Tannins (tannic acids) are naturally occurring complex chemicals found in plants. These proanthocyanidin polyphenols are particularly prevalent in a variety of vascular plants, including fruits (especially grapes), teas, legumes, and grasses. Their taste is quite sharp or caustic, providing the distinctive astringency that humans associate with red wines, teas, and unripe fruits. In addition, tannins are used in the dyeing of textiles and in the tanning of leather products. See also: Dyeing; Fruit; Leather and fur processing; Phenol; Taste; Tea; Wine

In their condensed, secondary-metabolite form, tannins are highly toxic and insoluble compounds, and they bind to proteins and enzymes. As such, in nature, tannins are essential products for plant defense, and they provide plants with a chemical mechanism with which to combat pathogens and herbivores. Because tannins are so bitter, most herbivorous predators are disinclined to consume them. Moreover, inside any herbivorous predator persistent enough to eat tannin-rich plants, the tannins will biochemically target digestive enzymes and inhibit digestion, sometimes to the extent that the animal is killed. Tannins also protect plants from ultraviolet radiation. See also: Chemical ecology; Herbivory; Plant defense against herbivorous insects; Plant defense against pathogens; Predator-prey interactions; Ultraviolet radiation

In 2013, investigators pinpointed the exact location in plant cells where tannins are manufactured. The source for these important compounds is a newly discovered organelle called the tannosome. This small organelle arises in chloroplasts (cell plastids occurring in the green parts of plants, which contain chlorophyll pigments and function in photosynthesis and protein synthesis) and is responsible for tannin formation. After the tannins are created, the tannosome is encapsulated in a membrane, allowing the structure to bud off from the chloroplast and to be shuttled to a plant vacuole for safe storage. (If not safely stored, the tannins would harm the plant itself.) See also: Cell plastids; Plant cell; Vacuole

The discovery of the tannosome has important implications for food engineering. By determining the location and method by which tannins are produced, it should be possible in the future to bioengineer food products with modified tannins, thereby altering, for example, the tannin content and taste of teas and wines. See also: Biotechnology; Food engineering; Genetically engineered plants