## 1 Introduction  
Essential oils, EOs, are concentrated liquids of complex mixtures of volatile organic compounds that can be extracted from leaves, flowers, or seeds of different plants, which have been used by humans throughout history due to their natural properties: from the fragrance to anti-bacterial/fungal activities, among others [1, 2, 3, 4, 5]. There are various methods available to extract EOs, steam distillation being the most commonly used in the industry, as it does not require the use of organic solvents and is still capable of achieving a high extraction yield. For research activities, hydrodistillation, HD, is the most used technique for EO extraction [6, 7, 8], a traditional method for the extraction of bioactive compounds from plants, where plant materials are packed with water in sufficient amount in a static compartment and brought to a boil. Alternatively, direct steam may also be injected into the plant sample. In order to further optimize the extraction of EOs, the application of pulsed electric fields, PEF, can be used as a pre-treatment before HD. The application of an electric field to a biologic cell induces the formation of pores in the cell membrane, named the electroporation effect [9]. Electroporation through PEF application is becoming a mature processing technique in several fields, from agriculture and food [10; 11; 12], biotechnology and health [13; 14], to waste and environmental applications [11; 15; 16]. Through PEF, very intense electric fields, in the order of a few to dozens of kV/cm, can be applied without a thermal effect [17]. Several authors have used PEF for increasing the efficiency in essential oil extraction from some plants, using HD, for example 4 kV/cm was applied to Rosa alba L. with 30-250 min extraction time by [7] and to _Nepeta transcalcascia_ with 120 min extraction time by [18], 0.8-2.5 kV/cm was applied to _Marribium vulgare_ with 30-60 min extraction time by [19], 10-30 kV/cm was applied to flowers of Damask rose with 30-180 min extraction time by [20], and 1-3 kV was applied to Artemisia herba alba with 30-60 min extraction time by [21], from which it was possible to extract higher EO quantities and to which the PEF parameters were optimized. Considering the potential of PEF, as a pre-treatment before HD, for increasing the EO extraction from plants and reducing the distillation time, the number of studies published, and plants investigated is still very limited [7; 18; 19; 20; 21]. Hence, this paper aims to contribute to increase the understanding of this application towards a future industrial acceptance of this technology, where three different plant materials were used, with two clear objectives in study: (a) PEF impact as a pre-treatment for eucalyptus and rosemary EOs extraction, using the plant leaves material some weeks after harvesting; (b) optimization of PEF parameters for extraction of thym essential oil, using dry plant leaves material. The first study aims to understand the impact of applying PEF, as a pre-treatment, to the extraction of EOs in similar conditions to what is used in the industry. The PEF protocol used was constant, and derived from the bibliography, where the distillation time was varied, to analyze its impact in both quantity and quality of the extracted EO, versus the EO extracted through the traditional method, for the same distillation times. The main goal is to study if PEF application to the plant material succeeds to achieve an EO with similar quality, and lower energetic costs, i.e., the same quantity and quality in less time. Eucalyptus and rosemary were selected as rosemary's EO is one of the EO of reference in the industry and eucalyptus is used primarily in the paper industry, in Portugal, and its leaves are seen as a waste, thus the extraction of EO from this plant material would contribute to the circular economy.