Impact of Postharvest Oxalic Acid Treatments on Quality Properties and Biochemical Composition of Medlar (Mespilus germanica L.) Fruit

This study was conducted to investigate the effects of oxalic acid (OA) applications at different concentrations on the quality traits and biochemical components of medlar (Mespilus germanica L.) fruit. Fruits harvested in Bolu province in 2024 were subjected to control (0) and 0.5, 1.0, and 1.5 mM OA applications and stored at 4°C and 90% ± 5% relative humidity for 45 days. Samples were taken from the fruits at certain intervals (0, 15, 30, and 45 days) and analyzed. Specifically, 1.0 and 1.5 mM OA applications significantly reduced fruit weight loss and decay rates. OA also inhibited increases in fruit respiration rate and pH and slowed the decline in total acidity, soluble solids content (SSC), vitamin C, phenolic compounds, and organic acids. Malic, citric, succinic, tartaric, and fumaric acids, vitamin C, and important phenolic compounds (chlorogenic acid, gallic acid, quercetin, catechin) were protected by OA applications, and this protection was most pronounced at 1.5 mM OA dose. The correlation analyses revealed significant relationships between quality parameters and biochemical contents. The weight loss and decay rate were negatively correlated with bioactive compounds such as gallic acid and ascorbic acid, indicating that quality deterioration progressed in parallel with the decrease in nutrient content. A strong positive correlation was found between titratable acidity and malic and citric acids. In principal component analysis (PCA), malic and citric acids were prominent in the early storage period, while weight loss, decay, and increased respiration became dominant in the late storage period. Phenolic compounds decreased significantly with storage time, and antioxidant capacity decreased. Heat map analysis visually supported the increase in spoilage parameters with the decrease in ascorbic acid and phenolic compounds. OA applications maintained the quality of medlar fruit and increased biochemical stability, extending postharvest life. 1.5 mM OA, in particular, stands out as an effective postharvest preservation method. These results demonstrate that OA offers significant potential for increasing the commercial value of perishable fruits like medlar and for developing preservation technologies.