Optimization of ultrasound pre-treatment and the effect of different drying techniques on antioxidant capacity, bioaccessibility, structural and thermal properties of purple cabbage

Abstract

Some of the challenges of purple cabbage post-harvest processing and shelf-life is the preservation of its bioactive compounds, especially anthocyanins– a water-soluble pigment of interest due to their health-promoting benefits. The current study proposes the application of ultrasound (US) as a pretreatment prior to convective drying (CD), freeze-drying (FD), spray drying (SD), or spouted bed drying (SB). For this, a Box-Behnken Design (BBD) evaluated the effect of the different drying techniques and selected US amplitudes (20-70%) and times (4-12 min) on the antioxidant capacity, bioaccessibility, and structural and thermal properties of purple cabbage. The results demonstrated that the amplitude of 80.35% and 8 min of US, yielded the highest total phenolic compounds (TPC) (80.46 mg GAE/100g), total anthocyanins (TA) (9.85 mg/100g), and antioxidant activity (29.74 µM Fe2SO4/g) levels. SD and SB stood out for preserving bioactive compounds, increasing solubility, and reducing powder hygroscopicity. In the Scanning Electron Microscopy (SEM) analysis, CD, FD, and SB exhibited irregular and rough surfaces, while SD displayed spherical particles. Changes in the intensities of Fourier-Transform Infrared Spectroscopy (FT-IR) spectra, especially in the bonds (C=C, 1020 cm−1) in SD and SB, suggest alterations in molecular interactions due to processing and the adjuvant. Differential scanning calorimetry (DSC) indicated that SD displayed greater thermal stability. In conclusion, US pre-treatment and appropriate selection of drying techniques significantly influenced the properties of purple cabbage powder, bringing insights for the food industry regarding the use of drying as a post-harvested treatment of purple cabbage.