Effect of pulsed electric field pre-treatment on microstructure and internal transport throughout osmotic treatment of organic kiwifruit.

Summary

Plant cellular structure could be considered a complex organized system where flows are carried out by different solutes or solvents transports systems.
Passive transports, which is based on the free energy gradients, involve symplastic, apoplastic and aquaporins transmembrane transports. During osmotic
dehydration (OD) treatment the semipermeable membranes such as plasma membrane and tonoplast, are forced to separate, due to the water losses from
the vacuoles, starting the process known as plasmolysis, characterized by the loss of the turgor pressure. In the last years, OD has been extensively studied for
the partial dehydration of fruits and vegetables in order to obtain semi-moist products; however, it presents some limitations such as the low dehydration rate
and the high solute content in the final product. Therefore, the use of pre-treatment such as Pulsed Electric Fields (PEF) has been reported to facilitate water
removal and to improve the quality of the dried or osmo-dried products. PEF is a non-thermal technology which involves the application of short and repeated
voltage pulses to a biological tissue placed between two electrodes; it induces changes and reorganization in the electric conformation of the cell membrane,
modifying the normal fluxes during drying process when it is used as a pre-treatment. In present work PEF has been applied as a pre-treatment prior OD of
organic kiwifruits (Actinidia deliciosa cv Hayward) in order to evaluate its effect on the internal structure and internal water transport. PEF pre-treatments
were performed using the following parameters: E= 100, 250 and 400 V/cm, 60 near-rectangular shape pulses, pulse width of 100 ± 2 μs and a repetition time of
10.0 ± 0.1 ms. The OD was carried out by immersing the samples in 61.5% sucrose solution at 25 °C for different time period (0-120 min). The samples were
analyzed in terms of microstructure by Cryo-SEM microscopy and internal water transport by Time Domain Nuclear Magnetic Resonance (TD-NMR). The
results showed that the application of a PEF pre-treatment before the OD produces a process of plasmolysis proportional to the electric field strength applied.
It is because the PEF removes the mobile charges of the medium, such as electrolytes, organic acids, amino acids; Ca+2 is the major culprit of the plasmolysis
because it fixes some of the junctions of the microtubules between the cell wall and the membrane. In addition, the process of plasmolysis induced by the
electric field changes the behavior of kiwifruit tissue during the OD process. In a standard OD without any pretratment, the main transport is the symplastic,
whereas if previously treated with PEF, the apoplastic transport is as important as the symplastic, considerably increasing the rate of dehydration.
Acknowledgements: Financial support for this project is provided by funding bodies within the FP7 ERA-Net CORE Organic Plus, and with cofounds from the
European Commission (No 618107).