## 3 Results  
### Selection of PEF Pre-Treatment Conditions for Tomatoes  
The degree of electroporation caused by Pulsed Electric Fields (PEF) in plant tissues is well represented by the decrease in tissue firmness. Before the application of PEF as a pre-treatment to Osmotic Dehydration (OD), a wide array of treatment conditions (1.8 kV/cm field strength, 0-1000 pulses of 15 \(\upmu\)s width and 20 Hz frequency) was applied in order to establish the limits within which the treatment remains effective, while over-processing is avoided. Figure 1a shows the dependence of tomato firmness on the number of applied pulses. At a number of pulses below 200, firmness drastically decreases as processing intensity increases. On the contrary, for a PEF treatment exceeding 500 pulses, tissue firmness does not significantly (\(p>0.05\)) vary with the increase in process intensity. This signifies that a maximal level of electroporation has been achieved that cannot be exceeded with further increase in pulse delivery. Therefore, a treatment at 300 pulses is adequate to cause the highest attainable degree of electroporation at 1.8 kV/cm field strength. Increasing pulse number above this value leads to unnecessary energy expenditure.  
The dependence of the total color difference of PEF treated tomatoes on the number of applied pulses is given in Figure 1b. According to the CIELab specification, a value of \(\Delta E\) exceeding 2.5 corresponds to a color difference perceivable by the human eye [25]. PEF treatment did cause a slight discoloration of tomatoes as processing condition intensity increases. A loss of redness was also reported by the sensory evaluation, as described below. This effect may be attributed to the fact that electroporation also disrupts chromoplasts among other organelles, leading to a diffusion and perceived \"dilution\" of red pigments across the peel.  
Figure 1c presents the dependence of the sensory acceptance on the number of applied pulses. An increase in processing condition intensity leads to deterioration of color, textural consistency and flavor. The value of 300 pulses can be considered the value above which neither textural properties (crispiness, bite hardness) nor color intensity further deteriorate. This outcome is not surprising since tissue softening and the repartitioning of water, solutes and pigments within the tissue are a natural consequence of plant cell electroporation. It is evident that the application of PEF alone on fresh tomatoes would lead to the production of an unacceptable product (overall acceptance falls below 5 for tomatoes treated at 300 pulses). Since PEF is to be applied as a pre-treatment to osmotic dehydration, it is expected that this deterioration will be offset by OD. This is based on two hypotheses: (a) the presence of calcium ions in the osmotic medium in combination with the increased cellular permeability of PEF treated tissues may increase tomato firmness due to the increased interaction of calcium ions with pectin substances within the cells; (b) the removal of water may restore part of the tissue's firmness and concentrate red pigments leading to a brighter red color. Red pigments of tomato (mainly lycopene and \(\beta\)-carotene) are fat soluble and thus are not expected to be lost to the osmotic medium with the loss of water. From the results of this preliminary screening, PEF treatment conditions as a pre-treatment to OD were narrowed down to the following: 0, 20, 50, 100, 300 pulses at 1.8 kV/cm. Even though 300 pulses were recognized to be an intense treatment condition leading to an unacceptable final product, it was selected nonetheless to serve as a benchmark for effective dehydration.  
Figure 1: Evolution of (**a**) tomato firmness, (**b**) color (total color difference: \(\Delta E\)) and (**c**) total sensory acceptance for tomatoes treated with PEF at 1.8 kV/cm and 0–1000 pulses of 15 μs width and 20 Hz frequency (average values \(\pm\) standard deviation).