Tannins are a diverse group of polyphenolic compounds found in various plants, known for their astringent properties. They are widely used in both traditional and modern applications due to their unique chemical characteristics and beneficial effects.

Chemical Composition and Structure:

• Polyphenolic Compounds: Tannins are primarily composed of phenolic groups, which contribute to their astringency and antioxidant properties.
• Catechins and Gallic Acid: Common types of tannins include hydrolyzable tannins (like ellagic acid) and condensed tannins (like catechins).
• Complex Polymers: The structure of tannins often involves complex polymeric forms, which can vary depending on the plant source.

Physical Properties:

• Appearance: Typically occur as powders or solutions, varying in color from light yellow to dark brown.
• Odor: Usually odorless or have a faint, characteristic smell.
• Texture: Powders are fine and can have a gritty texture; solutions are typically clear or slightly opaque.

Production Process:

The extraction and processing of tannins typically involve:

1. Extraction: Obtaining tannins from plant materials using solvents like water, ethanol, or acetone.
2. Purification: Removing impurities and concentrating the tannins.
3. Processing: Depending on the application, tannins may be processed into powders or solutions.
4. Formulation: Incorporating tannins into various products, including cosmetics, medicines, and industrial materials.

Applications:

• Cosmetics: Used in astringent formulations, such as toners and masks, for their skin-tightening, antioxidant, astringent and anti-inflammatory properties.
• Medical: Applied in treatments for wounds and diarrhea due to their astringent and antimicrobial effects.
• Food and Beverages: Found in teas, wines, and some fruits, where they contribute to flavor and preservation.
• Industrial: Utilized in leather tanning, dyeing, and as preservatives.

Environmental and Safety Considerations:

Tannins are generally regarded as safe for topical and dietary use. However, like all compounds, they should be used according to established guidelines to avoid potential adverse effects. Excessive consumption or application might lead to skin irritation or gastrointestinal issues. Ensuring the source of tannins is sustainable and free from harmful contaminants is important.

References__________________________________________________________________________

Ogawa S, Yazaki Y. Tannins from Acacia mearnsii De Wild. Bark: Tannin Determination and Biological Activities. Molecules. 2018 Apr 5;23(4):837. doi: 10.3390/molecules23040837. PMID: 29621196; PMCID: PMC6017853.

Abstract. The bark of Acacia mearnsii De Wild. (black wattle) contains significant amounts of water-soluble components acalled "wattle tannin". Following the discovery of its strong antioxidant activity, a wattle tannin dietary supplement has been developed and as part of developing new dietary supplements, a literature search was conducted using the SciFinder data base for "Acacia species and their biological activities". An analysis of the references found indicated that the name of Acacia nilotica had been changed to Vachellia nilotica, even though the name of the genus Acacia originated from its original name. This review briefly describes why and how the name of A. nilotica changed. Tannin has been analyzed using the Stiasny method when the tannin is used to make adhesives and the hide-powder method is used when the tannin is to be used for leather tanning. A simple UV method is also able to be used to estimate the values for both adhesives and leather tanning applications. The tannin content in bark can also be estimated using NIR and NMR. Tannin content estimations using pyrolysis/GC, electrospray mass spectrometry and quantitative 31P-NMR analyses have also been described. Tannins consists mostly of polyflavanoids and all the compounds isolated have been updated. Antioxidant activities of the tannin relating to anti-tumor properties, the viability of human neuroblastoma SH-SY5Y cells and also anti-hypertensive effects have been studied. The antioxidant activity of proanthocyanidins was found to be higher than that of flavan-3-ol monomers. A total of fourteen papers and two patents reported the antimicrobial activities of wattle tannin. Bacteria were more susceptible to the tannins than the fungal strains tested. Several bacteria were inhibited by the extract from A. mearnsii bark. The growth inhibition mechanisms of E. coli were investigated. An interaction between extracts from A. mearnsii bark and antibiotics has also been studied. The extracts from A. mearnsii bark inhibit the growth of cyanobacteria. Wattle tannin has the ability to inactivate α-amylase, lipase and glucosidase. In vivo experiments on anti-obesity and anti-diabetes were also reported. Several patents relating to these enzymes for anti-diabetes and anti-obesity are in the literature. In addition, studies on Acacia bark extract regarding its antitermite activities, inhibition of itching in atopic dermatitis and anti-inflammatory effects have also been reported. The growth of bacteria was inhibited by the extract from A. mearnsii bark, and typical intestinal bacteria such as E. coli, K. pneumoniae, P. vulgaris and S. marcescenes was also inhibited in vitro by extracts. Based on these results, the Acacia bark extract may inhibit not only the growth of these typical intestinal bacteria but also the growth of other types of intestinal bacteria such as Clostridium and Bacteroides, a so-called "bad bacteria". If the tannin extract from A. mearnsii bark inhibits growth of these "bad bacteria" in vivo evaluation, the extracts might be usable as a new dietary supplement, which could control the human intestinal microbiome to keep the body healthy.

Chung KT, Wong TY, Wei CI, Huang YW, Lin Y. Tannins and human health: a review. Crit Rev Food Sci Nutr. 1998 Aug;38(6):421-64. doi: 10.1080/10408699891274273. 

Abstract. Tannins (commonly referred to as tannic acid) are water-soluble polyphenols that are present in many plant foods. They have been reported to be responsible for decreases in feed intake, growth rate, feed efficiency, net metabolizable energy, and protein digestibility in experimental animals. Therefore, foods rich in tannins are considered to be of low nutritional value. However, recent findings indicate that the major effect of tannins was not due to their inhibition on food consumption or digestion but rather the decreased efficiency in converting the absorbed nutrients to new body substances. Incidences of certain cancers, such as esophageal cancer, have been reported to be related to consumption of tannins-rich foods such as betel nuts and herbal teas, suggesting that tannins might be carcinogenic. However, other reports indicated that the carcinogenic activity of tannins might be related to components associated with tannins rather than tannins themselves. Interestingly, many reports indicated negative association between tea consumption and incidences of cancers. Tea polyphenols and many tannin components were suggested to be anticarcinogenic. Many tannin molecules have also been shown to reduce the mutagenic activity of a number of mutagens. Many carcinogens and/or mutagens produce oxygen-free radicals for interaction with cellular macromolecules. The anticarcinogenic and antimutagenic potentials of tannins may be related to their antioxidative property, which is important in protecting cellular oxidative damage, including lipid peroxidation. The generation of superoxide radicals was reported to be inhibited by tannins and related compounds. The antimicrobial activities of tannins are well documented. The growth of many fungi, yeasts, bacteria, and viruses was inhibited by tannins. We have also found that tannic acid and propyl gallate, but not gallic acid, were inhibitory to foodborne bacteria, aquatic bacteria, and off-flavor-producing microorganisms. Their antimicrobial properties seemed to be associated with the hydrolysis of ester linkage between gallic acid and polyols hydrolyzed after ripening of many edible fruits. Tannins in these fruits thus serve as a natural defense mechanism against microbial infections. The antimicrobial property of tannic acid can also be used in food processing to increase the shelf-life of certain foods, such as catfish fillets. Tannins have also been reported to exert other physiological effects, such as to accelerate blood clotting, reduce blood pressure, decrease the serum lipid level, produce liver necrosis, and modulate immunoresponses. The dosage and kind of tannins are critical to these effects. The aim of this review is to summarize and analyze the vast and sometimes conflicting literature on tannins and to provide as accurately as possible the needed information for assessment of the overall effects of tannins on human health.

Rousserie P, Rabot A, Geny-Denis L. From Flavanols Biosynthesis to Wine Tannins: What Place for Grape Seeds? J Agric Food Chem. 2019 Feb 6;67(5):1325-1343. doi: 10.1021/acs.jafc.8b05768. 

Abstract. Phenolic compounds are among the most important quality factors of wines. They contribute to the organoleptic characteristics of wine such as color, astringency, and bitterness. Although tannins found in wine can come from microbial and oak sources, the main sources of polyphenols are grape skins and seeds. Since the 1960s, this subject has been widely studied by a large number of researchers covering different types of wine, climate conditions, growing practices, and grape varieties. As these works have been conducted under different conditions, the data collected can be conflicting. Moreover, even though the biosynthesis of the major proanthocyanidins units (+)-catechin and (-)-epicatechin is well-known, the mechanism of their polymerization remains unexplained. This is why the question remains: what factors influence the biosynthesis, the quantity, and the distribution of tannins in grape seeds and how can winemaking processes impact the extractability of seed tannins in wine?

Serrano J, Puupponen-Pimiä R, Dauer A, Aura AM, Saura-Calixto F. Tannins: current knowledge of food sources, intake, bioavailability and biological effects. Mol Nutr Food Res. 2009 Sep;53 Suppl 2:S310-29. doi: 10.1002/mnfr.200900039. 

Abstract. Tannins are a unique group of phenolic metabolites with molecular weights between 500 and 30 000 Da, which are widely distributed in almost all plant foods and beverages. Proanthocyanidins and hydrolysable tannins are the two major groups of these bioactive compounds, but complex tannins containing structural elements of both groups and specific tannins in marine brown algae have also been described. Most literature data on food tannins refer only to oligomeric compounds that are extracted with aqueous-organic solvents, but a significant number of non-extractable tannins are usually not mentioned in the literature. The biological effects of tannins usually depend on their grade of polymerisation and solubility. Highly polymerised tannins exhibit low bioaccessibility in the small intestine and low fermentability by colonic microflora. This review summarises a new approach to analysis of extractable and non-extractable tannins, major food sources, and effects of storage and processing on tannin content and bioavailability. Biological properties such as antioxidant, antimicrobial and antiviral effects are also described. In addition, the role of tannins in diabetes mellitus has been discussed.

Okuda T, Yoshida T, Hatano T. Pharmacologically active tannins isolated from medicinal plants. Basic Life Sci. 1992;59:539-69. doi: 10.1007/978-1-4615-3476-1_31.

Abstract. Starting with the isolation of a crystalline tannin (geraniin) of mild property from a popular herb medicine (Geranii herba), various polyphenolic compounds including those belonging to new classes of tannins (oligomeric hydrolyzable tannins, complex tannins, and other metabolites and condensates) have been isolated from various medicinal plants. Noticeable biological and pharmacological activities (inhibition of carcinogenesis, host-mediated antitumor activity, antiviral activity, and inhibition of active oxygen, such as inhibition of lipid peroxidation and lipoxygenase, xanthine oxidase, and monoamine oxidase) have been found for several of these polyphenolic compounds.