Effect of simultaneous dual-frequency ultrasound aided ethanolic pretreatment on drying kinetics, bioactive compounds, antioxidant activity, and physicochemical properties of apple slices using pulsed vacuum dryer

This work aims at investigating effect of different pretreatments and varied temperature on the drying kinetics, bioactive compounds, antioxidant activity, microstructure and functional group of apple slices using pulsed vacuum dryer. Pretreated apple slices (Ethanol, US + W, and US + E) dried at varied temperature (60, 70, and 80 degrees C) were analyzed to determine total phenolic content (TPC), total flavonoid content (TFC), 1,1-diphenyl-2-picrylhydrazyl (DPPH+), 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid (ABTS(+)), color, microstructure and FT-IR spectroscopy. The findings revealed that US + E pretreatment significantly (p < .05) decreased drying time (20-33.33%). Activation energy and moisture effective diffusivity were 17.60-29.86 kJ/mol and 1.52-3.60 x 10(-8)m(2)/s, respectively. Microstructure evaluation showed disruption of cells structure and microchannels formation but was more prominent in US + E pretreatment with noticeable larger pores and several microchannels. US + E pretreatment retained TPC (64.89-75.75 mg/100 g GAE dw), TFC (45.96-56.25 mg/100 g ER dw), DPPH+(66.85-73.34%) and ABTS(+)(62.58-71.17%) than Ethanol, US + W, and control and was further corroborated by the observed peaks as revealed by the FT-IR spectroscopy. Color (L*,a*,b*, whiteness) was better retained by Ethanol and US + E pretreatments. US + E pretreatment decreased drying time and also preserved bioactive compounds, antioxidant activity and color of apple slices. Practical application Apples are popularly consumed fruits and confer various nutritional and health benefits to humans. Drying is the most common approach for apple processing and shelf life extension. However, drying often leads to a reduction in quality parameters as well as degradation of nutritional components of the apple. Novel pretreatment techniques like ultrasonic aided ethanolic pretreatment preceding to apple drying provides an added advantage over conventional drying methods in that it reduces dehydration time as a result of faster moisture removal, lowers energy consumption, improves process efficiency leading to industrial competiveness and it also preserves the nutritional and sensorial parameters of dried apple slices.