Study on the formation mechanism of blackening in damaged lotus rhizome epidermis: Effects of polyphenols and iron.

Abstract

Lotus rhizome is an important aquatic vegetable, but the blackening of lotus rhizome epidermis (LRE) seriously affects its appearance and quality, which makes lotus rhizome products unmarketable. In this study, the effects of polyphenols and iron on the LRE color were studied to explore the possible mechanism of LRE blackening. Results indicated that the measurable total phenols contents in the mud treatment (MT) group were significantly reduced, and the total iron contents were significantly increased compared with the bruised treatment group (p<0.05). The high-performance liquid chromatography results showed that the main polyphenols in LRE were dopa, gallocatechin, and catechin, as well as a small amount of catechol, epicatechin, proanthocyanidin B2, and proanthocyanidin C1. Moreover, the results of color difference and ultraviolet adsorption spectroscopy showed that there were obviously black or brown-gray of dopa (525nm), gallocatechin (504.5nm), and catechin (550 and 504.5nm) with FeCl2. The simulated system treatment of LRE further confirmed that the chromaticity effect of dopa and iron in bruised LRE was similar to that of the MT group, whereas 1% (w/w) ascorbic acid, 2% (w/w) EDTA-2Na, or 3% (w/w) citric acid could solely prohibit the blackening. This suggested that the dopa in LRE and FeCl2 in mud may mainly combine into [2(DOPA-2H+)+Fe3+]- through non-covalent interaction, which leads to the blackening of bruised LRE under neutral conditions. These results can guide the storage of lotus rhizomes and improve the development of the lotus rhizome industry.