Drying rates of Rubi grapes submitted to chemical pretreatments for raisin production

Abstract

The objective of this work was to determine the most appropriated chemical treatment to be used to dry grapes cv. Rubi for raisin production. Drying curves for convective drying with air at 50^oC, in a tray drier, were obtained for grapes submitted to chemical pretreatments with different concentrations of potassium carbonate and olive oil, and different dipping times, according to factorial designs. Convective drying curves were also obtained for grapes pretreated in aqueous suspensions of soybean lecithin, at varied lecithin concentrations and dipping times. Page model was adjusted to the experimental drying curves, and the calculated drying times showed that the best pretreatment consisted in dipping grapes for 2 minutes in a 5% olive oil and 6% K₂CO₃ emulsion, at 50^oC, which resulted in a drying time close to that of the pretreatment with 2.5% of olive oil, but with a lower consumption of this substance. In addition, the immersion of grapes in an aqueous suspension of 2% soy lecithin, at 50^oC, for 5 minutes, resulted in a total drying time slightly higher than the most effective pretreatment.