Development and Characterization of a Zanthoxylum limonella Essential Oil Nanoemulsion for Oral Care Applications

Zanthoxylum limonella is an aromatic medicinal plant widely used in Thai traditional medicine and oral healthcare. This study aimed to characterize the chemical composition of Z. limonella essential oil (ZLO) extracted from fresh and dried fruits and to develop a nanoemulsion to enhance its stability and applicability in oral care products. Essential oils from fresh and shade-dried fruits were obtained by hydrodistillation and analyzed using gas chromatography–mass spectrometry (GC–MS). The major volatile constituents of the dried fruit oil were limonene (34.84%), terpinen-4-ol (15.75%), sabinene (11.02%), α-Phellandrene (7.10%), cis-β-Ocimene (4.74%) and γ-Terpinene (4.52%). Drying significantly altered the chemical composition of the essential oil, resulting in marked decreases in the relative abundance of α-phellandrene (19.75% to 7.10%), β-myrcene (3.54% to 2.16%), and linalool (2.88% to 1.78%), whereas the relative percentages of limonene (29.52% to 34.84%) and terpinen-4-ol (4.44% to 15.75%) increased substantially. An oil-in-water nanoemulsion containing the essential oil from dried fruits (NE-ZLO) was developed using an optimized mixed-surfactant system with a hydrophilic–lipophilic balance (HLB) of 12.0–15.0 and concentration of 20–26%. Eleven nanoemulsion formulations (NE-ZLO-1 to NE-ZLO-11) were characterized in terms of droplet size, polydispersity index, and zeta potential. NE-ZLO-9 showed the most favorable physicochemical properties while requiring the lowest surfactant concentration. Its stability was evaluated under refrigerated (4 °C), ambient, accelerated (45 °C), and light-exposure conditions compared with that of free essential oil (ZLO). NE-ZLO-9 showed higher apparent limonene-equivalent retention than free ZLO during storage, especially under accelerated and continuous light-exposure conditions. After incorporation into oral care prototypes, the benefits of nanoencapsulation were more pronounced in mouthwash, while the toothpaste matrix itself provided substantial protection against the loss of volatile constituents. These findings suggest that nanoencapsulation and the formulation matrix are important factors in protecting ZLO during storage and support its further development as a natural ingredient in oral care products.