To obtain better qualities of litchi fruits, fruit pulps were subjected to ultrasonic treatment (UT) followed by drying. Samples were subjected to UT at 3 W/g for 10 min with distilled or ice water and compared with non-UT dried samples. After drying, vitamin C, total phenolic content, color, texture, nutrition, microbial load, drying kinetics, and shelf life were assessed. Results suggest that shear stress plus increasing heat reduced drying time by about 50%, and retained 70% vitamin C and 60% total phenolic content. UT led to about 75% of vitamin C and 70% total phenolic content through inhibition of ultrasonic heat. No significant differences were found in redness, yellowness, and hardness. Inhibition of ultrasound heat resulted in about 27% glucose, 22% fructose, 17% sucrose, and prolonged storage time. Inhibition of increasing ultrasound heat allows low drying cost and high product quality of litchi fruit in air-drying. PRACTICAL APPLICATIONS: UT promotes drying efficiency and preserves product quality. However, this treatment triggers the loss of antioxidants and sugars of litchi fruits when water temperature arises in the treatment. Additional use of ice crystals can offset the thermal effect of the UT; this mechanism reduces the diffusion and loss of nutrients from the material to the solution. This strategy is simple and feasible to improve the drying rate and to retain the content of antioxidants, and further improve the flavor and storage quality of dried litchi fruits.
Keywords: litchi fruit; phenolic compounds; ultrasound; vitamin C.
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