Understanding Tannins: Nature's Astringent Polyphenols

Tannins, widely acknowledged for their astringent flavor, constitute a category of plant polyphenols with versatile applications and significant physiological impacts. These substances are celebrated for their capacity to associate with and cause the precipitation of proteins, inducing sensations of bitterness and astringency in comestibles and beverages. Grasping the essence and functionalities of tannins yields valuable insights across diverse domains such as nutrition, medicine, and industrial practices.

Primarily, tannins assume a pivotal role in determining the taste and tactile sensation of numerous consumables. Their interaction with proteins instigates a desiccating, puckering feel in the oral cavity, frequently encountered in red wines, teas, and specific fruits like grapes and persimmons. This distinctive astringent quality contributes to the intricacy and equilibrium of flavors in gastronomic encounters.

Furthermore, tannins harbor antioxidant characteristics that promise potential health advantages. Research indicates that the consumption of tannins through diet could potentially safeguard against oxidative stress, inflammation, and certain chronic ailments, encompassing cardiovascular disorders and cancer. Additionally, tannins demonstrate antimicrobial attributes, which have been investigated for their utility in food preservation and healthcare practices.

Apart from their roles in gastronomy and health maintenance, tannins find utility in various industrial applications. Their capability to precipitate proteins has been harnessed in the tanning of leather, where they bind with collagen fibers, rendering them more impervious to decay and augmenting the longevity of leather goods. Tannins also hold promise in wastewater treatment, facilitating the elimination of heavy metals and organic contaminants via intricate bonding and precipitation reactions.

In summary, tannins epitomize an enthralling category of plant polyphenols with multifarious functionalities and utilities. From sculpting the gustatory properties of edibles and beverages to proffering potential health advantages and bolstering industrial processes, their influence spans across an array of domains. By unraveling the intricacies of tannins' properties and mechanisms, we can further explore their potential in diverse fields, thereby enriching our comprehension of these remarkable natural compounds.
Understanding Tannins: Nature's Astringent Polyphenols

Tannins in plant food

Tannins are a group of phytochemicals that have been reported to be present in fruits and vegetables like bananas, sorghum, grapes, raisins, spinach, red wine, persimmons, chocolate etc. Beverages like tea and coffee have also been recognized to be rich source of tannins.

In their natural form, tannins are water-soluble phenolic compounds with molecular weights in the range of 300 to 500 and have the ability to precipitate gelatin, alkaloids, and proteins. Tannins are secondary metabolites widely distributed in plants: they are polymeric phenolic substances with astringency properties.

Tannins are classified into two classes. One group is comprised of hydrolysable polyhydric alcohol in which the hydroxyl groups are partially or completely esterified by gallic acid or similar kind of compounds. Three types of hydrolysable tannins: gallotannines, ellagitannines, and complex tannins (sugars derivatives—mainly glucose, gallic acid, and ellagic derivatives).

Hydrolysable tannins are mainly present in angiosperm and dicotyledons. Both gallotannins and ellagitannins may synthesize individually or in the form of a mixture in plants.

The other group is composed of tannins which are formed by condensation of phenolic compounds. Those are non-hydrolysable and are known as condensed tannins. Condensed tannins (also referred to as proanthocyanidins) are flavonoid units linked by carbon–carbon bonds that are not susceptible to cleavage by hydrolysis. These compounds are responsible for the intense pink, red, purple, or blue colors of many flowers, fruits, and leaves.

The antioxidant properties of tannins are widely utilized in the food and medical fields. Owing to its antioxidant capacity, such as preventing cardiovascular disease, cancer or osteoporosis, tannins have attracted much attention.
Tannins in plant food

Oligomeric proanthocyanidins – Food sources and health benefits

Proanthocyanidins are condensed tannins with various pharmacological properties. The compound forms a unique class of high-molecular weight oligomeric and polymeric secondary metabolites. Proanthocyanidins are present in flowers, nuts, fruits, bark, and seeds of various plants, as a defense against biotic and abiotic stressors. Their astringency protects the plants from pathogens and predators.

Proanthocyanidin degree of polymerization can range between 3 and 11. Usually, proanthocyanidins with a lower degree of polymerization (two to four monomers) are named oligomeric proanthocyanidins, whereas those with more than five monomers are called polymeric proanthocyanidins.

Oligomeric proanthocyanidins are one of the commonest bioflavonoids with a special molecular structure, formed by oligomerization or polymerization of subunits catechin, epicatechin, and their gallic acid esters primarily known for their excellent antioxidant activity.

Cocoa beans are a well-known source of antioxidant polyphenols. Especially individual oligomeric proanthocyanidins demonstrated a significant contribution to the total antioxidant activity of cocoa compared to monomeric compounds.

Chocolate contains flavonoids not found in tea, with high concentrations of oligomeric procyanidins. Grape seed and pine bark extracts are also rich with oligomeric proanthocyanidins. Oligomeric proanthocyanidins are also abundant in blackjack oak, horse chestnut, witch hazel and hawthorn, as well as in apples, berries, barley (and beer made from it), bean hulls, chocolate, rhubarb, rose hips and sorghum.

Proanthocyanidins are shown to be highly effective inhibitors of radical chain reactions. In cocoa, they consist of oligomers or polymers of (+)- catechin and/or (-)-epicatechin units. The size of proanthocyanidins is specified by their degree of polymerization (DP).

Oligomeric proanthocyanidins are primarily known for their antioxidant activity. It is widely reported that Oligomeric proanthocyanidin could prevent oxidative damage in tissues by reducing lipid oxidation and/or inhibit production of free radicals.

Oligomeric proanthocyanidin complexes also have been reported to demonstrate, antibacterial, antiviral, anticarcinogenic, anti-inflammatory, anti-allergic, and vasodilatory actions. In addition, they have been found to inhibit lipid peroxidation, platelet aggregation, capillary permeability and fragility, and to affect enzyme systems including phospholipase A2, cyclooxygenase, and lipoxygenase.
Oligomeric proanthocyanidins – Food sources and health benefits

Ellagitannins in food

Plants with high concentrations of ellagitannins have been attributed to a variety of health benefits and they are important contributors to color, flavor and stability of many foods.

Strawberries, blackberries, cranberries walnuts, and pecans are particularly good source of the polyphenol compound ellagic acid that the hydrolysis product of ellagitannins.

Ellagitannins are polyphenol naturally occurring water-soluble tannins found in many plant foods (including grapes, raspberries, strawberries, blackberries, blueberries and some nuts) and in oak-aged red wines.

Anti-carcinogenicity of ellagitannins and their metabolite ellagic acid has been shown along with induction of apoptosis in some cancer cells.

Canning, pureeing, and freezing had little effect on ellagitannins, but processing berries into non-clarified and clarified juices resulted in total ellagitannin losses of 70 and 82% respectively, due to removal of ellagitannin rich seeds in the presscake.
Ellagitannins in food

Tannins in tea

Tea is a popular beverage throughout the world. It is pleasant and mildly stimulating.

Tannins are a family of astringent compounds called polyphenols. Polyphenols constitute over 48.5% of the total solids in a cup of tea. They’re found in several kinds of plants, including dark sorghum, some apples and yerba mate, a popular South American drink made from the leaves and branches of an evergreen tree.

Tea is often included in the list of tannins-containing foods.   The tea leaves contain varying amounts of tannins: Green teas, which have unfermented leaves, contain the most tannins; black teas including pekoe, which are fermented, contain the fewest.

By definition, tannins are biologically active, that is tannins bond protein, a process that is pH dependent. Most of the phenolic groups in tannins are free. Tannins have a molecular weight between 500 and 3000, and generally are soluble in water.

Tannins are usually divided into two classes: those can be hydrolyzed (react with water) and those that cannot.  The nonhydrolysable tannins found in tea are condensation polymers of catechin.

The occurrence of tannins is reported in unripe fruits such as mango, dates and persimmons. The tannins diminish in amounts as the fruits ripen.

Other food sources of tannins are grapes (condensed tannins, on average 5000 mg/kg of grapes). Grape juice, wines (red wine: 1.2 – 4l4 g/L of wine) and some sorghum grain varieties.
Tannins in tea

What is proanthocyanidins?

Proanthocyanidins also called condensed tannins, are oligomers, and polymers of monomeric flavonoids. It represents a major group of phenolic compounds that occurs ubiquitously in woody and some herbaceous plants.

More specifically, they are polyflavans, condensed molecules of those flavonoids with a saturated ‘C’ ring. Linkage of the flavan-3-ol units occurs mainly through the C4  C8 bond, although a C4  C6 bond may also exits (B-type linkages).

Proanthocyanidins are considered secondary metabolites of plants, i.e. they are not required for the structural or metabolic integrity of the organism. The proanthocyanidins are classified according to the type of flavan-3-ols present, which vary in their OH patterns at C3 on the B ring.

The two major classes of proanthocyanidins found in berry fruit include procyanidins, composed exclusively of epi (catechin) units, and propelargonidins, composed exclusively of (epi)afzelechin units.

When tannins are consumed by man, they often have astringent properties. This properties cause by the cross-linkage of protein and glycoproteins and a corresponding loss of lubrication in the mouth and throat.

Proanthocyanidins are common in fruits, fruit pods, sees and shell, either as preracetates or in free phenolic forms.
What is proanthocyanidins?