Degradative pro-vitamin A active compounds of all-trans-β-carotene in dehydrated dark green leafy vegetables

Abstract

Dark green leafy vegetables (DGLV) are rich source of pro-vitamin A carotenoids, with all-trans-b-carotene as the main compound contributing over 90% of the vitamin A content. The other pro-vitamin A carotenoids present in DGLV are the cis isomers of b-carotene; the 9-cis and the 13-cis, and a-carotene in some vegetables. The dehydration processes of freeze-, solar- and sun-drying resulted in all-trans-b-carotene undergoing isomerization and oxidation to produce cis isomers and monoepoxides of b-carotene, which are pro-vitamin A active, and some volatile compounds. The isomerization process results in the reduction of the relative proportions of all-trans isomer and an increase in the relative proportion of the cis isomers. Oxidation of all-trans-b-carotene induced the formation of vitamin A active epoxides; 5,6- and 5,8-monoepoxides of b-carotene as intermediate products that decompose to smaller volatile compounds. The epoxides were detected in low but sometimes in measurable amounts on some dehydrated and/or stored vegetable samples. The vitamin A active degradative compounds of all-trans-b-carotene were monitored using isocratic HPLC procedures. The factors influencing degradation of pro-vitamin A carotenoids during dehydration are discussed.