Yacon (Smallanthus sonchifolius) root products are susceptible to oxidation, reduced quality and functional properties. The optimum water blanching process for yacon root slices was determined through a central composite design with variable temperature (80-100 degrees Celsius), blanching time (2-6 min), and citric acid concentration (0.04-0.20%). Phenolics and fructooligosaccharides of yacon slices were also evaluated after blanching. Yacon slices (3-4 mm) blanched at 90 degrees Celsius, 3.9 to 4.1min, and 0.05% to 0.07% citric acid showed the lowest polyphenol oxidase activity, highest whiteness value, and the highest sensory appearance scores. A second experiment showed that blanching at 100 degrees Celsius with 0.07% citric acid can maintain higher chlorogenic acid (3.52 mg/g more), inulin (5.41% more), and total sugar (34.9%) in yacon slices than blanching without citric acid. Thus, addition of less than 0.1% citric acid to boiling water can minimize loss of functional components of yacon slices during blanching.