Effect of modified atmosphere packaging on the flavonoids and vitamin C content of minimally processed Swiss chard (Beta vulgaris subspecies cycla)

Five flavonoids from Swiss chard leaves (Beta vulgaris subsp. cycla) cultivar "green" were isolated and identified as vitexin 2''-xyloside and its 6''-malonated derivative, kaempferol 3-gentiobioside, isorhamnetin 3-gentiobioside, and isorhamnetin 3-vicianoside. 6''-Malonyl-2''-xylosyl vitexin is a new naturally occurring flavonoid. In fact, this is the first time that a flavonoid C-glycoside acylated with a dicarboxylic acid is reported in nature. The flavonoid content of fresh leaves (green cultivar) was in the range 2.4-3.0 mg/g fresh weight (f.w.). Cultivar "yellow" contained only flavone C-glycosides (2.1-2.3 mg/g f.w.), while the flavonols were not detected. Their vitamin C content was between 0.4 and 0.5 mg/g f.w., and after domestic processing an 80% loss was observed. Modified atmosphere packaging (MAP) (7% O-2 and 10% CO2) had no effect on total flavonoid content after 8 days of storage, although it increased flavonoid extraction during cooking in boiling water. In contrast, Vitamin C content decreased, especially in MAP-stored Swiss chard, to reach levels below 50% of the intial content after 8 days of cold storage.