Sustainability of calcined oyster shell powder as a natural preservative for postharvest green onion (Allium fistulosum)

Calcined oyster shell powder (COSP), a natural antibacterial agent derived from marine waste, was used in this study to treat postharvest green onions (Allium fistulosum), which are susceptible to microbial contamination during the washing process after harvesting. The green onions were dipped in COSP solutions of varying concentrations (0%, 2%, 4%, 6%, and 8%). This study investigated the effects of COSP treatment on the physicochemical properties and microbial activities of green onions. Compared to untreated green onions, COSP-treated green onions exhibited an extended shelf life to 28 days, evidenced by reduced weight loss by up to 9.84%, reduced the total bacterial count by up to 1.20 Log₁₀ CFU/g, stabilized pH level (5.28 ± 0.38–6.06 ± 0.20), preserved chlorophyll a and b content (0.40 ± 0.01–0.46 ± 0.01 and 0.17 ± 0.003–0.22 ± 0.007, respectively), enhanced antioxidant activity (DPPH scavenging activity up to 12.47%, ABTS value up to 82.94%), and better color appearance. Furthermore, COSP treatment was effective in preserving allicin (35.03 ± 0.77–45.07 ± 2.37%) content and inhibiting the degradation of cycloalliin (13.58 ± 0.08–16.48 ± 0.15 g/kg dry weight) and methiin (17.14 ± 0.10–19.41 ± 0.47 g/kg dry weight) in green onions, thereby preserving their fresh aroma. This study offers a novel postharvest preservation method for green onions using marine waste-derived COSP dipping, which helps reduce the loss of quality in postharvest products.