Role of calcium and magnesium ions in the hardening of pressure-treated root vegetables

We investigated the effect of calcium and magnesium ions on the hardening of Japanese radish by the pressure treatment at 400 MPa. The pectins were separated into the three components. Water-soluble pectin (WP) and hydrochloric acid-soluble pectin (HP) were decreased by the pressure treatment, whereas hexametaphosphate-soluble pectin (PP), which is a metal binding type pectin, was increased. The degree of esterification of WP was decreased by pressurization. Variations with time after pressure release of all pectin fractions and the concentrations df calcium and magnesium ions contained in pectin fractions were small. We studied the hardness of the pressure treated samples by soaking them with or without EGTA and EDTA and the effect of the order reversal of the soaking and the standing. It was found that the magnesium ions had a great influence on the hardness of both non-treated and pressure-treated uncooked samples and that calcium ions affected the hardness of pressure-treated cooked samples. The suppressing effect of the calcium ions on the softening during cooking suggested that the interactions between such components as proteins and hemicellulose were promoted by the calcium ions. The mass transfer processes followed by the collapse of the cell membranes by the pressure treatment are considered to strengthen these interactions.