Pulsed electric fields effect on mechanical and sorption properties of dried apple tissue

Abstract

Electroporated and dried apples were characterised in terms of physical properties, such as sorption profile and mechanical test, in order to understand the treatment effects. Thermodynamic relationship between water activity and equilibrium moisture content at constant temperature and pressure has been mathematically expressed by sorption isotherm. High coefficient of determination of BET equation fitting was obtained, ranging from 0.974 to 0.990 (RMSE 0.07–0.08). The highest electric field strength (1.5 kV/cm) reveals a change in sorption isotherm shape. The III type of isotherm observed indicates a major presence of crystalline molecules increasing a water surface interaction. Compression test were applied to the samples at different hydration level and PEF intensity applied. The plasticizing effect was observed as a function of hydration level raise and involved a reduction of the maximum force from about 14.85 (±2.44) N to 8.45 (±0.59) N. The opposite anti-plasticizing effect was a consequence of electric filed strength increase. Particularly, at low hydration level, where plasticizing effect did not produce softening of structures, PEF treatment induced a toughness and stiffness increase, revealing the anti-plasticizing effect. Fermi equation showed high coefficient of determination (0.902–0.959) and Y0 value change from 17 to 35, as a function of PEF treatment intensity increase, confirming the structural differences in the analysed samples. Thus, main results revealed the modification of water holding capacity induced by PEF and drying. Sorption profile and mechanical properties well described plasticizing and anti-plasticizing induced effect, underling the enhanced mobility of the system promoted by treatments.