Impact of Mild Oven Drying on Colour Preservation and Phenolic Extractability of Mentha spicata L. from the Saloia Region

Mentha spicata L. is an aromatic herb valued for its sensory characteristics and phenolic compounds associated with antioxidant activity. Drying is widely used to preserve aromatic herbs by reducing moisture and microbial growth, extending shelf life, and facilitating storage and transport. However, drying conditions may affect visual quality and bioactive compound extractability. This study evaluated the effect of mild oven drying on the colour, total phenolic content, and antioxidant activity of M. spicata cultivated in the Saloia Region of Sintra, Portugal.

Fresh mint and samples oven-dried at 30 °C for 24 h were analysed. Colour was assessed using the CIE Lab* system. Aqueous extracts were analysed for total phenolic content and antioxidant activity using FRAP, DPPH, and ABTS assays. Results for fresh mint were converted to a dry matter basis, and all phenolic and antioxidant results were expressed per gram of sample dry weight. Data were analysed using an independent-samples t-test, with significance set at p < 0.05.

Mild oven drying did not significantly affect L*, a*, or b*, indicating preservation of mint colour. Dried mint showed significantly higher total phenolic content and FRAP values than fresh mint, with increases of approximately 46% and 172%, respectively, on a dry-weight basis. No significant differences were observed for DPPH or ABTS.

To the best of our knowledge, no previous studies have simultaneously assessed the effects of mild oven drying at 30 °C on instrumental colour, phenolic content, and antioxidant activity of M. spicata from the Saloia Region of Sintra. Overall, drying at 30 °C for 24 h preserved visual quality while enhancing phenolic extractability and ferric reducing antioxidant power. These findings support mild drying as a preservation strategy for producing dried mint with retained colour and enhanced functional potential, while maintaining a low processing temperature that may be particularly relevant for small-scale producers and commercial processors of high-quality aromatic herbs.