Effect of microwave-assisted vacuum and hot air oven drying methods on quality characteristics of apple pomace powder

Abstract Apple pomace, which makes up 20–30% of all processed apples, is an accessible source of bioactive ingredients that could be used in the food industry. A research of the impact of drying techniques on the quality characteristics of apple pomace powder was carried out to efficiently utilize this waste. The pomace was dried at 50 °C and 60 °C in a vacuum-assisted microwave dryer and an oven dryer, respectively. The different temperatures chosen for the drying of apple pomace were selected based on preliminary tests. Microwave drying resulted in reducing the drying time and improving the physicochemical, functional and morphological properties of the powder. The TPC (Total phenolic content) and AA (antioxidant activity) of pomace powder were found to be considerably influenced by the drying technique. Maximum TPC, DPPH and FRAP values observed for the apple pomace powder dried in the microwave were 5.21 ± 0.09 mg GAE/g, 93 ± 1% and 3.22 ± 0.04 µg/mg, respectively while as in oven drying, the values were 3.14 ± 0.06 mg GAE/g, 89 ± 1% and 2.22 ± 0.02 µg/mg. Microwave drying led to increasing bulk density (0.55 ± 0.01 g/cc), water hydration capacity (3.35 ± 0.09 mL/g), oil binding capacity (0.95 ± 0.04 g/g), solubility index (14.0 ± 0.9%), and emulsion capacity (60.0 ± 1.0%) of the powder. Lower values for bulk density (0.50 ± 0.01 g/cc), water hydration capacity (3.04 ± 0.08 mL/g), oil binding capacity (0.70 ± 0.03 g/g), solubility index (10.0 ± 0.8%), and emulsion capacity (48.0 ± 0.9%) were observed in oven-dried powder. Microwave drying resulted in a more disordered, crystalline and porous structure of apple pomace powder as compared to oven-dried powder as confirmed by SEM (Scanning electron microscopy) and XRD (X-ray diffraction). Microwave-dried powder also had a higher vitamin C content (20.00 ± 0.12 mg/100 mg) than oven-dried powder (12.53 ± 0.08 mg/100 mg). This study may be helpful in the preprocessing of apple pomace for bioconversion processes and extraction of valuable components from apple pomace. Graphical abstract