Tannins are water-soluble, natural polyphenols mainly present in plant-based materials, including food. Polyphenol compounds were originally known as “vegetable tannins” due to their ability to interact with proteins of the skin in the process of tanning that transforms animal skins into leather. Biochemically, tannins are sort of secondary metabolites predominantly available in plant-based foods and beverages. Secondary plant metabolites are used in signalling, regulation of primary metabolic pathways and oversee the overall development of the plant.
Tannins can be classified into two broad groups - Hydrolysable tannins (Gallic acid or Ellagic acid) and Condensed tannins (Catechin and Epicatechin)/ Proanthocyanidins (PA) (procyanidins, prodelphinidins, and profisetinidins).
Hydrolysable tannins are usually present in low amounts while Condensed tannins/PA are more widespread in plant based foods. Foods like coffee, black tea, wine, grapes, chocolates, cocoa, soybean, kidney beans, cow pea, cranberries, strawberries, blueberries, kathaa, supari, apples, apricots, barley, jowar, peaches,
mint, basil, rosemary are rich in the condensed category of tannins. Pomegranate, strawberries, raspberries, amla, clove, barley, rice, oat, rye usually have a high hydrolysable tannins content.(Now you know why tea and coffee is a big no-no immediately after consumption of main meals or snacks).
Tannins are secondary compounds, which are formed in plant leaves, fruits and bark and they accumulate mainly in the bran section of the legumes. Tannins usually affect protein digestibility and lead to reduction of essential amino acid availability. When ingested, tannins form complexes with proteins, which cause inactivation of many digestive enzymes- pepsin, trypsin, chymotrypsin, elastase, thereby causing protein deficiency.
Other deleterious effects of tannins include damages to mucosal lining of gastrointestinal tract, and thus, increased excretion of proteins and essential amino acids. Some experiments show that tannins also decrease the activities of intestinal microflora, consequently less absorption of organic matter and soluble fiber that is
attributed to damage the mucosal lining of the digestive system.
Hydrolysable tannins are readily broken down during the digestion process. The breakdown products constitute a large amount of compounds, which can be toxic.
Tannins are complex, astringent and water soluble phenolic compounds known to reduce the bioavailability of nutrients in the gut. The astringency feeling is perceived by the tongue in the form of extreme dryness and roughness in the mouth.
Moreover, high doses of tannins like catechin used in supplements can cause renal failure, hepatitis, fever, hemolytic anemia, thrombocytopenia, and skin disorders.
The main negative effect of tannins as food are their antinutritional impacts, i.e., absorptions and binding with nutrients such as proteins, carbohydrates and minerals in the digestive system, thus, hindering their nutritional availability.
Major antinutrient activity of tannins that is binding with minerals can cause severe deficiency of essential minerals. In this series iron deficiency is the most prevalent.
Moreover, tannins not only affect iron availability, but also iron metabolism. Ferritin, an iron storage protein, is adversely affected by tannin binding in soybean seed ferritin (SSF). However, despite the antinutritional and toxic impacts, recent studies have explored and confirmed numerous health benefits like antioxidant, anti-cancerous, anti-allergic, anti-inflammatory and anti-microbial activities
Diets in developed countries are mostly based on highly‐digestible proteins of animal and vegetable origin, while those in developing countries are predominantly based on poorly‐digestible proteins from less refined cereals and legumes due to the presence of less‐digestible protein fractions, high levels of insoluble fiber, and high amounts of antinutritional factors, including phenolic compounds like tannins.
Therefore, several technological treatments have been developed to reduce tannin content of these foods and thus increase protein digestibility. The various treatments which are also easy household measures for reduction are:
Soaking in water (overnight) or alkaline solutions (like sodium bicarbonate)
- Pressure cooking
These pre-treatments are especially important for vegetarians, for whom nutrition is mainly characterized by the intake of lower‐quality plant proteins accompanied by the intake of high amounts of secondary plant metabolites (tannins) resulting from a predominantly plant‐food‐based diet.
Tannins are a chemically diverse group of compounds that are present, at variable levels, in most plant based foods, so their intake is almost universal. Nevertheless, the foods containing them cannot be completely avoided if you are plant based. If you have health issues though, then cutting down on high tannin foods would be a good idea.
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