The influence of ultrasound-assisted osmotic dehydration as a pre-treatment method on the quality of vacuum dried pineapple

This study investigated the effect of ultrasound-assisted osmotic dehydration (UAOD) as a pre-treatment before vacuum drying on the drying time, quality, and microstructure of pineapple. At first, the best conditions for water desorption during osmotic dehydration (OD) were optimized. Pineapple cubes were subjected to OD at different osmotic solution concentrations (30, 40, and 60°Brix), pre-treatment time (0.5 and 1 hr), and temperature (35ºC) using two types of osmotic agents (sucrose and fructose). According to the results, sucrose was the best osmotic agent because it reduced the sample's moisture content and water activity. Secondly, the influence of UAOD on sugar gain, water loss, and drying time was evaluated using sucrose as an osmotic agent. Pineapple cubes were subjected to UAOD at different osmotic solution concentrations (30 and 60°Brix), pre-treatment time (10, 20, and 30 mint), and temperature (40 ºC) in an ultrasonic bath (ultrasound frequency of 40 kHz and ultrasound power of 300 W). Results exhibited that the application of UAOD increased the sugar gain and water loss, consequently decreasing the drying time. Overall, a 15.31 % reduction in vacuum drying time on a self-designed vacuum dryer and a 9 % reduction in total processing time were observed when UAOD (30 ºBrix) was applied for 20 min. Moreover, the influence of UAOD on the quality of dried pineapple in terms of rehydration ratio, firmness, and color change was also determined. Results showed that the pre-treatment with UAOD (30 ºBrix) of 20 min provided a better rehydration ability, with minimum change in color and textural properties. However, an increase in the pre-treatment time beyond 20 min diminished the rehydration ability of the dried product. Scanning electron microscope (SEM) images of pretreated and dried pineapples were also analyzed to determine the effect of pre-treatments applied for different lengths of time on the microstructure of the pineapple. The study's results suggested that applying UAOD as a pre-treatment considerably reduced the processing time and better retention of the pineapple properties.