From Oak Bark to Black Coffee: How Polyphenols Interact with Proteins (Part 1)

In Brief

Polyphenols are plant compounds that influence colour, bitterness, astringency, oxidation and ageing in wine. Their ability to interact with proteins explains a wide range of phenomena, from the transformation of animal hide into leather to the softer perception of black coffee when milk is added. This article explores the main families of wine polyphenols, the structure of proteins and their binding regions, the role of salivary proteins in astringency, and the historical importance of vegetable tanning in Tuscany and Montalcino. It also examines how milk proteins, fat and sugar modify the sensory perception of coffee.

Keywords: wine polyphenols, tannins, proteins, protein binding, salivary proteins, astringency, active sites, binding sites, molecular weight, anthocyanins, flavan-3-ols, condensed tannins, ellagitannins, oak tannins, vegetable tanning, leather tanning, collagen, oak bark, Tuscany leather, Montalcino history, Brunello di Montalcino, black coffee, milk proteins, casein, coffee polyphenols, chlorogenic acids, sensory masking, sugar and bitterness

From Oak Bark to Black Coffee: How Polyphenols Interact with Proteins

Long before tannins became part of the vocabulary of wine tasting, they were practical tools used by human societies to transform animal skin into leather.

The same broad family of plant compounds that once stabilised hides in traditional tanneries is now associated with the colour, structure, bitterness and ageing potential of wine. Similar interactions can also be observed in an everyday cup of black coffee when milk is added.

Leather, wine and coffee may seem like unrelated subjects. Chemically, however, they are connected by one central phenomenon: the ability of certain plant polyphenols to interact with proteins.

Understanding this relationship provides a useful foundation for exploring astringency, texture and food pairing. It also reminds us that taste is not merely aromatic. It is shaped by physical interactions taking place between plant molecules, food components and the human mouth.

What Are Polyphenols?

Polyphenols are a large and diverse family of plant compounds characterised by the presence of one or more phenolic structures.

Plants do not produce them for wine drinkers. They use them as part of their own biological systems: for protection against ultraviolet radiation, pathogens and herbivores; for pigmentation; and as part of their responses to environmental stress.

In wine, polyphenols contribute to:

• Colour.
• Bitterness.
• Astringency.
• Oxidative behaviour.
• Mouthfeel.
• Aromatic evolution.
• Ageing potential.

The word polyphenol does not describe one single substance. It refers to numerous compounds with very different molecular sizes, structures and sensory effects.

Wine polyphenols are generally divided into two broad families: flavonoids and non-flavonoids.

The Principal Polyphenols in Wine

Anthocyanins

Anthocyanins are largely responsible for the red, purple and bluish colours of young red wines.

They are extracted primarily from grape skins during fermentation. Over time, they react with tannins and other wine compounds, forming more stable pigments and contributing to the gradual evolution of colour from purple towards ruby, garnet and brick tones.

Anthocyanins are not traditionally considered the principal source of astringency, but research shows that they may participate in interactions involving salivary proteins and influence the way tannin structure is perceived.

Flavan-3-ols

Flavan-3-ols include compounds such as catechin and epicatechin.

These relatively small molecules can contribute to bitterness. They also act as the structural units from which larger condensed tannins are formed.

Condensed Tannins

Condensed tannins, also called proanthocyanidins, are chains of flavan-3-ol units.

They are found mainly in grape skins, seeds and stems. Their precise composition depends on the grape variety, vineyard conditions, maturity and winemaking techniques.

Skin tannins and seed tannins do not have identical structures. Their degree of polymerisation—the number of connected molecular units—also varies.

Molecular size matters, but it does not alone determine sensory behaviour. The shape, flexibility and chemical accessibility of a tannin influence its affinity for proteins and its perception as bitter, drying, rough or velvety.

Flavonols

Flavonols include compounds such as quercetin, myricetin and kaempferol.

They are concentrated mainly in grape skins and help protect the berries from ultraviolet radiation. Although present in smaller quantities than some other phenolic groups, they contribute to wine colour, copigmentation and oxidative behaviour.

Phenolic Acids

Wine contains hydroxybenzoic and hydroxycinnamic acids.

These compounds are generally smaller than condensed tannins. They can participate in oxidation and aromatic evolution and may act as precursors to volatile compounds generated during fermentation or ageing.

Stilbenes

The most famous stilbene in wine is resveratrol.

Stilbenes attract considerable attention because of their potential biological properties, but their contribution to the immediate tactile structure of wine is relatively limited compared with that of tannins.

Oak-Derived Tannins

Oak barrels can introduce hydrolysable tannins, particularly ellagitannins.

These differ chemically from the condensed tannins originating in grapes. They can influence oxygen reactions, colour stability, structure and ageing, although their sensory role depends on the barrel’s origin, seasoning, toast level, age and duration of contact with the wine.

Wine therefore contains a complex phenolic network, not a single substance simply called “tannin”.

What Are Proteins?

Proteins are long chains of amino acids folded into specific three-dimensional structures.

Their molecular weights are commonly expressed in daltons or kilodaltons. One kilodalton, abbreviated as kDa, corresponds to approximately one thousand daltons.

Proteins can range from small peptides of only a few amino acids to enormous molecular assemblies composed of several connected subunits.

Their behaviour depends not only on molecular weight but also on:

• Amino-acid composition.
• Three-dimensional conformation.
• Electrical charge.
• Flexibility.
• Solubility.
• Hydrophobic regions.
• Exposure of potential binding areas.
• Environmental conditions such as pH and temperature.

A large protein does not necessarily bind more polyphenols than a smaller one. A compact protein may hide many of its potential binding regions inside its folded structure, while a flexible protein can expose numerous accessible surfaces.

Active Sites or Binding Sites?

The expression active site is often used too broadly.

An active site is properly the specific region of an enzyme where a chemical reaction takes place. Most proteins involved in tasting are not acting as enzymes.

When discussing interactions between tannins and food or salivary proteins, it is generally more accurate to refer to:

• Binding sites.
• Binding regions.
• Accessible amino-acid sequences.
• Exposed molecular surfaces.

Polyphenols tend to associate particularly well with proteins containing flexible, exposed and hydrophobic regions. Proline-rich sequences often have a strong affinity for tannins.

Hydrogen bonding and hydrophobic interactions are among the principal forces involved. Depending on the conditions, additional electrostatic and covalent interactions may also occur.

The Different Types of Proteins Relevant to Taste

Salivary Proteins

Saliva is not simply water. It is a complex biological fluid containing minerals, enzymes, lipids and several families of proteins.

Among the proteins involved in the perception of astringency are:

• Proline-rich proteins.
• Mucins.
• Histatins.
• Statherin.
• Amylase.

Proline-rich proteins are particularly important because their flexible structures and abundance of exposed proline residues offer multiple binding regions for tannins.

Mucins are much larger glycoproteins. They contribute to the lubricating film covering the mouth and help create the smooth movement of oral surfaces.

When tannins interact with salivary proteins, they may form complexes and aggregates. This can reduce lubrication and increase friction between the tongue, palate, cheeks and gums.

The resulting sensation is astringency.

Astringency Is Not Simply Bitterness

Bitterness is a taste detected by specialised receptors.

Astringency is primarily a tactile sensation. It may be described as:

• Drying.
• Puckering.
• Rough.
• Dusty.
• Grippy.
• Velvety.
• Chalky.

Astringency is therefore not exactly something we taste. It is something we feel.

Tannin–protein precipitation was historically presented as the complete explanation for astringency. Today, the mechanism is understood to be more complex. In addition to salivary protein interactions, oral surfaces, mucins, lipids, viscosity and sensory receptors may all be involved.

The perception also evolves during tasting. The first sip of a tannic wine may feel less drying than the third or fourth because lubrication has progressively changed.

Individual differences in salivary composition help explain why the same wine can appear smooth to one person and aggressive to another.

From Animal Hide to Leather

The historical word tannin is connected to tanning: the process through which animal hide is transformed into stable leather.

Fresh animal hide is rich in collagen. Without preservation, it is vulnerable to microbial decay and physical deterioration.

Collagen is a structural protein composed of three chains wound together into a triple helix. A complete collagen molecule has a molecular mass of approximately 300 kDa, although collagen exists within tissues as much larger organised fibres.

Vegetable tanning traditionally uses tannin-rich materials such as:

• Oak bark.
• Chestnut wood and bark.
• Mimosa.
• Quebracho.
• Sumac.
• Various leaves, woods and fruits.

During tanning, plant tannins penetrate the hide and associate with collagen. These interactions stabilise the protein network and reduce its susceptibility to decomposition, heat and water.

The process is much slower and more sustained than anything occurring during tasting, but the molecular affinity is conceptually related: plant polyphenols interact with animal proteins and change their physical behaviour.

Tanning demonstrates that tannin–protein affinity is not merely a metaphor invented by sommeliers. It is a material property that humans have used for centuries.

Tuscany and the Tradition of Vegetable-Tanned Leather

Tuscany possesses a long history of leatherworking and vegetable tanning.

The most famous modern tanning district lies in the area around Santa Croce sull’Arno, between Florence and Pisa. The deeper tradition, however, is much older and is connected to medieval towns, water systems, livestock farming, woodland resources and commercial routes.

Oak and chestnut woodlands provided tannin-rich raw materials. Hides came from animals raised for food, labour and transport. Leather was essential for shoes, harnesses, belts, containers, saddles, protective clothing and bookbinding.

The leather crafts therefore belonged to a circular rural economy long before the expression “circular economy” existed.

Animal husbandry produced hides. Woodland management produced bark and timber. Tanners transformed a perishable by-product into a durable material.

Montalcino Before Brunello

Montalcino is now known throughout the world for Brunello. Wine has become so dominant in its contemporary identity that earlier economic activities can easily disappear from view.

The territory was historically associated with extensive woodland, particularly holm oak. The name Montalcino is traditionally connected to the Latin expression mons ilcinus, often interpreted as the “mountain of holm oaks”.

Leatherworking, shoemaking and related crafts formed part of the town’s pre-modern economy. Montalcino’s location in southern Tuscany, not far from routes connected with the Via Francigena, supported the movement of pilgrims, animals, goods and artisan products.

It is tempting to construct an overly perfect story in which all Montalcino leather was tanned exclusively with local oak bark. Available historical accounts do not justify such a precise claim.

What can reasonably be said is that woodland resources, animal husbandry and artisan trades created favourable conditions for leather production, and that Montalcino was historically known for leatherworkers and shoemakers before its international association with Brunello.

The connection between Montalcino and tannins is therefore more than a play on words.

In an earlier economy, plant tannins helped transform hides into leather. In the modern economy, grape tannins and oak-derived polyphenols contribute to the structure and longevity of one of Italy’s most famous wines.

Oak Bark and Oak Barrels: Similar Origin, Different Functions

Oak bark used in vegetable tanning contains tannins capable of interacting strongly with collagen.

Oak used for wine barrels also contains tannins, but the objective is very different. A barrel is not intended to tan the wine.

During ageing, the barrel allows slow oxygen exchange and releases numerous compounds, including ellagitannins, volatile phenols, oak lactones, furfural derivatives and aromatic substances associated with seasoning and toasting.

These compounds can participate in:

• Colour stabilisation.
• Oxidative reactions.
• Aromatic development.
• Tannin evolution.
• Changes in perceived texture.

The relationship between oak and wine is therefore chemical, technological and cultural. It should not, however, be reduced to the addition of a generic “oak flavour”.

As in traditional tanning, the quality of the raw material, the preparation of the wood, time and human craftsmanship are fundamental.

Black Coffee and Milk: An Everyday Demonstration

A cup of black coffee offers a familiar demonstration of protein–polyphenol interactions.

Coffee contains a complex mixture of molecules, including:

• Chlorogenic acids.
• Caffeic acid derivatives.
• Melanoidins formed during roasting.
• Caffeine.
• Volatile aromatic compounds.
• Organic acids.
• Lipids.

Chlorogenic acids are among the main phenolic compounds in coffee. Roasting transforms part of them and produces new bitter, aromatic and coloured substances.

Black coffee can therefore present both bitterness and a drying sensation. These two perceptions overlap but do not have identical origins.

When milk is added, several changes occur simultaneously.

Milk proteins can associate with coffee polyphenols. Caseins are particularly important because of their flexible structures and accessible hydrophobic regions. Whey proteins can also participate, especially when processing or heating alters their conformation.

Milk fat increases richness and lubrication. Lactose brings a mild natural sweetness. The beverage is also diluted and its temperature may fall.

As a result, the coffee often seems:

• Less bitter.
• Less drying.
• More rounded.
• More viscous.
• Sweeter.
• Aromatically softer.

Research confirms that milk proteins and coffee phenolics can form molecular associations, although their consequences for taste, antioxidant measurement and bioavailability depend on the beverage and experimental conditions.

Milk does not simply “neutralise” coffee. It reorganises the entire sensory system.

The Role of Sugar as a Sensory Mask

Adding sugar to coffee produces a different phenomenon.

Sugar does not necessarily remove bitter or phenolic compounds. It changes how they are perceived.

Sweetness can suppress or mask bitterness and reduce the prominence of acidity. It may also make astringency appear less severe within the complete sensory experience, even though the underlying tannins or polyphenols remain present.

Sugar is therefore best understood as a sensory mask.

The same principle applies to wine. Residual sugar can make a wine appear:

• Softer.
• Rounder.
• More immediately fruity.
• Less acidic.
• Less bitter.
• Less tannic.

This can be positive when sugar is part of a balanced wine style. It can also hide structural weaknesses, excessive bitterness, high alcohol or aggressive extraction.

Perceived softness is not always proof that tannins are genuinely fine or mature. Sometimes sweetness simply redirects sensory attention.

One Molecular Principle, Many Human Uses

Leather tanning, wine tasting and adding milk to coffee are not identical reactions. Their timescales, materials and objectives are completely different.

Yet they all reveal the same broad affinity: certain plant polyphenols can associate with proteins and modify their behaviour.

In leather, the interaction helps stabilise collagen.

In wine tasting, tannins interact with salivary proteins and contribute to astringency.

In milk coffee, dairy proteins, fats and lactose reshape the perception of coffee phenolics.

This molecular principle connects agriculture, craftsmanship and gastronomy. It also reminds us that taste does not exist exclusively inside a glass or on a plate.

Taste emerges from interaction.

It is created between molecules, between food and saliva, between human techniques and natural materials—and between history and the way we continue to eat and drink today.

Improve our tasting skills with Jollie's activities:

Guided Cheese tasting at Formaggioteca Terroir

Guided Cheese and Wine tasting during Fabulous Tuscany Wine Tour with Grape Tours