Mechanism of red pigment formation in lotus rhizome soup during cooking: The role of polyphenols, iron and organic acids.

Abstract

Lotus rhizome soup, renowned as a unique delicacy made from lotus rhizome, is cherished by many consumers. However, the varying colors of soup are sure to influence individuals' appetites. This research focused on the Miancheng lotus Rhizome (MLR) and Elian No. 6 lotus rhizome (ELR) to investigate the relationship between the variations in polyphenols, organic acids and iron levels in MLR and ELR soups during the cooking process. The findings indicated that cooking MLR for 12 min resulted in a red soup with a* value of 8.65 ± 0.12, whereas the soup made from ELR remained white with a* value of 3.43 ± 0.08. The correlation analysis results indicated that polyphenols, specifically epigallocatechin (0.0169 ± 0.00029 μg/100 mL FW after cooking for 12 min) and epicatechin (0.0211 ± 0.00047 μg/100 mL FW after cooking for 12 min), exhibited a significant positive relationship with a* (p < 0.05). Moreover, lowering the pH, removing polyphenols and incorporating metal-chelating agents can also prevent the development of red pigment. The analysis from HPLC-MS, UV-Vis, FT-IR spectra and ESI-Q-TOF-MS indicated that the development of the red soup color involved i) the generation of epicatechin gallate through the enantiomeric reaction of epigallocatechin, ii) the co-chromic red interaction between anthocyanin cations and catechol structures, and iii) the formation of polyphenol oligomers (i.e., procyanidin A2, (+)-procyanidin B2, procyanidin C1 and prodelphinidin B4) due to heating, while the creation of phenol-iron chelates could inhibit the development of red coloration. In sum, this research introduces a new idea for managing the color of lotus rhizome soup and similar soup products.