Distillation time (DT) is a key parameter influencing yield and chemical composition, and its optimisation is crucial for production. This study is the first laboratory-scale investigation of the effect of distillation time (DT) on the leaf essential oils of Abies koreana, aiming to maximise oil yield and target bioactive components for cosmetic applications. Essential oils were obtained by hydrodistillation at 14 DTs (1, 3, 5, 10, 20, 40, 80, 120, 160, 200, 240, 280, 360, and 480 min), and the yields, chemical profiles, and fragrance characteristics were comparatively analysed. The control (exhaustive hydrodistillation: 20 h) yielded 2.82% and was dominated by D-limonene, bornyl acetate, and camphene. The contents of bioactive compounds associated with whitening and anti-wrinkle activities (α-pinene, D-limonene, borneol, and bornyl acetate) varied markedly with DT. The highest oil yield was obtained at 80 min (0.30 ± 0.01%), while the targeted components were achieved at 80–160 min. Heatmap-based multivariate analysis revealed distinct compositional differences between oils distilled at 80 min and 160 min, with DT shifting fragrance profiles from fresh, monoterpene-rich notes (linalyl acetate, camphor, and fenchol) to longer-lasting, sesquiterpene-dominated aromas (α-bisabolol and β-eudesmol). Therefore, distillation time significantly influenced A. koreana oil, with shorter distillation (80 min) maximising yield and longer distillation (80–160 min) enriching bioactive components for cosmetic applications.

Distillation is widely used for essential oils extraction, and distillation time (DT) influences the quality and quantity of oils. This study presents the effects of 14 DTs (1, 3, 5, 10, 20, 40, 80, 120, 160, 200, 240, 280, 360, and 480 min) on leaf oils from Thuja occidentalis L., aiming to maximise the yield and modulate chemical profiles. Also, a safety assessment of T. occidentalis oils obtained at different DTs was conducted based on thujone levels under International Fragrance Association (IFRA) guidelines. Oil yield varied with DT, reaching a maximum at 40–80 min (0.54 ± 0.04% and 0.59 ± 0.02%, respectively). Monoterpenes (sabinene, fenchone, and thujone) decreased with prolonged DT, whereas higher molecular weight compounds, including sesquiterpenes (caryophyllene) and diterpenes (hibaene and rimuene) increased. Principal component analysis (PCA) grouped samples into three stages, early (1–10 min), mid (20–80 min), and late (120–480 min), with clear compositional separation (PC1: 68.4%, PC2: 11.8%). As DT increased, the fragrance profile progressively shifted from monoterpene-rich characteristics in the early-stage oils to more persistent aromas associated with sesquiterpenes (caryophyllene, α-cadinol, and α-eudesmol) and diterpenes (hibaene and rimuene) in the late-stage oils. Safety assessment revealed that early-stage oils contained high thujone levels (67.19–71.88%), limiting their allowable use. In contrast, prolonged DTs (≥200 min) reduced thujone content, increasing permissible usage across multiple IFRA categories. Overall, DT significantly influences the oil yield, chemical composition, and regulatory applicability of T. occidentalis oils, with extended distillation enhancing formulation flexibility in cosmetic and fragrance applications.

The essential oil of Torreya nucifera, a coniferous tree native to East Asia, has notable bioactive properties with potential industrial applications. This study examined the effects of seasonal and regional factors on the yield and bioactive constituents of T. nucifera oils in South Korea. Leaf samples were collected in spring (March), summer (June), and autumn (September) of 2023–2024 from three ecologically distinct regions: Jeju Island, Jinju, and Hwasun. Oil yield was stable across seasons (0.9–1.6%) but varied significantly by region (p < 0.05), with Hwasun showing the highest yield. This stability across seasons may reflect the perennial evergreen nature of T. nucifera. Gas Chromatography-Mass Spectrometry (GC–MS) identified 32 volatile components, predominantly monoterpenes (83.7–90.4%) and sesquiterpenes (5.4–11.7%), with D-limonene and 3-carene as key chemical markers. Notably, 3-carene levels were significantly affected by region (p < 0.0001), with higher concentrations in Jeju oils, while D-limonene was influenced by season, region, and their interaction (p < 0.001), reaching peak levels in Hwasun during summer and autumn (up to 70%). Therefore, T. nucifera oil from Hwasun harvested in autumn can be optimised for commercial production by maximising oil yield and enhancing chemical markers.