Synergistic oxalate-phosphate complex inhibits blue stain and preserves freshness of postharvest lotus rhizomes

Lotus rhizome is valued for its nutritional benefits but often suffers from non-enzymatic blue stain, which deters consumers. To combat this, we developed a blue stain inhibitor (BSI) complex containing 10 g/kg oxalic acid, 10 g/kg NaH 2 PO 4 , 5 g/kg ascorbic acid, 10 g/kg tea saponin and 10 g/kg polyethylene glycol (PEG-400). This study examined the effects of BSI on the preservation and prevention of blue stain in lotus rhizomes. Compared to the control group (CK1), BSI significantly slowed the reduction in L * and b * values and the increase in ΔE ( p <0.05). After 60 d storage, it significantly slowed down the decrease in total phenolic content (0.56 ± 0.011 g/kg, p < 0.05 vs. CK1). Additionally, BSI reduced electrolyte leakage and inhibited the growth of bacteria, molds and yeasts, demonstrating improved preservation performance. HPLC analysis showed that after 60 d of storage, the BSI-treated lotus rhizomes (G2 group) had 1.2 to 2.2 times higher levels of L-dopa (8.92 mg/g DW), gallocatechin (6.60 mg/g DW), and epigallocatechin (7.88 mg/g DW) compared to CK1 ( p < 0.05). This suggests that BSI treatment lessened polyphenol leakage and oxidation. Furthermore, the content of iron ions (Fe 3+ ) decreased significantly, likely due to the formation of (FePO 4 ·Fe 2 (C 2 O 4 ) 3 ) precipitates with C 2 O 4 2- and H 2 PO 4 - and Fe 3+ , reducing polyphenol-iron chelate formation and inhibiting blue staining. In summary, this study offers new strategies for preventing blue stain and preserving lotus rhizomes. • 1% oxalic acid and 1% NaH 2 PO 4 as new ingredients for blue stain inhibitor (BSI). • C 2 O 4 2- and H 2 PO 4 -1 can enhance removal of Fe 3+ from lotus rhizome surfaces. • The blue stain was primarily due to binding of Fe 3+ with polyphenols. • BSI treatment reduced microbial contamination and conserved visual quality.