Effect of Calcium Ions on the Thermodynamics of Cucumber Tissue Softening

Calcium ion was found to cause inhibition of softening in blanched cucumber tissue under acidic conditions due to a dominant effect of the calcium upon the entropy of activation for the softening reaction. This resulted in slower rates of softening despite a decline in the enthalpy of activation. Enthalpy/entropy compensation was found to occur. The isokinetic temperature for softening was 260 ± 9 K. An assumption was made that the hyperbolic inhibition of softening by calcium was due to saturation of a binding site by calcium. The dissociation constant for calcium from the proposed site was determined to be 1.5 mM at 30°C. This was considerably stronger binding than should occur if calcium was binding to pectin carboxyl groups under similar conditions. The apparent enthalpy and entropy for calcium binding were determined.