Drying Kinetics and Mathematical Modelling of Raisin Production by Abrasive and Chemical Pre-Treatment of Grapes

One of the delicious dry fruit, raisin is obtained by drying of grapes with pre-drying treatment. Chemical pre-treatment is most commonly followed for the raisin processing. However, it has drawback like extended drying time and it also triggers the food safety concerns. Various alternatives are being explored for the production of safe raisins. In the present investigation, drying kinetics of grapes and modelling of raisin production have been studied. Grapes were pre-treated with conventional chemical method and novel abrasive method and compared. Treated grapes were dried in tray dryer and solar dryer. Drying kinetics and modelling of experimental data was examined with ten empirical drying equations. Drying rate was higher for grapes treated with abrasive method compared to chemical pre-treatment. Among the different drying conditions, tray drying (55°C) and solar drying (temperature range 43.33 to 58.33°C) was found suitable for superior quality raisin production with drying time of 56.66 h and 78.5 h, respectively. The browning index and firmness of raisins produced using abrasive pre-treatment was to some extent higher compared to chemical treatment whereas yellowness index was found slightly lesser in case of abrasive treated samples. The Modified Handerson-Pebis Model fitted best to predict the drying of abrasive treated grapes, whereas Two term model reported highest goodness of fit for chemically treated grapes. Effective diffusivity for moisture removal of grapes ranged between 1.03×10−9 to 5.52×10−10 m2/s and was maximum for abrasive treated grapes. Present findings of drying behaviour of grapes and modelling will be useful for the better design of drying systems for production of safe raisins using novel abrasive pre-treatment.