A smelly kitchen sink is usually caused by food debris trapped in the drain garbage disposal or P-trap. Run the disposal with cold water and clean it by grinding ice cubes and citrus peels. Pour baking soda and vinegar down the drain weekly. Clean the rubber splash guard on the disposal which collects food residue. If the smell persists check the P-trap for clogs. A disconnected or damaged vent pipe can also allow sewer gases into the kitchen.