Chemical profiles of aqueous or ethanolic extracts of 140, 170 and 200 °C-heated perilla meal were identified by GC-MS, and antioxidant properties of the extracts were observed via in vitro assays and in bulk oil or oil-in-water (O/W) emulsion. A total of 22 and 27 chemicals were found in aqueous and ethanolic extracts from non-heated samples, respectively. As the heating temperature increased to 200 °C, the carbohydrate and derivative contents decreased significantly (P < 0.05), whereas rosmarinic acid concentration decreased in both extracts. Ethanolic extracts possessed higher antioxidant activities than aqueous extracts based on the results of radical scavenging and ferric-reducing antioxidant power assays and the Rancimat assay, but there were no significant differences among samples (P > 0.05). In the case of O/W emulsions, aqueous extracts inhibited lipid oxidation more efficiently than ethanolic extracts at 50 °C. In particular, heat treatment decreased the antioxidant activities of ethanolic extracts and not aqueous extracts in the O/W emulsion system. Aqueous extracts are recommended in moisture-rich emulsionbased foods while ethanolic extracts are more suitable in a lipid-rich environment for enhancing oxidative stability.

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Solubility or content of oxygen molecules in oil matrices could be a critical factor to determine oxidative stability in lipid-rich products. In this study, a method to determine oxygen content in oil was developed by gas chromatograph with a thermal conductivity detector (GC-TCD), and factors affecting oxygen solubility or oxygen content were evaluated in bulk oil. Validation parameters were confirmed for headspace oxygen molecules using a GC-TCD. Oxygen gas was spiked into headspace of air-tight 10-mL vials with 70% sample content produced more reliable results on oxygen molecules for 6 h duration. The ratio of 7:3 (w/w) between sample content and headspace was selected considering precision and sensitivity. Addition of lecithin significantly decreased oxygen solubility in bulk oil, while monoacylglycerols did not show such decreasing effects. Medium chain triacylglycerols (MCT) had significantly high oxygen solubility compared to conventional edible oils partly due to the relatively low surface tension. Heavily oxidised bulk oils had higher surface tension and lower oxygen solubility. Therefore, oils with relatively low surface tension had higher oxygen solubility. The results of this study can help to explain the oxidative stability of edible oils containing high amphiphilic compounds.

Volatile profiles and degree of oxidation in corn oils after microwave treatment for 50 min were compared to oils with conventional heating at 180°C for 4 h. Oil microwaved for 10 and 50 min showed a similar degree of oxidation compared to the oil heated at 180°C for 1 and 3 h, respectively, based on the primary and secondary oxidation products. However, the total volatile contents of 50 min-microwaved oil were significantly lower than that of corn oil heated for 1 h at 180°C (P < 0.05). Microwave treatment accelerated the rates of lipid oxidation by 4.6 times compared to 180°C thermal oxidation. Relatively short treatment with microwave irradiation may not provide sufficient time to generate volatiles from oil. Microwaved corn oil for 50 min had higher contents of hexanal and t-2-heptenal and lower contents of 2,4-decadienal and nonanal than the oils heated at 180°C for 3 h. Reactive oxygen species may be involved in volatile formation in microwave-irradiated oils.

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