Effect of reversible permeabilization in combination with different drying methods on the structure and sensorial quality of dried basil leaves

Abstract: In this work the impact of different drying techniques and the stomata aperture induced by pulsed electric field (PEF) prior drying on quality of the dried products was assessed. The conditions of the electroporation treatment were chosen to induce an irreversible opening of the stomata of the basil leaves. The drying times of the different drying techniques used (air-, vacuum- and freeze drying) were adjusted to obtained products with a moisture content of about 7.5 % and a water activity between of about 0.3-0.5. Dried samples were compared according to sensorial quality, the degree in cell collapse, the degree in color changes and the rehydration capacity. The results showed that the quality properties of dried basil were influenced by both, drying technique and the irreversible stomata aperture induced by pulsed electric field treatment. The different drying techniques and the PEF treatment showed an influence on drying time, where the PEF treatment reduced the drying time needed to reach the same amount of moisture content. The degree of cell collapse was assessed with scanning electron microscopy. It was shown that the choice of drying technique influences the degree of cell collapse, with freeze dried samples showing the best-preserved cell structure. The PEF treatment influenced the degree of cell collapse with an exception in freeze dried samples, which were not different from each other. In contrast the rehydration capacity assessment did not show differences depending on the drying technique or PEF treatment between the samples. The preservation of the aromatic oil cavities, containing the aroma compound were assessed with SEM and showed that there was no influence of aroma regarding different drying techniques observed. PEF treatment significantly enhanced the aroma compound retention, with an exception of the freeze dried samples. When assessing the aroma compounds through the sensory evaluation, there was an influence on aroma compound retention between different techniques and PEF pre-treatments. The PEF treated basil were the samples with more preserved aroma compounds. An influence of the drying technique used according to aroma compound retention was perceived, where for a dried basil and pesto the vacuum dried samples were shown to retained most aroma compounds, as assessed by sensory evaluation. An influence of the different drying techniques on the color change after drying was observed. Air dried samples resulted in a yellower green product when compared the fresh sample and the vacuum and freeze dried samples changed towards a darker green. The PEF treatment showed a slight influence in the color change, but to a much lesser extent than the drying techniques. The sensory panel detected clear color differences of the air dried samples compared to the samples dried with the other techniques and a rather clear difference between the untreated and PEF treated air dried sample. However, differences between untreated and PEF treated sample of the other two techniques were difficult to detect. The industrial implementation of PEF treatment prior to drying was suggested for air- and vacuum drying and was found to be inadvisable to freeze drying.