The effects of ultrasonic pretreatment and structural changes during the osmotic dehydration of the 'Starking' apple (Malus domestica Borkh)

During the osmotic dehydration (OD) of fruit, the cell membrane displays a high resistance to mass transfer, thereby reducing the dehydration rate. To reduce thermal damage to cell membranes, alternative methods have recently been introduced to reduce the initial moisture content and/or modify the structure of fruit tissue. The aim of this work was to evaluate the effects of an ultrasound (US) pretreatment for OD on the effective diffusion coefficients and to observe the changes in the molecular structure of 'Starking' apple cubes by Fourier-transform infrared spectroscopy (FTIR) during a 3 h process using a 45°Bx sucrose solution at 60°C. In the pretreatment step, apple samples were immersed in an ultrasonic bath at 45 kHz for 20 min. The effective diffusion coefficients for water (D _ew) and solids (D _es) were calculated from the observed osmotic kinetics according to Fick's second law for the transient state. The solids coefficients were higher than the water coefficients in both processes due to the concentration difference (D _e = 7.7 × 10 ^-9 and 9.7 × 10 ^-9 m ² s ^-1 for OD _US). The structural changes were determined by FTIR by measuring the molecular vibration frequency for sucrose. The 1,500-900 cm ^-1 region of the infrared spectra was used to monitor the effect of sucrose concentration on fruit structure. We observed that the first bonds formed were C-H and C-O-C stretching (at 920 and 1,129 cm ^-1, respectively) in the sucrose skeleton and glycoside bonds among sucrose molecules. The water concentration affected the diffusion coefficient significantly due to its dependence on the physical structure of the food.