Influence of drying temperature on quality attributes of basil leaves (Ocimum basilicum L.): Drying kinetics and modelling, antioxidant analysis, colour, FT-IR analysis, and surface morphology

This work explores the impact of various drying techniques on basil leaves, including microwave drying at different power levels (300 W, 600 W, and 900 W), hot-air drying at 60 °C, and freeze-drying at –40 °C. The study evaluates changes in phytochemical composition, antioxidant potential, functional group profiles, and surface morphology under these treatments. Elevated drying temperatures accelerated moisture removal by improving water diffusivity, with the Midilli model providing the most accurate description of the drying kinetics. The choice of drying method strongly influenced leaf coloration: hot-air drying produced the most pronounced changes, while freeze-drying maintained pigment stability to the greatest extent. Chlorophyll and related green pigments were generally retained at higher levels in freeze-dried samples, whereas microwave drying at 600W showed high phenolic content (10.16±0.4 g gallic acid equivalent/kg dry-matter), flavonoids (5.01±0.01 g quercetin/kg dry-matter), and antioxidant activity (81.26±0.61 %) but reduced vitamin C concentration. Rutin concentrations were 21.28±0.89, 63.08±1.2, and 35.19±0.96 mg/g dry-matter for the different microwave power levels, while dihydroxybenzoic acid levels were 2.13±0.04, 5.48±0.12, and 3.19±0.08 mg/g dry-matter. Morphological analysis revealed that freeze-drying preserved cell structure more effectively, producing smooth and intact surfaces, while hot-air and microwave methods induced structural disruption and surface roughness. Spectral analysis confirmed the presence of polyphenols, flavonoids, and catechin, with the N-H functional group of gallic acid identified at freeze-dried and microwave-dried 600W samples and a peak at 1397 cm -1 in freeze-dried samples indicating -C-H alkenes in apigenin. The findings highlight that drying techniques greatly influence the color, structure, nutritional profile, and bioactive compounds of basil. Freeze-drying provides the best retention of pigments and cellular integrity, microwave drying at medium power boosts antioxidant and phytochemical levels while lowering vitamin C, and hot-air drying leads to the most noticeable alterations in appearance and surface properties. • Systematic evaluation of the drying kinetics of basil leaves. • Impact of drying techniques on moisture diffusivity, activation energy, and drying efficiency. • Identifying optimal kinetic behaviour under thermal and non-thermal conditions. • Comprehensive analysis of bioactive compounds: polyphenols, flavonoids, vitamin C, etc. • Structural and functional modifications caused by drying through SEM and FTIR.