Drying Kinetics and Quality of Grapes ( Vitis vinifera L. cv. Alphonse Lavallée): Needle Micro-Perforation vs. Alkaline Treatment

Grapes are among the most widely cultivated and exported fruits in Turkey. However, their high moisture and sugar contents make them highly perishable, limiting their shelf life and requiring drying as a preservation method. Hot air drying is generally preferred due to its controllable conditions and shorter processing time. To reduce quality losses during drying, chemical or physical pretreatments are commonly applied.

This study investigates needle micro-perforation as a physical alternative to the widely used alkaline dipping pretreatment in grape drying. For this purpose, Kavacık grapes (Vitis vinifera L. cv. Alphonse Lavallée) were pretreated by means of alkaline dipping and needle micro-perforation using needle lengths of 0.5, 1.0, and 1.5 mm, followed by convective drying at 60, 70, and 80 °C. Micro-perforation was applied to the entire surface of the grapes, with an average pore diameter of 0.46 mm. Drying behavior was modeled using Fickian and Non-Fickian diffusion models, and physico-chemical properties such as water activity, rehydration ratio, pH, titratable acidity, and color were analyzed.

Drying kinetics showed that the Non-Fickian model better fitted the data, indicated by higher R² and lower RMSE and χ² values. The results also revealed super-diffusion in alkaline-dipped and sub-diffusion in micro-perforated grapes. Micro-perforated grapes exhibited a water activity of 0.44 ± 0.05, rehydration ratio of 2.00 ± 0.07, pH of 4.34 ± 0.07, titratable acidity of 0.94 ± 0.16 g tartaric acid/100 g, and ΔE of 4.33 ± 1.41. At 60 °C, both pretreatments resulted in statistically similar physico-chemical characteristics (P > 0.05). Since similar drying behavior and physico-chemical properties were obtained, needle micro-perforation can be considered a potential alternative to alkaline dipping.