Enhanced Grape Drying Using Indirect Solar Dryers: Improved Quality and Safety of Raisins

Abstract

The production of raisins, a method of grape preservation since antiquity, has evolved with various drying techniques that significantly impact the quality and safety of the final product. This study evaluates the efficacy of a solar indirect dryer compared to traditional sun-drying methods for drying Centennial Seedless and Sultanina grape cultivars in Crete, Greece. Key parameters assessed include environmental conditions, drying time, grape color, fungal contamination, and Ochratoxin A (OTA) levels. Grapes were dried in a controlled solar chamber and under open sun conditions. The solar chamber maintained higher average temperatures (34 °C) and lower relative humidity (39.7%) than outside conditions (24.2 °C and 58.7%, respectively), significantly reducing the drying time from 12 to 7 days. Raisins dried in the solar chamber exhibited improved color quality, with higher Lightness (L*), Hue Angle (h), and Chroma (C*) values, attributed to minimized enzymatic and nonenzymatic browning. Mycological analysis revealed a substantial reduction in Aspergillus section Nigri contamination in chamber-dried raisins, with mean colony-forming units per gram significantly lower than those of sun-dried raisins. Consequently, OTA levels were also significantly reduced in chamber-dried raisins, with Centennial Seedless showing a mean concentration of 1.01 µg/kg compared to 2.66 µg/kg in sun-dried samples, and Sultanina showing 0.70 µg/kg versus 2.05 µg/kg, respectively. These findings underscore the advantages of using solar indirect dryers to enhance drying efficiency, improve color quality, and reduce fungal and OTA contamination, highlighting the importance of adopting controlled drying technologies for safer, higher-quality raisins.