The word “tannins” in foods induce a scare to all health conscious men. We know tannins are needed to produce leather and ink. Consumption of a cup of “Kadak meethi chai ” (strong sweet tea) is known for reducing hunger and hindering iron absorption in stomach and that is the reason many people abhor tea. But it is to be remembered that tannins, similar to all good things like sugar, are harmful in high concentrations only and form friendly components of our foods with many health benefits.
Tannins are polyphenols having an astringent taste and ability to convert putrescible animal hydes to hard stable leather. Two groups of tannins, hydrolysable and condensed, are recognised in plants. The former group is soluble in water, contains simple phenolic acids esterified with one or more sugar molecules and are hydrolysed by dilute acids to component compounds. Condensed tannins are polyphenols insoluble in water, which on treatment with acids yield complex products of unknown composition called “tannin reds” or “phlobaphenes”.
Proanthocyanidins
Proanthocyanidins (known earlier as ‘leucoanthocyanins”) contain ‘flavans’ like catechins (flavan-3-ols) and flavan–3, 4-diols. A proanthocyanidin molecule may be a dimer of flavan 3, 4-diol or contain a catechin and a flavan-3, 4-diol. The linkage between the two monomers, most often, is a C-C linkage. Proanthocyanidins may contain 3 and 11 of these monomers. These compounds, as the name signifies, yield anthocyanidins (25%) on acidic or alkaline hydrolysis along with complex polymeric structures known as ‘phlobaphenes’ or ‘tannin reds’. Based on the type of anthocyanidin produced, they can be procyanidins, prodelphinidins, propelargonidins etc.
A simple test for proanthocyanidins is the development of red color when the plant material is boiled in acidic solution(2N HCl) for 30 min. On shaking this extract with amyl alcohol, the red colour will be extracted in amyl alc. This red extract can be concentrated and analysed for Anthocyanidins by PC or TLC.
Proanthocyanidins are responsible for the puckering (dry) sensation when a plant part is tasted. They are seen in many 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. But some like selligueain A, from the rhizome of the fern Selliguea feei is intensely-sweet. Almost all plants, where bark, wood or roots and even leaves are used as medicines, are bound to contain these compounds.
The most common sources of proanthocyanidins are ;
1. Berries like Lingonberry, cranberry, black elderberry, black chokeberry, black currant and blueberry
2. fruits like Persimmon, banana, carob bean, and Chinese quince, chokeberries, rose hips, and cocoa products, medlar, mulberry, plum, apricot, walnut, silverberry and pomegranate
3. Grape seed
Proanthocyanidins can impart astringency, bitterness, sourness, sweetness, salivary viscosity, aroma, and color formation. It’s savor taste can be used in fruit juices, teas, beverages and -also for enhancing the shelf life of several food products like wine . They are used as additives in food preparations for enhancing microbial stability, foamability, oxidative stability, and heat stability.
Proanthocyanidins are found to possess a large number of medicinal properties such as:
1. Antioxidant : Grape seed proanthocyanidins are consumed as dietary supplements
2. Protects heart. By Causing vessel relaxation and the inhibition LDL oxidation and inhibiting
the binding of oxidized LDL to the lectin-like oxidized LDL receptor. They also modulate NO/cyclic
3. Lipid lowering properties ;
Proanthocyanidins causes lowering of triacylglycerol by repressing lipoprotein secretion. They also reduce oxidized LDL level, lowering the risk of arteriosclerosis .
4. Protects the nerves
Proanthocynidins caused neuroprotective effect in Alzheimer’s disease (AD) via suppressing the amyloid-beta aggregation, decreasing amyloid-beta production, amy loid plaques and microgliosis, and preventing from amyloid-beta neurotoxicity in brain of mice. Food rich in proanthocyanidins (30, 60, and 90 mg/kg) ameliorated the learning and memory abilities impaired by D-galactose as well as significantly lowered the concentrations of MDA, NO, content of β-amyloid peptide, and total NO synthase alongside enhancing concentrations of glutathione peroxidase and SOD in brain.
5. Immunomodulatory
These compounds stimulate lymphocyte trans formation, enhance lysosomal enzyme activity and phagocytic cap ability of peritoneal macrophages. They promoted the production of TNF-α andreduced inflammatory cells, Th2 cytokines and serum IgE levels
6. Antidiabetic
Grape seed proanthocyanidins increased normal insulin content , inhibited lipid peroxidation, restored endothelial function, and reduced the risk of vascular disease in diabetes.
7. Anticancer
Being anticancer agents, oligomeric proanthocyanidins (OPCs) alter genes involved in cell cycle and DNA replication. They significantly decreased xenograft tumor growth, and inhibited the formation of organoids in colorectal cancer . They are also found to be effective against skin-tumour, prostate cancer, breast cancer etc.
10. Antimicrobial
Being polyphenolic, these compounds are excellent antimicrobials.
Reference:
Abdur R. et al. (2019) Proanthocyanidins: A comprehensive review, Biomedicine and Pharmacotherapy, 116, August 2019, 108999
Mammen Daniel
