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

Blueberries: Rich Antioxidant Source

Polyphenols or phenolic compounds emerge as secondary metabolites in the shikimic acid pathway of plants and pentose phosphate via phenylpropanoid metabolization. Comprising benzene rings adorned with one or more hydroxyl substituents, they range from straightforward phenolic molecules to intricately polymerized compounds.

These compounds are recognized for their vital antioxidant properties and showcase various activities, such as antiglycemic, antiviral, anticancer, anti-inflammatory, antiallergic, and antimicrobial effects. Plant phenolics assume a crucial role as defense mechanisms against environmental stresses, such as high light, low temperatures, pathogen infections, herbivores, and nutrient deficiencies, which can instigate an elevated production of free radicals and oxidative species in plants.

Blueberries, often dubbed the "longevity fruit," are celebrated for their significant antioxidant capacity against free radicals and reactive species. They stand out as one of the most abundant sources of antioxidants among fruits and vegetables. Recent research has unveiled additional health benefits unrelated to antioxidants, including a reduced incidence of major modern diseases like cardiovascular disease, diabetes, and cancer.

The blueberry plant hosts a diverse array of antioxidant and antimicrobial molecules, specifically polyphenol compounds, spanning a broad spectrum of anthocyanins. Among various fruits and vegetables, blueberries boast the highest total phenolic content (TPC, 9.44 mg gallic acid/g DW), total flavonoid content (TFC, 36.08 mg rutin/g DW), and total anthocyanidin content (TAC, 24.38 mg catechin/g DW).

Blueberries are extensively documented for their abundance in phenolics, encompassing ellagic, gallic, ferulic, catechin, chlorogenic, and quercetin. Notably, catechin, a crucial phytochemical within flavonoids, is particularly prevalent in these fruits.

The genesis and accrual of bioactive compounds in fruits are subject to influences like genotype, environmental conditions, ripeness, and storage. Additionally, phenolic compounds prove highly responsive to environmental stress, leading to rapid alterations in their content under such influences.

Studies indicate that blueberry leaves harbor a significantly higher concentration of polyphenols compared to the fruits. However, fresh green leaves do not contain anthocyanins.
Blueberries: Rich Antioxidant Source

Properties of ellagitannins

Ellagitannins occur naturally in certain fruits, herbs and seeds. They are abundant in some berries (especially raspberries, blackberries, currants, and strawberries), as well as walnuts, pistachios, cashews, chestnuts, acorns, and pecans.

Ellagitannin is a hydrolysable polymer contrary to the rest of the family of tannins and can be hydrolyzed to more simple monomers that can be eventually metabolized and that can become bioavailable with subsequent exposition of the body to these metabolites.

Ellagitannins along with condensed tannins, comprise one of the two large groups of tannins, and have the hexahydroxydiphenoyl (HHDP) group, or a similar group derivable from the HHDP group, in their molecules. The HHDP group is biosynthetically formed through intramolecular, oxidative C-C bond formation between neighboring galloyl groups in galloylglucoses.

Condensed tannins are formed through the condensation of flavan-3-ols (catechins) and are often referred to as proanthocyanidins.

The ellagitannins include:
(1) monomeric ellagitannins,
(2) C-glycosidic ellagitannins with an open-chain glucose core,
(3) condensates of C-glycosidic tannins with flavan-3-ol (complex tannin), and
(4) oligomers which are produced through intermolecular C-O or C-C bonds between monomers

Ellagitannins form a diverse group of bioactive polyphenols with anti-inflammatory, anticancer, antioxidant and antimicrobial (antibacterial, antifungal and antiviral) activity.

The consumption of ellagitannins has often been associated with positive effects on many pathologies, including cardiovascular diseases, neurodegenerative syndromes, and cancer. Although multiple biological activities (antioxidant, anti-inflammatory, chemo preventive) have been known for ellagitannins, their limited bioavailability prevents reaching significant concentrations in systemic circulation.

There may be monomeric (nupharin A, geraniin, tellimagrandin II), oligomeric (nupharin E, nupharin C, hirtellin A), or C-glycosidic (vescalagin, castalagin). Ellagitannins tend to form high molecular weight dimers and oligomers.
Properties of ellagitannins

A-type pro-anthocyanidins

Pro-anthocyanidins, also called condensed tannins, are oligomers and polymers of monomeric flavonoids. Pro-anthocyanidins are present in flowers, nuts, fruits, bark, and seeds of various plants, as a defense against biotic and abiotic stressors.

Pro-anthocyanidins can be differentiated into B-type or A-type depending on their interflavanic linkages.

A-type pro-anthocyanidins containing double interflavanyl linkages (for example, procyanidin A2: epicatechin (2b→7,4b→8)-epicatechin) compared to B-type pro-anthocyanidins that have a single interflavanyl bond, typically between C4→C8 (for example procyanidin B2: epicatechin–(4b→8)-epicatechin.

The most common A-type compounds are A1 and A2. A-type pro-anthocyanidins were found in only three fruits (cranberry, avocado and plum), one nut (peanut), and two spices (cinnamon and curry).

Study shows that cinnamon, which contains a series of unique trimeric and tetrameric procyanidins with A-type linkages, significantly decreased plasma levels of triglycerides and total and LDL cholesterol, when administered (1–6 g=day) just for 20 days.

Pro-anthocyanidins found in cranberry juice with A-type linkages prevented adhesion of uropathogenic P-fimbriated E. coli suggesting they may help to maintain a healthy urinary tract.
A-type pro-anthocyanidins

Flavonoids in dark chocolate

The beneficial plant compounds in chocolate are called flavonoids, which contribute to the dark pigment in chocolate. Flavonoids are polyphenolic compounds found abundantly in cocoa.

Dark chocolate contains a high percentage (equal to 70 percent) of cocoa solids, and little or no added sugar. As a result, dark chocolate contains more health benefits. At the contrary, milk chocolate contains less flavonoids as evidenced by its lighter color.

Cocoa flavonoids are classified as flavanols and include the monomers, (-)-epicatechin, (+)-catechin, and procyanidins, the oligomers of these monomeric units. Flavonoids occur naturally in the cocoa plant as a way of protecting the plant. Their presence affects the taste of chocolate and they are responsible for the astringency in the unprocessed cocoa bean.

The flavonoid found in dark chocolate has been linked to decreases in blood pressure, improvements in endothelial dysfunction that promote vascular homeostasis, improvements in antiplatelet activity, improvements in insulin sensitivity, increases in high-density lipoprotein (HDL) cholesterol concentrations, improvements in cognitive, and antioxidant properties.

Furthermore, an increase in plasma antioxidant capacity and a decrease in plasma oxidation products are associated with elevated epicatechin concentrations. Research recently reported that the cocoa flavonoids decrease plasma leukotriene-prostacyclin ratios in human plasma and aortic endothelial cells (Journal of the American College of Nutrition, Vol. 23, No. 3, 197–204, 2004).
Flavonoids in dark chocolate