Pinus pumila, an evergreen shrub of the Pinus genus (family Pinaceae), was widely distributed across northeastern China, the Russian Far East, Japan, and the Korean Peninsula [1]. In nature, P. pumila forests produced approximately 200–320 kg of cones per hectare, of which the seed scales accounted for about 150–240 kg [2]. As these seed scales were usually discarded as forestry residues, substantial resource wastage occurred. Therefore, the development and utilization of seed scales were considered crucial for reducing waste and enhancing their economic value. Plant essential oils exhibited a broad range of biological activities, including antioxidant [3], antibacterial [4], anti-inflammatory [5], and antiviral effects [6]. Owing to these properties, they were widely used in the pharmaceutical, cosmetic, food, and chemical industries [7]. Given the increasing interest in plant essential oils as antimicrobial agents, exploring the potential of seed scales as a novel source of essential oils was of significant interest.

The main methods for obtaining essential oils from plant materials included hydrodistillation (HD), steam distillation, microwave-assisted hydrodistillation (MAH), supercritical fluid extraction (SFE), and liquid-liquid extractions [8]. Among these, HD remained the most widely used due to its simple operation and low equipment costs. However, HD had several drawbacks, including long extraction times (often exceeding 180 min), high energy consumption, and the potential thermal degradation of heat-sensitive compounds, which negatively affected the extraction cost and quality of the essential oil [9]. While SFE could avoid thermal degradation, its high equipment and operating costs made it unsuitable for developing low-value-added products from forestry waste [10]. Additionally, liquid-liquid extraction involved the use of large amounts of organic solvents, raising concerns about the potential health risks of solvent residue [11]. In light of these issues, an approach to MAH was developed with a short extraction time, high yield, and high quality of essential oil while having low operation costs. MAH rapidly rotated the polar molecules inside and outside plant materials through dipole-dipole interactions in an electric field, achieving efficient and uniform heating of both the interior and exterior of the raw material simultaneously, which accelerated the heating rate of the system, reduced mass transfer resistance, and thereby enhanced the separation of essential oils [12]. MAH had been widely used in the extraction of plant essential oils such as Rosmarinus officinalis L [13], Thymus vulgaris [14], Santalum album [15], and Salvia fruticose [16], which had higher yields and superior bioactivity. The MAH method offered significant advantages, including a substantial reduction in extraction time, energy consumption, and CO2 emissions, as well as improved essential oil quality [17]. These benefits made MAH a research hotspot in the field of plant essential oil extraction.

A review of the literature revealed that no previous studies had reported the use of MAH for extracting essential oils from P. pumila seed scales. In this study, we focused on extraction kinetics, yield, energy efficiency, and essential oil composition. The components of the obtained essential oils were characterized using Fourier Transform Infrared Spectroscopy (FT-IR) and Gas Chromatography–Mass Spectrometry (GC–MS). In addition, their antibacterial efficacy was assessed through minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and inhibition zone diameter (IZD) assays. The antibacterial mechanism of PSEO against Escherichia coli was explored by measuring alkaline phosphatase (AKPase) activity, changes in membrane conductivity, and leakage of intracellular components. Furthermore, molecular docking analyses were performed on OmpF with the 19 compounds that showed higher contents in PSEO extracted by the MAH method compared with the HD method. This study provided data to support the extraction, development, and application of essential oil from P. pumila seed scales, a forestry residue.