Drying characteristics and quality of grape under physical pretreatment

• Wax abrasive pretreatment on drying kinetics of raisins at 40–70 °C was investigated.
• Abrasive pretreatment decreased of about 1/3 the drying time.
• Raisins at 50 °C were less shrinked, with best rehydration capability and color.

Grape drying is a slow and energy intensive process because the waxy peel has low permeability to moisture. Therefore, peel chemical and physical pretreatments are considered before drying in order to facilitate water diffusion. However, they cause heterogeneity in the waxes removal and problems during shelf-life.In this paper an alternative abrasive pretreatment of grape peel, for enhancing the drying rate and preserving the samples, was applied to Red Globe grapes. Convective drying experiments were carried out at 40–70 °C and at 2.3 ms−1 air velocity. The effect of wax abrasive pretreatment on the drying kinetics and quality parameters of raisins was investigated. The results were compared with those of samples pretreated by dipping in alkaline ethyl oleate solution and untreated grapes. All the dried samples are darker than fresh one and shrunked. The samples pretreated by peel abrasion and dried at 50 °C showed the lowest color changes, less shrinkage and the best rehydration capacity. The drying kinetics and shrinkage curves were also analyzed using some commonly available empirical models.