Kinetics of thermal softening of potato tissue (cv. Monalisa) by water heating

The temperature dependence of rheological parameters as firmness indicators for potato tissue was determined within the temperature range 50-100 °C using four different objective methods. The rate of thermal softening of potato tissue by water treatment at 50 °C, 90 °C, and 100 °C was consistent with one pseudo first-order kinetic mechanism, while at 70 °C and 80 °C the rate of softening was consistent with two simultaneous pseudo first-order kinetic mechanisms. Kinetic theory was suitable to detect an increase of firmness through heating at 60 °C, mainly between 20 min and 40 min, presumably by pectinesterase activation. This study shows that two substrates S_a and S_b may be involved in providing firmness to potato tissue at 70 °C and 80 °C. For these temperatures, mechanism 1 is more probably due to gelatinization and light cooking, whereas mechanism 2 is more likely to represent the changes of the pectic substances in the cell wall and interlamellar region. At 90 °C and 100 °C the gelatinization process was fast and therefore the simple mechanism fitted presumably reflects the degree of solubilization of the pectic substances. At 50 °C and 60 °C there was practically no gelatinization, so that the simple mechanism fitted presumably represented incipient solubilization of pectic material. In water heating, gelatinization contributes less than the cell wall structure to potato tissue firmness on the basis of either kinetic parameters or microscopic observations. Maximum breaking compression force and modulus of rigidity were the most suitable rheological parameters for studying the softening of potato tissue in water heating.