Miracle fruit
The berry has a low sugar content and a mildly sweet taste. When the fruit is eaten, miraculin binds to the tongue's taste buds, causing sour foods to taste sweet. At neutral pH, miraculin binds and blocks the receptors, but at low pH (after eating sour food) miraculin binds protons and activates the sweet receptors. This is what causes the sweet taste. This effect lasts until the protein is washed away by saliva (up to about 60 minutes).