Application of HPLC coupled with DAD, APcI-MS and NMR to the analysis of lutein and zeaxanthin stereoisomers in thermally processed vegetables

A method for the simultaneous determination of lutein and zeaxanthin stereoisomers by HPLC-DAD was developed. For this purpose, ( Z )-isomers of lutein and zeaxanthin were prepared by iodine-catalyzed photoisomerization and their structures elucidated by 1D- and 2D-LC-NMR spectroscopy, by APcI-MS in the positive mode, and by UV/Vis spectroscopy. Near baseline separation was achieved for (13- Z )-lutein, (13′- Z )-lutein, ( all - E )-lutein, (9- Z )-lutein, (9′- Z )-lutein, (13- Z )-zeaxanthin, ( all - E )-zeaxanthin, and (9- Z )-zeaxanthin. The influence of selected thermal treatments on degradation and isomerization of lutein and zeaxanthin was assessed. Sweet corn and spinach were sterilized ( T max = 121 °C, F = 5) and blanched ( t = 2 min, steam), respectively. Heating resulted in decreases in total lutein content in sweet corn by 26% and in spinach by 17%. Total zeaxanthin content in sweet corn decreased by 29%. The amount of ( Z )-isomers of lutein and zeaxanthin increased in sweet corn from 12% to 30% and 7% to 25%, respectively, whereas in fresh spinach a decrease in lutein ( Z )-stereoisomers from 21% to 14% was observed.