Reversible electroporation of Thai basil leaves as a pretreatment prior to drying

Abstract: As commercial dried herbs are of lower quality than fresh herbs, it is of key importance to understand the effect of pre-drying treatments and drying techniques on the quality of the dried product. Several technologies are reviewed, focusing on their effects on aroma and color, with the goal of providing an overview of various technological strategies developed for improving the quality of aromatic herbs for industrial drying. One of the pretreatments, pulsed electric field (PEF), can be used to enhance the drying rate of plant leaves. The electroporation of guard cells on the plant leaf surfaces results in sustained stomata opening during the drying process that, in consequence, increases the drying rate. The effect of electroporation parameters on the reversible permeabilization of cells in Thai basil leaves, specifically cells on the leaf surface, was investigated. Various PEF parameter combinations were used. Microscopic observations were used to assess the effect of these parameters on the electroporation of the leaf surface. The results showed that the electroporation of epidermal cells increased with increasing treatment time. After homogeneous electroporation of epidermal cells, guard cells were electroporated. Electroporation of epidermal cells on the leaf surface increased with voltage, pulse width, and number of pulses. Six specific PEF parameter combinations were found to electroporate the guard cells on the leaf surface while maintaining the leaves' viability. In this study, one of the six established electroporation combinations (200 monopolar, rectangular pulses of 50 µs pulse duration, 760 µs between pulses, and nominal field strength of 650 V/cm) was used, followed by a 24-hour resting period in humid conditions before hot air drying at 40 °C. This treatment helped some cells in Thai basil leaves to survive different levels of dehydration (moisture ratio = 0.2 and 0.1). We show that resting after the application of reversible PEF may allow a hardening phase to exert a protective effect on the cells, thus reducing damage during subsequent drying. Cell vitality preservation would be associated with a more turgid and fresh-like rehydrated product. Furthermore, the properties of dried and rehydrated Thai basil leaves were assessed with two different drying methods, convective drying at 40 °C and vacuum drying at room temperature. Vacuum drying caused more cell damage and tissue collapse than convective air-drying. Remarkably, reversible electroporation followed by resting resulted in greater trichome preservation, showing that this pretreatment protects trichomes even after complete dehydration (water activity, aw < 0.6).