Variation in ultrasonic frequency and time as pre-treatments to air-drying of carrot

Vegetable drying is an alternative method to curb post harvest decay of vegetables and a process to produce dried vegetables, which can be directly consumed or used as ingredients for the preparation of soups, stews, baby foods, cakes, puddings, and many other foods. In this study, the effect of ultrasound frequency and sonication time as pre-treatment prior to air drying at 70°C at an air velocity of 0.5 m/s, on carrot drying kinetics, flavour, colour, and non-enzymatic browning was investigated using a 3-level factorial response surface method. The result showed that an increase in sonication frequency and time did not significantly increase moisture diffusivity but in comparison with the control, water diffusivity increased after ultrasound application and the overall drying time was reduced. Application of the ultrasound pre-treatment in distilled water resulted in water gain and sugar loss, indicating that the ultrasonic pre-treatment can be an important step to produce low sugar content dried products. The brightness and redness to yellowness values of the ultrasonically pre-treated dried carrots were better than those without ultrasound application. In comparison with the flavour of the control, the ultrasound pre-treated samples recorded higher flavour response signals, indicating that the application of ultrasound improved the flavour of the dried carrot. The variation in sonication frequency and time did not significantly affect the non-enzymatic browning index of the dried products but were better than the control.