Effect of Freeze-Thaw and Ultrasonic Pretreatment on the Drying Rate and Quality of Lemon (Citrus limon) Slices

Abstract

The drying of lemon slices requires considerable time and often leads to unwanted alterations in their characteristics, such as discoloration and the degradation of bioactive compounds. Pretreatment has been widely applied to address these issues. This study aimed to investigate the effects of freeze-thaw and ultrasonic pretreatments on the drying rate and quality attributes of dried lemon slices. Lemon slices were subjected to ultrasonic pretreatment (40 kHz, 200 W) for 0, 10, and 20 min, either alone or combined with freeze-thaw (-20°C for 12 h), and subsequently dried using a convection oven at 60°C. The 10 min ultrasonic pretreatment without freeze-thawing was identified as the optimal approach based on process efficiency and quality retention. This treatment increased the drying rate (p < 0.05) and rehydration ratio (p < 0.05), reduced the browning index (p < 0.05) and total color difference (ΔE) (p < 0.05), maintained the total phenolic and total flavonoid contents of the dried lemons, and exhibited the lowest reduction in vitamin C content and antioxidant activity compared to the other pretreatments.