Drying kinetics and quality of Monukka seedless grapes dried in an air-impingement jet dryer

Drying kinetics and quality of Monukka seedless grapes were investigated in an impingement dryer under different drying temperatures (50,55,60 and 65 degrees C) and air velocities (3,5,7 and 9 m s(-1)). Results indicated that the effect of drying temperature on drying time was more distinct than air velocity. The moisture effective diffusivity ranged from 1.82 x 10(-10) to 5.84 x 10(-10) m s(-2) calculated using the Pick's second law of diffusion. The activation energy determined from Arrhenius equation was 67.29 kJ mol(-1). The hardness of dried Monukka seedless grapes changed from 9.53 to 17.16 N showing an increasing trend as drying temperature increased. The retention ratio of vitamin C of the samples varied from 10.26 to 39.73% compared to the fresh one. The results also illustrated that the drying temperature was the major factor controlling the retention of vitamin C, while there was no direct correlation between air velocity and vitamin C retention. (C) 2009 IAgrE. Published by Elsevier Ltd. All rights reserved.