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 (Tmax = 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.