The term 'MOST SOUR-TASTING' can be described as the BITTEREST, which is a nine letter word. Bitter is a taste sensation that is often associated with sourness, and is usually described as having a sharp, unpleasant taste. It is often found in foods such as coffee, dark chocolate, and some fruits and vegetables. Bitter flavors are often used to balance out sweet flavors in food, and can also be used to add complexity to dishes.