Wine lovers may differ on how tannic they like their wines, but there’s no getting around the fact that all wines contain tannins. The chemical compounds, which give wine structure, are naturally occurring and come from grape skins, seeds and stems. They also come from the oak in which the wine is aged or from powdered tannins some winemakers choose to add. But scientists have never had a clear picture of how plants produce tannins. Now an international team of scientists has discovered tannin-producing organelles in plant cells they have dubbed “tannosomes."
The research team, working at the French National Institute for Agricultural Research (INRA) at Montpellier, reported the discovery in an article in the Annals of Botany. Organelles are specialized parts of cells with their own functions. You may remember the names of some of the more well-known from biology class: Chloroplasts are involved in photosynthesis and mitochondria in energy production. Hoping to find the sources of tannins, the scientists were examining organelles in cells from multiple species of plants when they noticed the tannosomes. At first, they believed they were looking at chloroplasts, but then realized these strange organelles were producing tannins.
Scientists believe that bitter, astringent tannins serve as a first line of defense against predators and disease in the plant kingdom. Nearly all vascular plants contain tannins to some degree, often concentrated in the skins and seeds. Their bitter taste serves to deter creatures from munching on the plant’s vulnerable parts. Imagine the mouth feel and taste of an underripe persimmon or the bitter red skin around a peanut.
Tannins are generally water soluble, which means tannins from grape skins, seeds and stems end up in the wine, along with the tannins found in oak barrels many wines are aged in. Chemical reactions of tannins over time affect how a wine ages in the bottle.
The discovery of tannosomes will likely open the door to new ideas in the vineyard and winery. “The manner in which tannin pre-cursors are assembled leads to different qualities in grape tannins and finished wines," said Matt Brain, lecturer and cellar master at California Polytechnic State University’s newly formed department of viticulture and enology. "Winemakers and growers might better understand how to more precisely impact tannin quality.”