Comparison of methods for the hydrolysis of flavonoids and phenolic acids from onion and spinach for HPLC analysis

For the quantitative determination of individual flavonoid glycosides in plant materials, the glycosides are normally hydrolysed and the resulting aglycones are identified and quantified. However, the hydrolysis conditions which result in optimal breakdown of glycosides are too harsh for some of the other phenolic compounds present in the same plant material. Therefore, the effects of different hydrolysis conditions and different antioxidants on pure flavonoid glycones and aglycones were studied. On the basis of the results obtained with standards, suitable hydrolysis methods for red spring onion and spinach were developed. The best results from these vegetables were obtained by refluxing at 80 °C for 2 h with 1.2 M HCl in 50% aqueous methanol with addition of 2 mg ascorbic acid as an antioxidant. The method developed in this study is suited to the screening of flavonoids in vegetables and leafy vegetables.