Ultrasound-assisted solvent-free extraction of rosemary absolute oil: A green paradigm for high-efficacy natural antioxidant in frying oil stabilization

• Ultrasound-assisted oleo-extraction (UOE) yielded the best rosemary absolute oil (RAO). • UOE-RAO outperformed conventional methods with the highest yield and TPC. • UOE-RAO could enhance rosemary antioxidants via carnosic acid conversion. • UOE-RAO showed comparable DPPH scavenging capacity to BHT in vitro . • 0.1% (w/w) UOE-RAO extended the frying stability from 31 min to 63 h. This study developed a green extraction paradigm for rosemary absolute oil (RAO) as a sustainable natural antioxidant to inhibit lipid oxidation in frying oils. RAO was prepared via three methods: heat reflux extraction (HRE) with n-hexane, solvent-free oil extraction (OE) using refined soybean oil, and ultrasound-assisted solvent-free oil extraction (UOE). Under optimized UOE conditions (solid-to-liquid ratio 1:10, 400 W, 25°C, 13 min), RAO yield reached 19.09%, with total phenolic content (TPC) of 347.16 ± 2.81 mg GAE/g, significantly exceeding values from HRE and OE. UOE-RAO contained elevated concentrations of bioactive compounds: carnosic acid (CA, 89.45 ± 1.89 mg/g), ursolic acid (UA, 7.43 ± 0.02 mg/g), and carnosol (16.81 mg/g), accompanied by a pronounced CA-to-carnosol conversion. In vitro antioxidant assays (DPPH) demonstrated UOE-RAO’s potent radical scavenging activity (IC 50 : 71.26 ± 0.81 μg/mL), on par with synthetic butylated hydroxytoluene (BHT). When fortifying soybean oil with 0.1% UOE-RAO, oxidative stability during 180°C frying was markedly enhanced: total polar materials (TPM) accumulation was retarded, extending oil usability from 31 h (control) to 63 h, while acid value (AV) remained consistently lower throughout frying. Differential scanning calorimetry (DSC) further validated improved oxidative resistance, with induction times for primary (12.75 ± 3.51 → 14.14 ± 0.20 min) and secondary oxidation (40.47 ± 6.02 → 61.41 ± 3.10 min) significantly prolonged. Collectively, these findings highlight that UOE enables the production of RAO with superior antioxidant efficacy, positioning it as a green, high-performance alternative to synthetic antioxidants for lipid-rich food processing, particularly high-temperature frying applications.