Oxalic Acid–MAP Synergy in Maintaining Postharvest Grape Quality and Biochemical Content

This study was conducted to investigate the effects of oxalic acid (OA) and modified atmosphere packaging (MAP) applications, both alone and in combination, on the preservation of postharvest quality characteristics of grapes. All samples were stored at 4 ± 1 °C and approximately 80–85% relative humidity for 28 days. The OA + MAP treatment most effectively limited weight loss (10.49%), soluble solids content (SSC; 20.62°Bx), and decay rate (1.78%), while providing the best preservation of total phenolic content (68.25% change), antioxidant activity (53.19% change), and individual phenolic compounds at the end of the storage period. A decrease in color parameters, including L* and a* values, was observed, with OA treatment particularly prominent in preserving the red color. Principal component analysis and hierarchical heatmap analysis confirmed distinct clustering of treatments from the control, highlighting the process efficiency of packaging-based interventions. The combination of OA and MAP treatments was more effective than either treatment alone in reducing quality losses in grapes during storage. Therefore, the widespread use of OA + MAP in commercial storage and marketing processes is recommended.