Effect of hydrothermal–microwave pretreatment on tocopherol retention and acidity evolution in canola oil during storage

Processing conditions applied before canola oil extraction may influence tocopherol content and the oil´s acidity during storage. In this context, this work evaluated the effect of combined hydrothermal–microwave (HTMW) pretreatment on tocopherol content and acidity evolution in mechanically pressed canola oil under different storage conditions.

Untreated and hydrothermal–microwave (HTMW) pretreated canola seeds were mechanically pressed, and the resulting oils were stored for 108 days under different temperature and light conditions (8 or 37 °C, and with or without light exposure). Tocopherol content was determined by HPLC, while acidity was expressed as a percentage of oleic acid. All determinations were performed in duplicate (n = 2). Data were analysed using ANOVA followed by Tukey's Test (p < 0.05).

The HTMW treatment resulted in higher initial tocopherol levels in the extracted oil (1061 µg/g) than those measured in oil obtained from untreated seeds (956 µg/g). Tocopherol concentrations remained unchanged throughout storage in oils kept at 8 °C, regardless of seed pretreatment. Similarly, no significant losses were detected in oil obtained from untreated seeds and stored at 37 °C under light-protected conditions. In contrast, total tocopherols declined significantly in oils from HTMW-treated seeds stored at 37 °C, both with and without light, as well as in oil from untreated seeds stored at 37 °C with light exposure. Despite these reductions, oils produced from HTMW-treated seeds retained higher tocopherol concentrations than their corresponding untreated counterparts after 108 days of storage.

Regarding changes in acidity, free fatty acids gradually increased after 14 days of storage onward. In contrast, oils obtained from HTMW-treated canola seeds generally showed higher acidity values throughout storage. 

These results show that the hydrothermal–microwave treatment increased the oil's initial tocopherol content and contributed to higher levels of these antioxidants throughout storage, whereas temperature and light exposure increased tocopherol losses during storage.