Effect of preheating temperature on vegetable hardening and extent of cell membrane damage based on the measurements of potassium ions eluted and electrical properties

The temperature dependence of vegetable hardening due to preheating was found to be similar for Japanese radishes, carrots and potatoes. The hardening phenomenon occurred slightly at 50–55 °C and markedly at 60–70 °C, especially at 65 °C, and decreased at 80 °C. However, temperature dependence of the cell membrane damage that triggered hardening was different across the vegetables, as revealed by the measurements of eluted potassium ions and electrical properties. The highest heat sensitivity of the cell membrane was observed in carrots while the lowest was in potatoes. The maximum hardening at 65 °C was confirmed by a complete loss of cell membrane integrity, followed by adequate leakage of intracellular potassium ions, resulting in high PME activity despite the effect of heat inactivation. A sample size of 1 cm cubic used this study was beneficial for distinguishing the effect of different temperatures, because the inner temperature quickly matched the preheating temperature.