If you have ever eaten a piece of fruit that makes your mouth pucker, you've experienced tannins at work. Tannins, a type of plant polyphenol, are natural chemicals found in fruits that produce an astringent or bitter taste when you eat them. These chemicals also affect the nutritional value of foods by binding to and precipitating proteins.
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Medical research indicates that the tannins in cranberries have several medicinal properties. For example, cranberries help prevent urinary tract infections in women by keeping E. coli from adhering to the urinary tract walls, as suggested by the US Forest Service. There is also evidence that tannins in cranberries reduce bad cholesterol levels and improve cardiac. In addition to cranberries, other berries such as strawberries and blackberries also contain tannins.
The tannins in grapes concentrate in the skin and seeds and are an important part of the winemaking process. Unripe grapes that are small and green have high levels of tannins, but tannin levels decrease as the fruit ripens. Using unripe grapes to produce wine contributes to a very dry and strongly astringent taste. According to a November 2006 article in "Scientific American," tannins are the key to heart health from wine because they suppress the production of peptides that cause hardening of the arteries.
Bananas, which originated in the Indo-Malaysia region, are the fourth largest fruit crop in the world. The levels of tannins in green bananas range from 122.6 mg to 241.4 mg. As bananas ripen, the tannin content decreases and becomes part of the pulp. Current research indicates that condensed tannins are also in the cell walls, which are a suitable source of natural antioxidants that are biologically accessible in the stomach.
Native to China and common in Japan and the eastern United States, persimmons are round orange or purple fruits with high levels of tannins. In order to avoid the bitter and astringent taste of tannins, eat persimmons when the skin wrinkles, which indicates that the fruit is ripe and the tannins levels have dwindled. As tannins condense, they produce other polyphenols such flavonoids and catechins, which have antioxidant, anti-inflammatory and antiviral properties.
Mangos are an ancient fruit that is native to south Asia. Polyphenols, which are present in the pulp, provide antioxidant properties. While many enjoy the sweet, juicy flesh of the mango and discard the seed, research suggests that tannins in mango kernels, particularly Thai varieties, have additional health benefits. For example, extract from mango seeds can potentially be used as antibacterial agents.