Physicochemical Stability of Peppermint Essential Oil under Variable Storage and Packaging Systems

Peppermint (Mentha piperita L.) essential oil stability is strongly influenced by the storage environment and container material. the aim of this study was to evaluate the physicochemical stability and chemical composition of peppermint essential oil stored for up to 12 months in plastic, aluminium, and amber glass vials under room-temperature, dark, and refrigerated conditions. Fresh aerial parts collected from Barabanki (Uttar Pradesh, India) in July 2023 were steam-hydrodistilled using a Clevenger apparatus for 6–8 h. Oil aliquots (50 mL) were stored in plastic, aluminium, and amber glass vials under room-temperature (27 ± 3 °C), dark (27 ± 3 °C), and refrigerated (<1 °C) conditions for 12 months. Initial physicochemical properties included an optical rotation of −33.03°, a refractive index of 1.4543, and a specific gravity of 0.8920. These parameters showed minimal variation, with refrigeration and amber glass providing the greatest stability, whereas plastic containers under ambient conditions showed greater drift, with specific gravity reaching 0.8986. GC–MS analysis revealed l-menthol as the major constituent (76.55%), increasing to 82.69% in plastic vials under ambient storage, whereas menthone declined from 7.88% to 5.72%. Methyl acetate decreased markedly from 2.26% to approximately 1.2%, while iso-menthone (approximately 3.2–3.7%) and neo-menthol (approximately 2.2–2.3%) remained relatively stable. Refrigeration and storage in amber glass or metal containers best preserved chemical integrity.