The softening of a French fry-type product prepared from‘Jewel’ sweetpotatoes decreased linearly (P ≤ 0.01) when the tissue pH was incrementally lowered from ca. 6 to 3.8 prior to blanching. Tissue pH was decreased by vacuum infiltration of a series of organic and mineral acids of 0.01 to 0.4M. Firmness retention decreased when the ionic strength of the acidulant was increased by addition of NaCl. The structures of the pectic substances were affected by the tissue pH and starch hydrolysis decreased as tissue pH decreased from its normal value of ca. 6, reflecting partial inactivation of endogenous amylolytic enzymes.