Quercetin | A phenol which occurs naturally in vegetables and fruits, including wine grapes and grape stems. It protects grapes from ultraviolet light damage. Pinot Noir, Sangiovese and Cesanese are examples of grape varieties whose quercetin levels can be high. Levels of quercetin in wine are linked to how much exposure to sunlight the grape receive. Quercetin reacts with anthocyanins to give wine deeper and more vibrant colours.