Ultrasound-assisted osmotic dehydration of litchi: effect of pretreatment on mass transfer and quality attributes during frozen storage

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Litchi (Litchi chinensis Sonn.), fruits being the perishable and high amount of moisture content, are very susceptible to fruit deterioration and reduced shelf life. A novel method of ultrasound-assisted osmotic dehydration can improve fruit quality by increasing mass transfer rate and maintaining texture characteristics. Therefore, this study aimed to investigate the effect of ultrasound-assisted osmotic dehydration on the mass transfer parameters, physicochemical and microbiological qualities of litchi fruits during frozen periods. For this purpose, litchi fruits were subjected to ultrasound-assisted osmotic dehydration as a pretreatment prior to the freezing step. The pretreatment consisted of three processing conditions: the use of osmotic dehydration at 50% (w/w) of sucrose (OD); ultrasound in an osmotic solution for 10 min before osmotic dehydration (US10 + OD); and ultrasound in an osmotic solution for 15 min before osmotic dehydration (US15 + OD). All samples were frozen and stored at − 18 °C. The obtained results indicated that ultrasound pretreatment effectively increases water loss (28.73 ± 3.25%) and solid gain (45.44 ± 6.85%) levels of litchi fruit. During the frozen storage for 90 days, the use of ultrasound, remarkably, US10 + OD treatment reduced the moisture content (76.46 ± 1.38%), drip loss (15.93 ± 1.01%), and microbial load (2.58 ± 0.06 log CFU/g) of the frozen litchi while increasing the firmness (1.46 ± 0.24 N), and a total soluble solid (17.50 ± 0.87 oBrix) amount of the samples in comparison to their controls (p < 0.05). Therefore, these results demonstrated that ultrasound-assisted osmotic dehydration could significantly improve the quality of litchi fruit in freezing and thawing processes.