Ascorbic acid-catalyzed degradation of cyanidin-3-O-beta-glucoside: Proposed mechanism and identification of a novel hydroxylated product

by Stebbins, Nathan B.; Howard, Luke R.; Prior, Ronald L.; Brownmiller, Cindi; Liyanage, Rohana; Lay, Jackson O.; Yang, Xiaoyu; Qian, Steven Y.

BACKGROUND: Many brightly colored fruits and vegetables owe their pigmentation and beneficial health effects to anthocyanins. Unfortunately, anthocyanins in the presence of ascorbic acid are readily degraded over juice processing and storage, which adversely affects color stability and potential health benefits. OBJECTIVE: This project focused on the effect of ascorbic acid as a catalyst in anthocyanin degradation. METHODS: The project involved searching for novel pigmented compounds in a simple model system composed of the most common anthocyanin cyanidin-3-O-beta-glucoside and ascorbic acid, and a second model system consisting of blackberry extract supplemented with ascorbic acid. Degradation products were identified by HPLC-PDA and HPLC-MS. ESR was used to monitor hydroxyl radical formation in the model systems. RESULTS: Over 72 hours at ambient temperature, 67% of cyanidin-3-O-beta-glucoside was lost in the model system during which time an unknown pigmented compound was formed. The unknown compound was also formed in a more complex model system consisting of blackberry extract and ascorbic acid. HPLC with PDA monitoring at 510 nm was used to detect a novel compound and HPLC-ESI-MS3 allowed a proposed structure to be built based on the fragmentation patterns. The unknown structure formed via oxidation of cyanidin 3-O-beta-glucoside by ascorbic acid was identified as 6-hydroxy-cyanidin-3-O-beta-glucoside. The mechanism was substantiated with malvidin-3-O-beta-glucoside and ascorbic acid, which produced a hydroxylated malvidin-3-O-beta-glucoside. Production of hydroxyl radical in the base and blackberry model systems was confirmed by ESR. CONCLUSIONS: We propose that the pigmented compound is formed from hydroxyl radicals via the Haber-Weiss reaction. The addition of food grade hydroxyl radical scavengers to juices may be a viable treatment to prevent ascorbic acid-catalyzed degradation of anthocyanins.

Journal of Berry Research
Start Page
1878-5123; 1878-5093