EFFECTS OF ULTRASOUND PRE-TREATMENT ON THE COLOUR CHANGES KINETICS OF KIWIFRUIT SLICES DURING OSMOTIC DEHYDRATION AND HOT-AIR DRYING

Abstract

In this study, kiwifruit slices were osmotically dehydrated in different hypertonic sucrose solutions  and assisted with or without ultrasound. Sucrose concentrations (in three levels of 20, 30, and 40 °Brix), sonication power (in three levels of 0, 75 W, and 150 W), and treatment time (in 8 time intervals: 10, 20, 30, 40, 50, 60, 70, and 80 min) were the factors investigated concerning colour parameters including a* (redness: green to red), b* (yellowness: blue to yellow), L* (lightness), and ?E (total colour difference), and also surface area changes of the kiwifruit slices during osmotic dehydration (OD). As the sonication power (40 kHz) increased from 0W to 150W, the lightness index values of kiwifruit slices reduced from 64.8 to 60.6 (30°Brix). Also, with increasing sucrose solution concentration from 20°Brix to 40°Brix the lightness index values of kiwifruit slices decreased from 61.8 to 59.3 (power=150W). The results showed that the mean of ?E index values of dehydrated kiwifruit slices increased during OD period. The highest ?E index (8.03) and lowest surface area (8.64 cm²) values were for the kiwifruit slices treated by the lowest ultrasound power (0W) and the highest sucrose solution (40°Brix). The average surface area of fresh kiwifruit slices, dehydrated samples, hot-air drying samples, and rehydrated samples were 9.50 cm², 9.05 cm², 6.44 cm², and 7.82 cm², respectively. The results illustrated that the MMF (Morgan-Mercer-Flodin) equation was the best equation to explain the ?E index changes of untreated and treated kiwifruit slices during OD (the average R²=0.995, the average SSE=0.171, and the average RMSE=0.177).