Oxidative stability of edible vegetable oils enriched in polyphenols with olive leaf extract

Commercially available oils (olive oil, sunflower oil, palm oil, and a vegetable shortening) were enriched in polyphenols, by adding olive leaf extract. Addition of the extract was performed in such way that the oils were enriched with 200 mg/kg polyphenols. Total polyphenols of both enriched and commercial oils were estimated by the Folin-Ciocalteau assay, while identification and quantification of individual simple phenolic compounds was performed by GC/MS and of oleuropein by HPLC analysis. The enrichment resulted in the supplementation of the commercial oils mainly with oleuropein, hydroxytyrosol, and quercetin. Antioxidant capacity and oxidative stability of the enriched oils and the commercial ones were assessed by the 1,1-diphenyl-2-picrylhydrazyl radical (DPPH center dot) scavenging assay and the Rancimat method, respectively. Both antioxidant capacity and oxidative stability were substantially improved for all the oils studied after supplementation. By the procedure adopted, oils rich in polyphenols, mainly in oleuropein, can be produced with olive leaf extract supplementation.