The pectin methyl esterase enzyme system was shown to be involved in firmness of thermally treated onion in the temperature range 50–70C. Thermal softening of onion at 90 and 100C showed an initial steep negative slope with a shallow negative slope at longer heating time. Low-temperature blanching at 70C was effective to maintain firm onion tissue exposed to excessive heating. Physical strength of onion was substantially diminished when exposed to a commercial sterilization condition where F0, extent of thermal sterilization, was 3, and/or more. Blanching in water for 120 min at 70C resulted in a maximum value for the firmness of commercially sterilized onion. Firmness of onion, blanched in calcium brine at a concentration range of 0.0–1.0% (wt) prior to heat treatment, decreased with increasing severity of thermal sterilization treatment. At F0 = 6, blanching in 0.5% calcium brine resulted in maximum firmness of thermally sterilized onion, approximately 70% of that of raw onion.