Drying Grapes for Raisin Production

Objective: The aim of this study was to investigate the influence of bleaching grapes prior to drying for raisin production. Theoretical Framework: Removing the waxy coating from the surface of the grapes reduces their resistance to heat and mass transfer. Method: Candy Crunch™ grapes, produced by Outro Chão – Agricultura Biológica; Lisbon – Portugal, were blanched for 2 minutes in water at a temperature of 92.0ºC and their drying performance was verified by comparing them with fresh grapes. The grape moisture content was obtained in an oven at 105ºC, achieving an average of 82.1% (wb) and a sphericity of 0.83. The average initial masses of six blanched and unblanched grapes, before drying, were: 42.28 and 40.24 g. Results and Discussion: After drying, under the same time and operating conditions, the average masses were: 24.88 and 34.90 g. Therefore, the mass reduction after drying was 41.3% and 13.3% for the blanched and unblanched grapes, respectively. Research Implications: There was a 30% reduction in the heat exchange area and mass of the grapes at the end of drying when blanching is carried out beforehand; for unbleached grapes, the area reduction was 9%. Originality/Value: Considerations are made regarding the heat and mass transfer coefficients, comparing the start of drying with the result after 340 minutes of processing. These results are important in grape processing studies and necessary for dryer design.