Nitrogen-doped Graphene Quantum Dots as a Novel Pre-Harvest Tool for Enhancing Raisin Quality and Yield Across Different Drying Ecologies

Improving our understanding of the effects of nitrogen-doped graphene quantum dots (N-GQDs) on raisin quality and yield is essential for developing innovative approaches in viticulture. However, knowledge about how N-GQDs applications during grape development and ripening stages influence raisin characteristics remains limited. We investigated the effects of three repeated treatments of 0 (Control), 0.10 and 0.25 g L⁻¹ N-GQDs on bunch and berry characteristics of raisins produced using traditional drying techniques from two cultivars: cv. Ekşi Kara (traditional, sun-dried) and cv. Gök Üzüm (shade-dried after dipping in a hot dipping solution). Cultivar genotype and N-GQDs concentration significantly influenced all measured parameters (p ≤ 0.05). Treatments were evaluated by analysing cluster and berry size, berry volume, skin thickness, °Brix, pH, titratable acidity (TA), total phenolic content (TPC), antioxidant activity (AA), color, pedicel separation force, skin deformation force, and raisin yield. Principal Component Analysis (PCA) revealed that the first two principal components explained 62.4% and 23.9% of the total variance, respectively, demonstrating clear differentiation among treatments. N-GQDs treatments resulted in significant improvements across all parameters examined, with cultivar differences playing a significant role in treatment efficacy. Notably, 0.25 g L⁻¹ N-GQDs in cv. Ekşi Kara and 0.10 g L⁻¹ N-GQDs in cv. Gök Üzüm yielded the highest raisin yield increases of 7.94% and 6.30%, respectively, alongside improved antioxidant activity and total phenolic content. We conclude that repeated N-GQDs treatments during the grape growth season represent a promising strategy for producers seeking higher yields and for consumers seeking raisins with improved nutritional value. The 0.10 and 0.25 g L⁻¹ N-GQDs concentrations identified in this study can be recommended as initial applications for raisin varieties grown in different ecologies, with potential for further optimization to achieve even superior results.