Effect of a pulsed electric field and osmotic treatment on freezing of potato tissue.

This work discusses the effects of pulsed electric field (PEF) and osmotic pre-treatments on potato tissue structure and on the freezing and freeze-drying behaviour of this tissue. Potato samples (26-mm diameter, 10-mm height) were treated by PEF (400 V/cm) to high level of disintegration (conductivity disintegration index Z was ≈0.95) and were subjected to osmotic treatment in an aqueous solution of NaCl. The samples were either frozen in an air-blast freezer at air temperature of −80 °C and velocity of 2 m/s or freeze-dried at 0 °C and 0.04-mbar pressure. The scanning electron microscope (SEM) images evidenced similarity in structure of the cell walls and area and morphology of starch granules for untreated and PEF-treated potato tissues. However, sequential (PEF + osmotic) pre-treatment of potato tissue resulted in starch granules with rougher surface. The profiles of freezing curves were strongly dependent on pre-treatment. The longest effective freezing time t f was observed for untreated tissue, and the values of t f were decreasing in the following sequence: untreated > PEF pre-treated > PEF + osmotically pre-treated. The faster freezing and freeze drying and visually better quality of the dried samples were observed for PEF or sequential PEF + osmotic pre-treatments. The SEM analysis revealed also a noticeable disorder of starch granule surface morphology inside the cells of the freeze-dried potatoes after sequential PEF + osmotic pre-treatment.