Quality analysis and drying characteristics of turmeric (Curcuma longa L.) dried by hot air and direct solar dryers

The present study evaluates the effects of hot air drying (HAD) and direct solar drying (DSD) on drying kinetics and properties of turmeric. Obtained results unveiled higher effective diffusivity (1.98 × 10−8 - 13.1 × 10−8 m2/s) and lower drying time (3.5 h) for DSD turmeric than HAD (2.19 × 10−8 - 2.67 × 10−8 m2/s and 4.5 h). Page and diffusion models were the best-suited models for turmeric drying. Browning index was reduced by 25.65 and 21.53% for DSD and HAD samples resulting in better color retention in DSD. The differences in hue angles between DSD (67.53° ± 0.30) and HAD (68.22° ± 0.11) were significant revealing superior yellow color in solar-dried turmeric (p < 0.05). Scanning electron microscopy revealed smoother and spherical particles in DSD indicating better flow-ability. Elemental diffraction x-ray spectroscopy exhibited higher atomic and weight percentage of potassium from DSD by 3.86 and 1.38%, respectively as compared to HAD. Atomic force microscopy revealed higher porosity in DSD with lower surface skewness (−0.347 ± 0.02) and surface kurtosis (2.910 ± 0.01) implying smooth and unimpaired surface. Infrared peaks showed higher curcumin in DSD than HAD samples as correlated with low intensity C–O–C bond stretching vibrations. Thermogravimetric analysis confirmed lesser weight loss of curcuminoids for DSD (42.60%) in comparison with HAD (44.77%).