Heat-Induced Cell Membrane Injury of Vegetable Tissues -An applied study on potatoes

Heat processing of vegetables involves the use of temperatures above the physiological limit which affect the cellular structures of the tissue and induce a series of events at cell level that will in turn be reflected in the transport properties of the tissue. One of the most important cellular structures from the mass transport point of view is the cell membrane since it represents the physical barrier to the transport of nutrients into and out of the cell. The effect of heat treatment above physiological limits on the cell membrane of vegetable tissues was studied using potatoes as a model tissue. In vitro studies of the thermal deactivation kinetics of the plasma membrane H+-ATPase showed a pseudo-first-order model characterized by the sum of two independent and simultaneous processes occurring at different rates. Study of the temperature dependence of the activity loss showed that the potato cell membrane undergoes functional breakdown at 55°C, which is characterized by a sharp drop in the H+-ATPase activity and a drastic 7-fold decrease in the energy of activation (Ea). As a result, the proton-motive force needed to provide the energy for the active solute transport is greatly affected and the ability of the cell to retain cytoplasmic solutes decrease. Since the H+-ATPase plays a key role in the transport of solutes across the cell membrane, results indicate that processing temperatures above 55°C affect both mediate and active transport at cellular level. The cell membrane integrity of potato tissue was measured at different depths from the surface to the centre of a sample exposed to different heat treatments. Computer simulations using the finite-element method were performed in order to estimate the temperature distribution within the tissue during heat treatment. Treatment conditions were found to affect the degree of cell membrane integrity in the tissue, especially after heat treatment above 55°C. In similarity to the thermal deactivation of the plasma membrane H+-ATPase, the kinetics of the reduction in cell membrane integrity was found to follow a pseudo-first-order model indicating theta the cell membrane integrity of the tissue is coupled to the thermal stability of the plant plasma membrane H+-ATPase. The temperature dependence of the cell membrane integrity after heat treatment did not follow Arrhenius’ law but a sigmoid pattern, as demonstrated by a slight decrease at low tissue temperatures followed by a sudden sharp decrease after the tissue reached temperatures above 50°C. The absolute residual pectin methylesterase activity of the tissue was also measured at different depths and related the simulated temperature of the tissue. The activity of the enzyme was found to be strongly dependent on the degree of heat penetration and temperature distribution within the tissue. Thermally induced activation of the enzyme was detected at tissue temperatures above 52°C. The simultaneous occurrence of thermal activation of the enzyme and the sharp decrease in cell membrane integrity of the tissue in the same temperature range suggest a coupling of the two processes. The activation of pectin methylesterase enzyme in the tissue may be the consequence of cell membrane disruption and subsequent leakage of cytoplasmic solutes towards the cell wall.