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

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.