Tannins

Solvents: smaller tannins soluble in hot water, EtOH; larger tannins relatively insoluble. Excellent solubility in glycerin. Fair solubility in oil.

23 Foods With Tannins And Their Health Effects

Tannins are a class of complex organic compounds that are widely distributed in the plant kingdom. They are a type of phytochemical, which are naturally occurring chemical compounds in plants that often contribute to the plant's color, flavor, and resistance to pests. Tannins are particularly notable for their astringent properties, which can cause a dry and puckering sensation in the mouth.

What Are Tannins?

Tannins are polyphenolic compounds that have the ability to bind and precipitate proteins. This characteristic is what gives them their astringent quality. They are found in various parts of plants, including leaves, bark, wood, and fruit. Tannins play a crucial role in the plant's defense mechanism against herbivores and pathogens due to their bitter taste and ability to inhibit microbial growth.

Types of Tannins

Tannins are generally classified into two main types:

1. Hydrolyzable Tannins: These tannins can be broken down into simpler molecules by hydrolysis. They are composed of a central core of polyol, such as glucose, esterified with gallic acid or ellagic acid. Examples include gallotannins and ellagitannins.

2. Condensed Tannins: Also known as proanthocyanidins, these tannins are polymers of flavonoid units. They are more resistant to hydrolysis compared to hydrolyzable tannins. Condensed tannins are commonly found in fruits like grapes and berries.

Key Practical Implications

• In beverages: Red wine and black tea owe much of their astringency to condensed tannins, while some herbal infusions (e.g., from oak galls) feature hydrolyzable ones.
• Nutrient interactions: Both inhibit non-heme iron absorption, but condensed tannins form tighter, more stable complexes.
• Toxicity: Hydrolyzable tannins can be hepatotoxic in excess (due to free gallic acid), whereas condensed tannins are generally safer at dietary levels.

In summary, condensed tannins dominate in common foods/beverages and provide durable, polymeric astringency, while hydrolyzable tannins are more characteristic of specific medicinal/tanning plants and are chemically more reactive.

Sources of Tannins

Tannins are abundant in a variety of plant-based foods and beverages. Some common sources include:

• Tea: Both black and green teas are rich in tannins, contributing to their characteristic astringency.
• Wine: Red wine, in particular, contains high levels of tannins derived from grape skins, seeds, and stems.
• Fruits: Grapes, cranberries, and pomegranates are notable for their tannin content.
• Nuts: Walnuts and pecans contain significant amounts of tannins.
• Spices and Herbs: Cloves, cinnamon, and bay leaves are also sources of tannins.

Plant galls (particularly oak galls or nutgalls, such as those from Quercus infectoria or Chinese gallnuts) contain the highest levels of tannins among natural plant sources.These abnormal growths on plants (often oaks) form as a defense against insects and are exceptionally rich in hydrolyzable tannins (primarily gallotannins). Historical and industrial sources report tannin contents up to 60–77% by dry weight in high-quality gallnuts, far exceeding other plants or plant parts.For comparison:

• Quebracho wood (Schinopsis spp.): ~20–30% tannins (condensed type, a major commercial source).
• Chestnut wood (Castanea sativa): 7–20% tannins (hydrolyzable type).
• Oak bark (Quercus spp.): Typically 10–20% tannins.
• Tara pods (Caesalpinia spinosa): High but generally lower than galls.
• Other sources like sumac leaves, myrobalan fruit, or mimosa bark are in the 10–30% range.

Gallnuts have been prized historically for their superior tannin concentration, used extensively in tanning, inks, and medicine due to this richness. While tannins occur widely in plants (bark, wood, leaves, fruits), galls represent the most concentrated natural reservoir.

Black tea (brewed infusion): Strongly brewed black tea often has the highest perceived and effective tannin levels in a beverage serving, contributing to its robust bitterness and dryness. Dry black tea leaves contain ~24–37 mg/g tannic acid equivalents, and a typical cup (2–3g leaves in 200–250mL water) extracts significant amounts, leading to high astringency—often used as a reference for "tannic" sensation (similar to oversteeped tea matching high-tannin red wines).

Red wine: Tannin levels average ~500–600 mg/L (ranging 100–1900 mg/L depending on grape variety, e.g., higher in Cabernet Sauvignon, lower in Pinot Noir). A 150mL glass provides ~75–300 mg, giving structure and dryness but generally less intense per sip than strong black tea.

Walnuts: Contain ~400–1100 mg/100g dry weight (mostly in the pellicle/skin), but as eaten (kernels, ~30g serving), this is lower per portion and less bioavailable/extractable in the mouth compared to beverages. The astringency is noticeable (e.g., from skins) but milder than in tea or wine.

Applications of Tannins

Tannins have a wide range of applications due to their unique properties:

• Leather Tanning: Historically, tannins have been used in the leather tanning process to convert animal hides into leather. The astringent properties of tannins help to stabilize the collagen fibers in the hides.

• Food and Beverage Industry: Tannins are used to add flavor and astringency to various food products. They also act as natural preservatives due to their antimicrobial properties.

• Pharmaceuticals: Tannins are used in some medicinal formulations for their antioxidant and anti-inflammatory properties.

Health Benefits of Tannins

Tannins have been studied for their potential health benefits, which include:

• Antioxidant Activity: Tannins can neutralize free radicals, reducing oxidative stress and potentially lowering the risk of chronic diseases.

• Antimicrobial Properties: Tannins have been shown to inhibit the growth of various bacteria, fungi, and viruses, contributing to their potential use in treating infections.

• Cardiovascular Health: Some studies suggest that tannins may help improve heart health by reducing blood pressure and cholesterol levels.

• Cancer Prevention: The antioxidant properties of tannins may also play a role in reducing the risk of certain types of cancer.

Considerations and Potential Side Effects

While tannins offer several health benefits, excessive consumption can lead to some adverse effects:

• Iron Absorption: Tannins can inhibit the absorption of non-heme iron from plant-based foods, potentially leading to iron deficiency in susceptible individuals.

• Digestive Issues: High intake of tannins may cause stomach irritation and affect digestion.

• Allergic Reactions: Some individuals may experience allergic reactions to tannin-rich foods and beverages.

Key Considerations

• Moderation is key: Benefits outweigh risks in typical dietary amounts (e.g., a few cups of tea or glasses of red wine daily). To minimize iron interference, avoid consuming tannin-rich drinks with iron-rich meals—wait 1–2 hours.
• Individual variation: Effects depend on diet composition, gut microbiota, and health status. Those with iron deficiency should monitor intake.
• Sources matter: Condensed tannins (e.g., in tea, wine) are often more beneficial than hydrolysable ones in some contexts.

Overall, tannins from whole foods and beverages are generally health-promoting due to their antioxidant and anti-inflammatory roles, with risks primarily relevant in excess or specific vulnerabilities

Plant remedies traditionally used for tannin constituents

• Hamamelis (witchhazel), Potentilla tormentilla (tormentil), Quercus (oak), Agrimonia (agrimony), Geum (avens), krameria (rhatany), Geranium (cranesbill), Cnicus benedictus (blessed thistle), Acacia catechu (catechu), Bidens (bur-marigold), Alchemilla (ladies mantle), polygonum (bistort), Sanguisorba (burnet)

Indications for tannins

• Inflammation of the upper digestive tract
• Diarrhea following gastrointestinal inflammation
• Topically: open, discharging lesions, wounds, hemorrhoids and third-degree burns

Contraindications for tannins

• Constipation
• Iron deficiency anemia
• Malnutrition

Traditional Therapeutic Insights into the use of tannins

Tannins were used throughout history for forming leather from animal tissues and as effective cauterizing agents for burns and other open wounds. The primary traditional indication for the digestive tract was probably diarrhea.

Application

Tannins should be taken after food in most cases. For some lesions of the upper digestive tract, short-term use between meals or before food is justifiable.

Long-term therapy with high doses of tannins is to be avoided

Source: Bone & Mill

Articles/Studies

Tannins and human health: a review 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.

Tannins in Wine

Pharmacological effects and mechanisms of tannic acid.

Cancer

Antitumor activity of certain plants due to tannins -

COVID-19

Natural tannins as anti-SARS-CoV-2 compounds.

HIV

Tannin inhibits HIV-1 entry into target cells.