Effect of pulsed electric field treatment on water distribution of freeze-dried apple tissue evaluated with DSC and TD-NMR techniques

This work aimed to study pulsed electric fields (PEF) effect on the water distribution of freeze-dried apple. Apple (var. Cripps Pink) was treated in 15% trehalose and 1% ascorbic acid solution (388 μS/cm) at 3 Hz and various electric field strengths 0.3; 0.6; 0.9 and 1.2 kV cm− 1 for 5, 10 or 15 pulses. The samples were frozen at − 45 °C and freeze-dried. The analyses were performed after rehydration. Differential Scanning Calorimetry (DSC) and Nuclear Magnetic Resonance in the domain of time (TD-NMR) were performed to assess thermal properties of freezable water and water distribution in apple tissue, respectively. PEF changed the integrity and continuity of the cell structure shown by the water redistribution between different compartments. The water in vacuoles and extracellular spaces had higher TD-NMR relaxation times as water molecules can diffuse in larger volumes before relaxing, even if the overall solutes concentration in the tissue increases.