Impacts of microwave non-thermal effect on the aroma formation of fragrant rapeseed oil

Flavor has been one of the most important quality indicators of rapeseed oil. The chemical aroma essence of strongly flavored rapeseed oil dominates its flavor control on the consumer choice. It is of great significance to promote the product quality of the rapeseed oil industry. Microwave treatment of raw materials can be expected to serve as the pretreatment for the preparation of aromatic rapeseed oil. This study aims to explore the contribution rates of the microwave non-thermal and thermal effects to the flavor formation during microwave processing. The "Dadi 199" rapeseed was taken as the raw material. A series of experiments were performed on the microwave and 150 ℃ seed roasting pretreatment and pressing. The heating temperature of stir-frying and microwave was controlled to balance the thermal effects during the microwave process. A comparison was made to clarify the influence of the non-thermal effects on the flavor formation of coriander oil. The browning index (BI) was then measured to compare the Maillard reaction between microwave and stir-fried modes. The browning indices were 8.32, 8.32, 12.84, 14.16, 14.65, and 18.05, respectively, at 0, 1, 2, 3, 4, 5 min under microwave; By contrast, the browning indices were 8.32, 9.45, 9.45, 11.48, 11.28, and 15.73, respectively, at 0, 1, 2, 3, 4, 5 min during frying. The Maillard reaction after 0-1 min microwave treatment was very weak in the initial stage of pretreatment. There was no increase in the final product of the Maillard reaction (melanoidin-like). The more intense Maillard reaction enhanced the accumulation of the final products after 2 min. The Maillard reaction caused by the pure thermal effect was faster than that of the microwave. The black-like semen accumulated higher than that of the microwave at 1 min. There was an increasing content of black-like semen produced by microwave and frying, with the increase of the processing time. The content of black-like semen produced by microwave treatment was higher than that of frying treatment. The non-thermal effect of microwaves effectively promoted the final products of the Maillard reaction. The key flavor components in the aromatic rapeseed oil were quantitatively analyzed using headspace solid-phase microextraction and gas chromatography-mass spectrometry olfactory technology. Among them, Furan was a unique substance that was produced by microwave non-thermal effects during the microwave pretreatment stage. Meanwhile, the microwaves promoted the production of pyrazines, phenols, alcohols, acids, aldehydes, isothiocyanates, and sulfur-containing compounds. The contribution rate of the microwave non-thermal effect was 2-15 times that of the heating of stir-fried seeds alone at 5 min. The total amount of the key aromas was 266.54 and 68.83 mg/kg, respectively, in the fragrant rapeseed oil that was promoted by microwave and stir-frying seeds. The contribution rates of the microwave non-thermal effect to the flavor of strong aromatic rapeseed oil at 150 ℃ for 1, 2, 3, 4, 5 min were 14.7%, 49.5%, 18.73%, 66.26%, and 74.2%, respectively. As such, the microwave non-thermal effect at the same temperature can dominate the key flavor formation of fragrant rapeseed oil.