Research background. The worldwide demand for healthy and sulphur-free dried vegetables and fruits has grown. Combined ultrasound-assisted osmotic dehydration (UOD) and application of active coatings incorporating natural preservatives represents an attractive alternative for sulphuring process to preserve the sensorial and nutritional quality of dried fruits. The aim of this study was to investigate the effect of osmotic dehydration (OD) and UOD and the use of pectin coatings (alone or with citric acid, CA or ascorbic acid, AA) on physical, textural and microstructural properties of hot air-dried apricot.
Experimental approach. Fresh apricot cubes (1 cm3) were pre-treated with either OD at a temperature of 55 °C for 30 and 45 min or UOD at two ultrasonic frequencies of 25 and 35 kHz for 30 and 45 min followed by application of active coatings with pectin alone, pectin + CA or pectin + AA for 10 min. All pre-treated coated samples were then hot-air dried at a temperature of 60 °C until a final moisture content of 20 % (wet basis) was reached. Physical (shrinkage, apparent and bulk densities), chemical (browning value, water activity), textural (firmness and shrinkage), microstructure and microbial load of dried apricot was studied.
Results and conclusions. Application of OD and UOD improved physical and textural properties of the dried apricots. Moreover, apparent and bulk densities, rehydration capacity of OD and UOD pre-treated samples were increased. While, shrinkage, water activity and microbial load were decreased. Firmness of UOD pre-treated samples was significantly (p < 0.05) lower than that of OD ones. Likewise, increasing ultrasound frequency from 25 to 35 kHz led to a significant decrease in Fmax values of dried apricots. Furthermore, coating of the OD and UOD processed samples with pectin + CA increased Fmax and decreased rehydration capacity of dried apricots. Scanning electron microscopy of both OD and UOD samples illustrated improvement of textural properties. The utilization of both OD pre-treatment and pectin edible coatings resulted in a decrease in browning values. However, UOD increased browning values of the dried apricots. Coating of UOD samples with pectin + AA resulted in substantial discoloration in hot air-dried apricot.
Novelty and scientific contribution. This study advances the knowledge in the field of fruit drying by combined application of OD or UOD pre-treatments with post-treatments with active edible coatings on different properties of hot-air dried apricot.
Effect of ultrasound-assisted osmotic dehydration pretreatment on the drying characteristics and quality properties of Sanhua plum (Prunus salicina L.)