Improved firmness in calcified diced tomatoes by temperature activation of pectin methylesterase

The effects of temperature and calcium on pectin methylesterase (PME) activity and texture in tomato pericarp material were examined. Heating thin slices of pericarp to temperatures between 50 degrees C and 75 degrees C led to the rapid evolution of methanol from the material, indicating an activation of PME. This activity was further stimulated when CaCl2 (up to 2.0% w/v) was added. When applied to half-inch diced tomato pericarp, the same conditions that led to the activation of PME also improved firmness. Diced tomatoes treated for 5 min with 0.5% CaCl2 at 70 degrees C were 2.5 times firmer than diced tomatoes treated with CaCl2 at room temperature. This improvement in texture by treating with CaCl2 at elevated temperatures was only apparent when the tomatoes received a subsequent 100 degrees C treatment. Heating tomatoes to 70 degrees C either before or after the CaCl2 treatment also improved firmness through a subsequent high-temperature treatment, but to a lesser extent than heating during the CaCl2 treatment. These results are consistent with the model that heating to 70 degrees C greatly increases PME activity, leading to extensive pectin de-esterification and increased calcium cross-linking of the pectins in the middle lamella. Production of thermally processed diced tomatoes with improved firmness should be possible by increasing the temperature during and after calcium treatment.