Preparation of onion powder by freeze-thaw pretreatments: Influence of different thawing methods on drying kinetics and quality properties

Abstract

Onion is one of the foremost vital vegetables/spices delivered as a source of cash salary and for flavoring nourishments. In any case, postharvest misfortune amid capacity remains a major challenge. Therefore, in this study the effects of freeze-thaw pretreatments on drying kinetics and qualitative properties (color, total phenol content, antioxidant activity, total flavonoid content) of hot air dried onion was studied. Three pretreatment methods, including ultrasound (U), freezing-water thawing (F-T, 25°C), and Freezing- ultrasonic- assisted thawing (U-T) were individually used. FTIR spectra of the prepared powder samples showed the sample treated with F-T has a similar spectrum to the raw onion sample, demonstrating the least modification in sample structure relative to the alternative prepared samples. Results showed that the use of F-T pretreatment before grape drying reduced the color changes, total phenol content (from 16.92 ± 0.31 to 12.95 ± 0.13 mg GAE/g DW), total flavonoid content (3.8 ± 0.15 to 2.83 ± 0.09 mg QE/g sample), antioxidant activity effective moisture diffusion coefficient and activation energy (from 12.43 (control samples) to, 13.51 (U), 16.31 (F-T), and 20 ± 3 kJ/mol (T-U)). The optimum model for the drying kinetics of treated and controlled onions was the Modified Page model, with the greatest R2 and the lowest RMSE and χ2. Generally, the appropriateness of F-T pretreatment is employed in onion hot air drying to enhance the quality properties of dried onion.