Optimized Copper Pretreatment Protocols for Premium Green Raisin Production via Vacuum Impregnation and Metallo‐Complex Stabilization

Green raisin production is limited by rapid chlorophyll degradation during drying. This study evaluated copper metallo‐complex formation to stabilize green pigmentation in Vitis vinifera cv. Thompson Seedless grapes. Pretreatments with copper sulfate (CuSO 4 ) pentahydrate (2%–6% w/v) were applied via dipping and vacuum impregnation (2–10 min), followed by drying at 50°C–70°C for 6–10 h. A Taguchi orthogonal array design (25 trials) enabled simultaneous optimization of process parameters. Vacuum impregnation with 6% CuSO 4 for 4 min, combined with drying at 65°C–68°C for 8–9 h, yielded optimal outcomes: superior color retention, reduced browning index (BI ≈ 123.9 vs. 134.7 in dipping, p < 0.05), 78%–82% moisture reduction, and improved ascorbic acid preservation (2.18 ± 0.12 mg/100 g vs. 1.96 ± 0.15 mg/100 g). Mechanistic analysis confirmed copper–chlorophyll complexes of pheophytin and pyropheophytin as stabilizers of greenness. Compared to dipping, vacuum impregnation achieved deeper ion penetration, improved pigment stability, and enhanced physicochemical attributes (higher TSS, lower acidity, and elevated pH). These findings establish vacuum impregnation with copper pretreatment as a scalable protocol for producing nutritionally superior, visually appealing green raisins. While physicochemical improvements are clear, confirmation of consumer preference and export‐market acceptance requires targeted sensory and market studies.