Retention and distribution of natural antioxidants (α-tocopherol, polyphenols and terpenic acids) after shallow frying of vegetables in virgin olive oil

Potatoes, green peppers, zucchinis and eggplants were shallow fried in virgin olive oil (VOO) according to the Mediterranean traditional culinary practice. Zucchinis and eggplants were also blanketed with wheat flour or batter prior to frying. Polyphenols and hydroxy pentacyclic triterpene acids (HPTAs) were determined by GC/MS, while α-tocopherol was determined by high-performance liquid chromatography. Among 12 polyphenols determined, tyrosol predominated in frying oils and zucchini samples, while chlorogenic acid was the major phenolic species in the other vegetable samples. The triterpene acids maslinic, oleanolic and ursolic were determined in frying oils and fried vegetables, while α-tocopherol was present in all samples. Besides water loss and oil absorption, shallow frying resulted in partial loss of all the antioxidants studied in frying oils and enrichment of fried vegetables with olive oil antioxidants, which was in some extent affected by the type of vegetable fried and the culinary practice followed. The overall retention of the antioxidants in oil and food ranged from 32% to 64% for α-tocopherol, 25% to 70% for polyphenols and 35% to 83% for HPTA. It appears that vegetables fried in VOO provide an additional intake of α-tocopherol, terpenic acids and polyphenols as tyrosol and chlorogenic acid.