Investigation of microwave drying efficiency through ultrasonic pretreatment: a study on drying kinetics and quality retention in Capsicum annuum L. (Red bell pepper)

This study examined the influence of ultrasound-vacuum pretreatment prior to microwave drying of red bell pepper slices. Ultrasound-vacuum and water bath-vacuum pretreatment (50 and 70 °C) of red bell pepper slices for microwave drying was studied. Use of ultrasound-vacuum as a pretreatment prior to microwave drying of red bell peppers could improve effective moisture diffusivity up to 3.61 × 10 − 8 (m 2 /s) ( p > 0.05). Furthermore, use of ultrasound could increase kinetics of drying, decreasing the overall drying time. Quality assessments demonstrated that applying ultrasound-vacuum as a pretreatment before microwave drying resulted in superior retention of bioactive compounds, including total phenolic content, carotenoids, and ascorbic acid, along with elevated antioxidant activity (measured via DPPH) in dried red bell peppers. The use of ultrasonic (50 and 70 °C) before microwave-drying of red bell pepper slices showed lower browning index (197.50–33.90), color change (42.9–45.4) and higher rehydration rate (67.26–64.98) compare to control (506.80, 4.70 and 63.43) significantly ( p > 0.05). Ultrasound pretreatment as a non-thermal technology suggests a favorable pretreatment to improve overall characteristics and marketability of dried red bell pepper slices.