in presence of EGCG, FeCl3 interacted with EGCG via chelating interactions, preventing salivary protein aggregation. This resulted in less desorption of the salivary film, retaining the lubrication ability of salivary proteins.

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.

Paper chromatography approach for the assessment of interaction between red wine and whole saliva

In-mouth interaction of red wine compounds with salivary proteins is a primary event allegedly responsible for eliciting the mouth-feel sensation of astringency. Those interactions have been currently associated with precipitation of salivary protein/polyphenol complexes. However, such single physicochemical evidence for interaction does not account for the complexity of astringency. This study aimed to develop a paper chromatography method to assess interactions between red wine and the salivary protein fraction using stepwise series of red wine/saliva binary mixtures from 100% wine to 100% saliva ("Alpha and Omega series"). Aliquots of each one of the mixtures were spotted on a cellulose membrane to scrutinize independently the distribution areas of wine components (naturally pink-colored) and salivary protein (stained blue in Coomassie Brilliant R-250). This double target detection revealed interactions between saliva and red wine components along most of the quantitative Alpha and Omega series, a point of equivalence corresponding to maximum interactivity for both complex reactants and a non-diffusible sub-fraction of saliva displaying the highest interactivity. The results indicate a novel way to assess quantitatively physicochemical interactions between red wines and human saliva but also provide new lights to approach the identification of molecular salivary structures involved in triggering astringency.

Prevention of Dental Biofilm Formation with Polyphenols: A Systematic Review

Polyphenols are plant secondary products with health-promoting properties against various degenerative or infectious diseases, and thus may help in the prevention of oral diseases. The aim of the present systematic review was to investigate polyphenols as a possible adjuvant in inhibiting dental biofilm formation, which is an important precondition for the most prevalent oral disease - caries and periodontitis. A literature search was conducted using the databases PubMed, CENTRAL and Scopus. Only studies with oral healthy participants and plaque level as outcome were included. Data search and extraction was conducted by two authors independently. Of the 211 initially identified studies, only six met all inclusion criteria. Meta-analysis was performed with five studies using the random effect model. Treatment with polyphenols reduced the plaque level in comparison to a negative control, but not significantly. Strong evidence of heterogeneity was observed. The diversity and complexity of polyphenols and their preparation need to be considered. There is no clear evidence that clinical use of polyphenols can prevent dental biofilm formation. Additional research with more and larger randomized controlled trials are required.

The perception and influencing factors of astringency, and health-promoting effects associated with phytochemicals: A comprehensive review

Astringency as the complex sensory of drying or shrinking can be perceived from natural foods, including abundant phenolic compounds. Up to now, there have been two possible astringency perception mechanisms of phenolic compounds. The first possible mechanism involved chemosensors and mechanosensors and took salivary binding proteins as the premise. Although piecemeal reports about chemosensors, friction mechanosensor's perception mechanisms were absent. There might be another perception way because a part of astringent phenolic compounds also triggered astringency although they could not bind with salivary proteins, however, the specific mechanism was unclear. Structures caused the differences in astringency perception mechanisms and intensities. Except for structures, other influencing factors also changed astringency perception intensity and aimed to decrease it, which probably ignored the health-promoting effects of phenolic compounds. Therefore, we roundly summarized the chemosensor's perception processes of the first mechanism. Meanwhile, we speculated that friction mechanosensor's probably activated Piezo2 ion channel on cell membranes. Phenolic compounds directly binds with oral epithelial cells, activating Piezo2 ion channel probably the another astringency perception mechanism. Except for structure, the increase of pH values, ethanol concentrations, and viscosity not only lowered astringency perception but were beneficial to improve the bioaccessibility and bioavailability of astringent phenolic compounds, which contributed to stronger antioxidant, anti-inflammatory, antiaging and anticancer effects.

Investigating wine astringency profiles by characterizing tannin fractions in Cabernet Sauvignon wines and model wines

Contribution of various phenols on wine astringency profiles was far from clear explanations. To effectively describe wine astringency profiles and determined the function of tannins/matrix (pH and ethanol), multiple chemical analyses combined RATA (Rate-all-that-apply) sensory method were applied in Cabernet Sauvignon and model wines. Results showed that polymeric flavanols determined the bulk of wine astringency intensity, oligomeric tannins enriched the smoothness and periodontium astringency, and monomeric phenol enhanced overall astringency intensity through synergistic effect. Astringency balance was effectively quantification, and its potential correlation relationship with epicatechin extension subunit (0.83) and fluorescence peak shift (0.75) cannot be ignored. The astringency profiles of condensed tannins with anthocyanins were enhanced. Low-pH (from 3.8 to 3.0) enhanced astringency by increasing the tannins affinity to proteins, while ethanol (from 10.0 % ∼ 15.0 %) decreased the hydrophobicity bond between tannins-protein interaction. This paper provided new insights to explain wine astringency profiles and a reference for astringency modification during winemaking.

Mouthfeel subqualities in wines: A current insight on sensory descriptors and physical-chemical markers

Astringency and more generally mouthfeel perception are relevant to the overall quality of the wine. However, their origin and description are still uncertain and are constantly updating. Additionally, the terminology related to mouthfeel properties is expansive and extremely diversified, characterized by common traditional terms as well as novel recently adopted descriptors. In this context, this review evaluated the mention frequency of astringent subqualities and other mouthfeel attributes in the scientific literature of the last decades (2000-August 17, 2022). One hundred and twenty-five scientific publications have been selected and classified based on wine typology, aim, and instrumental-sensorial methods adopted. Dry resulted as the most frequent astringent subquality (10% for red wines, 8.6% for white wines), while body-and related terms-is a common mouthfeel sensation for different wine types, although its concept is still vague. Alongside, promising analytical and instrumental techniques investigating and simulating the in-mouth properties are discussed in detail, such as rheology for the viscosity and tribology for the lubrication loss, as well as the different approaches for the quantitative and qualitative evaluation of the interaction between salivary proteins and astringency markers. A focus on the phenolic compounds involved in the tactile perception was conducted, with tannins being the compounds conventionally found responsible for astringency. Nevertheless, other non-tannic polyphenolic classes (i.e., flavonols, phenolic acids, anthocyanins, anthocyanin-derivative pigments) as well as chemical-physical factors and the wine matrix (i.e., polysaccharides, mannoproteins, ethanol, glycerol, and pH) can also contribute to the wine in-mouth sensory profile. An overview of mouthfeel perception, factors involved, and its vocabulary is useful for enologists and consumers.

The Influence of (Poly)phenol Intake in Saliva Proteome: Short- and Medium-Term Effects of Apple

The relationship between salivary proteome and dietary habits was studied in previous works, where a relationship between salivary proteins like cystatins and polyphenol/tannin levels in diet was observed. However, it remains to be elucidated if this association results from an effect of polyphenol-rich food ingestion on saliva composition. The aim of this work was to test the effects of apple intake on the saliva proteome, both in the short and medium term (after 4 days of continuous intake). By incubating saliva samples with apple phenolic-rich extract, protein bands containing α-amylase, S-type cystatins, and proline-rich proteins (PRPs) appeared in the fraction that precipitated, showing the potential of these (poly)phenols to precipitate salivary proteins. Among these, it was salivary cystatins that presented changes in their levels both in the saliva samples collected immediately after apple intake and in the ones collected after 4 days of intake of an extra amount of apple. These results support the thought that intake is reflected in the salivary proteome. The effect of a polyphenol-rich food, like the apple, on salivary cystatin levels is in line with results observed in animal models and, due to the involvement of these proteins in oral food perception, it would be interesting to explore in future studies the effect of these changes on sensory perception and acceptance of polyphenol-rich food.

Novel oral microscope gives mechanistic insights into colloidal drivers of friction in oral biofilms

Texture and mouthfeel are central to the sensory enjoyment of food and beverages. Yet our incomplete understanding of how food boluses are transformed in the mouth limits our texture prediction ability. As well as thin film tribology, the interaction of food colloids with the oral tissue and salivary biofilms plays a key role in texture perception via mechanoreceptors in the papillae. In this study we describe the development of an oral microscope capable of quantitative characterization of the inactions of food colloids with papillae and their concurrent saliva biofilm. We also highlight how the oral microscope revealed key microstructural drivers of several topical phenomena (oral residue formation, coalescence in-mouth, grittiness of protein aggregates and finally microstructural origin of polyphenol astringency) in the domain of texture creation. The coupling of a fluorescent food grade dye with image analysis enabled specific and quantitative determination of the microstructural changes in mouth. Emulsions either underwent no aggregation, small aggregation, or extensive aggregation depending on whether their surface charge facilitated complexation with the saliva biofilm. Quite surprisingly cationic gelatin emulsions that were already aggregated with saliva in mouth underwent coalescence if subsequently exposed to tea polyphenols (EGCG). Large protein aggregates were found to aggregate with the saliva coated papillae, increasing their size tenfold and possibly explaining why there are perceived as gritty. An exciting observation was the oral microstructural changes that occurred upon exposure to tea polyphenols (EGCG). Filiform papillae shrunk, and the saliva biofilm was seen to precipitate/collapse, exposing a very rough tissue surface. These tentative early steps are the first in vivo microstructural insights into the different food oral transformations that are drivers of key texture sensation.

Performance of an Expert Sensory Panel and Instrumental Measures for Assessing Eating Fruit Quality Attributes in a Pear Breeding Programme

Breeding programmes count on stable trained panels that support breeding evaluation selections. This work aimed to evaluate the performance of a small expert panel in the join IRTA-PFR breeding programme to validate its use in the sensory assessments of fruit pear genotypes during the selection process. A breeding F1 population of 80 pear seedlings from this programme was used. Descriptors and standard references used for sensory evaluations of pear attributes were previously defined by the four members of the expert panel. A General Procrustes Analysis (GPA) was applied to analyse the relations between instrumental and sensory traits. The results showed a good relationship between sensory attributes such as firmness and crispness with penetrometer measures. A high correlation was also found between sensory sourness and titratable acidity (TA). Panel performance was evaluated in terms of reproducibility, homogeneity, and panel consonance. The results indicated that the experts were very consistent and had a good performance. The work demonstrates, for the first time, that a small expert trained panel could be efficiently used in pear breeding programmes and allows for the selection process in a more economical and available way in contrast to the larger sensory panels conventionally used.

FAPD: An Astringency Threshold and Astringency Type Prediction Database for Flavonoid Compounds Based on Machine Learning

Astringency is a puckering or velvety sensation mainly derived from flavonoid compounds in food. The traditional experimental approach for astringent compound discovery was labor-intensive and cost-consuming, while machine learning (ML) can greatly accelerate this procedure. Herein, we propose the Flavonoid Astringency Prediction Database (FAPD) based on ML. First, the Molecular Fingerprint Similarities (MFSs) and thresholds of flavonoid compounds were hierarchically clustering analyzed. For the astringency threshold prediction, four regressions models (i.e., Gaussian Process Regression (GPR), Support Vector Regression (SVR), Random Forest (RF), and Gradient Boosted Decision Tree (GBDT)) were established, and the best model was RF which was interpreted by the SHapley Additive exPlanations (SHAP) approach. For the astringency type prediction, six classification models (i.e., RF, GBDT, Gaussian Naive Bayes (GNB), Support Vector Machine (SVM), k-Nearest Neighbor (kNN), and Stochastic Gradient Descent (SGD)) were established, and the best model was SGD. Furthermore, over 1200 natural flavonoid compounds were discovered and built into the customized FAPD. In FAPD, the astringency thresholds were achieved by RF; the astringency types were distinguished by SGD, and the real and predicted astringency types were verified by t-Distributed Stochastic Neighbor Embedding (t-SNE). Therefore, ML models can be used to predict the astringency threshold and astringency type of flavonoid compounds, which provides a new paradigm to research the molecular structure-flavor property relationship of food components.

Influence of tannic acid concentration on the physicochemical characteristics of saliva of spider monkeys (Ateles geoffroyi)

Tannins are a chemical defense mechanism of plants consumed by herbivores. Variations in salivary physicochemical characteristics such as pH, total protein concentration (TP), and presence of proline-rich proteins (PRPs) in animals have been reported as a mechanism to protect the oral cavity when consuming food with variations in pH and tannins. Variations in salivary physiochemistry as adaptations for consuming tannin-rich foods have been found in omnivorous and folivorous primates, but have not yet been reported in frugivorous species such as spider monkeys. We therefore assessed changes in pH using test strips, TP concentration by measuring absorbance at 595 nm in a spectrophotometer and salivary PRPs using the SDS-PAGE electrophoresis technique in the saliva of nine captive spider monkeys in response to the consumption of solutions with different concentrations of tannic acid. The results showed variations in pH, TP concentration and the presence and variation of possible salivary PRPs associated with tannic acid concentration. These findings suggest that spider monkeys may tailor their salivary physicochemical characteristics in response to the ingestion of potentially toxic compounds.

Roughness perception: A multisensory/crossmodal perspective

Roughness is a perceptual attribute typically associated with certain stimuli that are presented in one of the spatial senses. In auditory research, the term is typically used to describe the harsh effects that are induced by particular sound qualities (i.e., dissonance) and human/animal vocalizations (e.g., screams, distress cries). In the tactile domain, roughness is a crucial factor determining the perceptual features of a surface. The same feature can also be ascertained visually, by means of the extraction of pattern features that determine the haptic quality of surfaces, such as grain size and density. By contrast, the term roughness has rarely been applied to the description of those stimuli perceived via the chemical senses. In this review, we take a critical look at the putative meaning(s) of the term roughness, when used in both unisensory and multisensory contexts, in an attempt to answer two key questions: (1) Is the use of the term 'roughness' the same in each modality when considered individually? and (2) Do crossmodal correspondences involving roughness match distinct perceptual features or (at least on certain occasions) do they merely pick-up on an amodal property? We start by examining the use of the term in the auditory domain. Next, we summarize the ways in which the term roughness has been used in the literature on tactile and visual perception, and in the domain of olfaction and gustation. Then, we move on to the crossmodal context, reviewing the literature on the perception of roughness in the audiovisual, audiotactile, and auditory-gustatory/olfactory domains. Finally, we highlight some limitations of the reviewed literature and we outline a number of key directions for future empirical research in roughness perception.

Colloidal dynamics of emulsion droplets in mouth

The interaction of emulsions with the tongue is key to the sensory appeal of food and can potentially be exploited for oral/buccal pharmaceutical delivery. Whilst there is good understanding of the different mucoadhesive forces governing emulsion interaction with the tongue, their relative importance is not well understood. In addition, the physical location of emulsions within the saliva papillae on the tongue is not understood at all. A combination of ex vivo salivary film, and in vivo oral coating experiments were used to determine the importance of different mucoadhesive forces. Mucoadhesion of cationic emulsions was largely driven by electrostatic complexation. SDS-PAGE of the in vivo saliva coating highlighted that mucins were largely responsible for cationic emulsion mucoadhesion. Anionic emulsions were bound via hydrophobic/steric interactions to small salivary proteins typically located away from the mucin anchor points. The physical location and clustering of emulsions relative to the salivary film/papillae was probed via the invention of a fluorescent oral microscope. Cationic emulsions were densely clustered close to the papillae whilst anionic emulsions were suspended in the salivary film above the papillae. Interestingly, non-ionic emulsions were also trapped within the salivary film above the papillae as individual droplets. These findings highlight that whilst electrostatic complexation with saliva is a powerful mucoadhesive force, hydrophobic and steric interactions also act to induce oral retention of emulsions. The differences in physical location and clustering of emulsions within the salivary film hint at the 3D locations of the different salivary proteins driving each mucoadhesive interaction. This novel understanding of emulsion saliva/papillae interactions has potential to aid efficacy of buccal pharmaceutical delivery and the reduction of astringency in plant-based foods.

Wine astringency: more than just tannin-protein interactions

Red wines are characterized by their astringency, a very important sensory attribute that affects the perceived quality of wines. Three mechanisms have been proposed to explain astringency, and two theories describe how these mechanisms work in an integrated manner to produce tactile sensations such as drying, roughening, shrinking and puckering. The factors involved include not only tannins and salivary proteins, but also anthocyanins, grape polysaccharides and mannoproteins, as well as other wine matrix components that modulate their interactions. These multifactorial interactions could be responsible for different sensory responses and therefore need to be further studied. This review presents the latest advances in astringency perception and its possible origins, with special attention on the interactions of components, their impact on oral perception and the development of astringency sub-qualities. Future research efforts should concentrate on understanding the mechanisms involved as well as on the limiting factors related to the conformation and stability of the tannin-salivary protein complexes. © 2021 Society of Chemical Industry.

Astringency Sensitivity to Tannic Acid: Effect of Ageing and Saliva

Astringency is an important sensory characteristic of food and beverages containing polyphenols. However, astringency perception in elderly people has not been previously documented. The aim of the present work was to evaluate sensitivity to astringency as a function of age, salivary flow and protein amount. Fifty-four panellists, including 30 elderly people (age = 75 ± 4.2 years) and 24 young people (age = 29.4 ± 3.8 years), participated in this study. Astringency sensitivity was evaluated by the 2-alternative forced choice (2-AFC) procedure using tannic acid solutions. Whole saliva was collected for 5 min before and after the sensory tests. The results showed that the astringency threshold was significantly higher in the elderly group than the young group. No correlation was observed between the salivary protein amount and threshold value. However, a negative correlation between salivary flow and threshold was observed in the young group only. These results showed a difference in oral astringency perception as a function of age. This difference can be linked to salivary properties that differ as a function of age.

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

How Different Snacks Produce a Distinct Effect in Salivary Protein Composition

Saliva secretion changes in response to different stimulation. Studies performed in animals and humans suggest that dietary constituents may influence saliva composition, although the dynamics of these changes, and how they are specific for each type of food, are little known. The objective of the present study was to access the short-term effects of different foods in salivation and salivary protein composition. Twelve participants were tested for four snacks (yoghurt, bread, apple and walnuts). Non-stimulated saliva was collected before and at 0′, 5′ and 30′ after each snack intake. Flow rate, total protein, alpha-amylase enzymatic activity and salivary protein profile were analyzed. Yoghurt and apple were the snacks resulting in higher salivary changes, with higher increases in flow rate and alpha-amylase activity immediately after intake. The expression levels of immunoglobulin chains decreased after the intake of all snacks, whereas cystatins and one pink band (proline-rich proteins—PRPs) increased only after yoghurt intake. Walnut’s snack was the one resulting in lower changes, probably due to lower amounts eaten. Even so, it resulted in the increase in one PRPs band. In conclusion, changes in saliva composition varies with foods, with variable changes in proteins related to oral food processing and perception.

Protein-saliva interactions: a systematic review

Food industries are challenged to reformulate foods and beverages with higher protein contents to lower fat and sugar content. However, increasing protein concentration can reduce sensory acceptability due to astringency perception. Since the properties of food-saliva mixtures govern mouthfeel perception, understanding how saliva and protein interact is key to guide development of future protein-rich reformulations with optimal sensory attributes. Hence, this systematic review investigated protein-saliva interaction using both model and real human saliva, including a quality assessment. A literature search of five databases (Medline, Pubmed, Embase, Scopus and Web of Science) was undertaken covering the last 20 years, yielding 36 604 articles. Using pre-defined criteria, this was reduced to a set of 33 articles with bulk protein solutions (n = 17), protein-stabilized emulsions (n = 13) and protein-rich food systems (n = 4). Interaction of dairy proteins, lysozyme and gelatine with model or human saliva dominated the literature. The pH was shown to have a strong effect on electrostatic interaction of proteins with negatively-charged salivary mucins, with greater interactions occurring below the isoelectric point of proteins. The effect of protein concentration was unclear due to the limited range of concentrations being studied. Most studies employed a 1 : 1 w/w protein : saliva ratio, which is not representative of true oral conditions. The interaction between protein and saliva appears to affect mouthfeel through aggregation and increased friction. The searches identified a gap in research on plant proteins. Accurate simulation of in vivo oral conditions should clarify understanding of protein-saliva interaction and its influence on sensory perception.

Influence of Age and Individual Differences on Mouthfeel Perception of Whey Protein-Fortified Products: A Review

Protein needs are considered to increase with age, with protein consumption being associated with many positive outcomes. Protein-fortified products are often used to improve nutritional status and prevent age-related muscle mass loss in older adults. Accordingly, older adults are commonly provided with products fortified with whey protein; however, such products can cause mouthdrying, limiting consumption and product enjoyment. Currently, the extent to which age and individual differences (e.g., saliva, oral health, food oral processing) influence the perception of whey protein-derived mouthdrying is relatively unclear. Previous research in this area has mainly focused on investigating mouthdrying, without taking into account individual differences that could influence this perception within the target population. Therefore, the main focus of this review is to provide an overview of the relevant individual differences likely to influence mouthfeel perception (specifically mouthdrying) from whey protein-fortified products, thereby enabling the future design of such products to incorporate better the needs of older adults and improve their nutritional status. This review concludes that age and individual differences are likely to influence mouthdrying sensations from whey protein-fortified products. Future research should focus more on the target population and individual differences to maximise the benefits from whey protein fortification.

A review of the flavor profile of metal salts: understanding the complexity of metallic sensation

The oral sensation of metallic is a complex experience. Much of our current understanding of metallic sensation is from the investigation of metal salts, which elicit diverse sensations, including taste, smell, and chemesthetic sensations, and therefore meet the definition of a flavor rather than a taste. Due to the involvement of multiple chemosensory systems, it can be challenging to define and characterize metallic sensation. Here, we provide a comprehensive review of the psychophysical studies quantifying and characterizing metallic sensation, focusing on metal salts. We examine the factors that impact perception, including anion complex, concentration, nasal occlusion, and pH. In addition, we summarize the receptors thought to be involved in the perception of metallic sensation (i.e., TRPV1, T1R3, TRPA1, and T2R7) either as a result of in vitro assays or from studies in knock-out mice. By enhancing our scientific understanding of metallic sensation and its transduction pathways, it has the potential to improve food and pharmaceuticals, help identify suppression or masking strategies, and improve the ability to characterize individual differences in metallic sensation. It also has the potential to translate to clinical populations by addressing the disparities in knowledge and treatment options for individuals suffering from metallic taste disorder (i.e., phantom taste or "metal mouth"). Future psychophysical studies investigating the sensory perception of metal salts should include a range of compounds and diverse food matrices, coupled with modern sensory methods, which will help to provide a more comprehensive understanding of metallic sensation.

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.

Cell Wall Mannoproteins from Yeast Affect Salivary Protein-Flavanol Interactions through Different Molecular Mechanisms

It is known that interactions between wine flavanols and salivary proline-rich proteins (PRPs) are one of the main factors responsible for wine astringency. The addition of commercial yeast mannoproteins (MPs) to wines has been pointed to as a possible tool to modulate the excessive astringency due to a lack of phenolic maturity at harvest time that might occur as a consequence of global climate change. The aim of this work was to study by isothermal titration calorimetry and molecular dynamics simulation the molecular mechanisms by which mannoproteins could modulate astringency elicited by tannins and if it can be influenced by mannoprotein composition. Results obtained indicate that the MPs assayed had an important impact on astringency through the formation of ternary aggregates with different solubilities or by preventing the flavanol-PRP interaction by a competitive mechanism, although in a different strength, depending on the size and the compositional characteristic of the mannoprotein.

An Investigation of the Influence of Age and Saliva Flow on the Oral Retention of Whey Protein and Its Potential Effect on the Perception and Acceptance of Whey Protein Beverages

Protein fortified products are regularly recommended to older adults to improve nutritional status and limit sarcopenia. However protein fortification can elicit negative sensory attributes such as mouthdrying. Sensitivity to mouthdrying can increase with age, yet the influence of saliva flow and mucoadhesion remain uncertain. Here, two studies tested different whey protein beverages (WPB); 22 healthy younger volunteers completed a pilot and 84 healthy volunteers from two age groups (18-30; 65+) completed the main study. In both studies salivary flow rates (mL/min) were measured and saliva samples were collected at time intervals post beverage consumption to measure mucoadhesion to the oral cavity, where protein concentration was analysed by Bradford Assay. Volunteers rated perception and acceptability of WPBs in the main study. WPB consumption resulted in significantly increased protein concentration (p < 0.0001) in saliva samples compared with a control whey permeate beverage. Older adults had significantly lower unstimulated saliva flow (p = 0.003) and significantly increased protein concentration (p = 0.02) in saliva samples, compared with younger adults. Heating of WPB significantly (p < 0.05) increased mouthdrying and thickness perception and reduced sweetness compared with unheated WPB. Mucoadhesion is concluded to be a true phenomenon in WPBs and increases with age.

Sensorial Perception of Astringency: Oral Mechanisms and Current Analysis Methods

Understanding consumers' food choices and the psychological processes involved in their preferences is crucial to promote more mindful eating regulation and guide food design. Fortifying foods minimizing the oral dryness, rough, and puckering associated with many functional ingredients has been attracting interest in understanding oral astringency over the years. A variety of studies have explored the sensorial mechanisms and the food properties determining astringency perception. The present review provides a deeper understanding of astringency, a general view of the oral mechanisms involved, and the exciting variety of the latest methods used to direct and indirectly quantify and simulate the astringency perception and the specific mechanisms involved.

Using Multiple Sensory Profiling Methods to Gain Insight into Temporal Perceptions of Pea Protein-Based Formulated Foods

The food industry is focused on creating plant-based foods that incorporate pea protein isolates. However, pea protein isolates are often described as having persistent beany, bitter, and astringent notes that can decrease the desirability of the resulting foods and make static sensory profiling difficult. To obtain more realistic descriptions of the sensory experiences associated with this category of products, researchers should consider using temporal methods and multi-intake methods, which allow consumers to evaluate whole food portions. This study aimed to understand better how product composition affected the sensory perception of pea protein-based beverages using three different sensory profiling methods. Particular focus was placed on beany, bitter, and astringent notes. Twelve pea protein-based beverages were formulated; they varied in pea protein type (pellet vs. isolate) and their content of gellan gum, salt, sunflower oil, sugar, and soy lecithin. They were evaluated by 16 trained panelists using three sensory profiling methods: static block profiling, mono-intake temporal dominance of sensations (TDS) profiling, and multi-intake TDS profiling. The static block and mono-intake TDS profiling methods yielded complementary results about the impact of beverage composition on attribute perceptions. Static block profiling revealed that beaniness was mainly affected by gellan gum and oil content and that bitterness and astringency were mainly affected by protein type and gellan gum content. Mono-intake TDS profiling highlighted the dynamics of beaniness and the strong persistence of astringency, and its results suggested that higher gellan gum and salt contents could limit this persistence. Multi-intake TDS profiling found that, throughout the consumption of a full product portion, beaniness and bitterness decreased, indicating an adaptation effect, while fattiness increased, indicating a build-up effect. This study has increased the understanding of how pea protein-based beverages are perceived under conditions that more closely resemble those associated with real-life consumption. It has also revealed how product formulation can reduce bitterness and astringency.