Changes in Phenolic Composition and Antioxidant Activity of Virgin Olive Oil during Frying

Abstract

The concentration of hydroxytyrosol (3,4-DHPEA) and its secoiridoid derivatives (3,4-DHPEA-EDA and 3,4-DHPEA-EA) in virgin olive oil decreased rapidly when the oil was repeatedly used for preparing french fries in deep-fat frying operations. At the end of the first frying process (10 min at 180 degrees C), the concentration of the dihydroxyphenol components was reduced to 50-60% of the original value, and after six frying operations only about 10% of the initial components remained. However, tyrosol (p-HPEA) and its derivatives (p-HPEA-EDA and p-HPEA-EA) in the oil were much more stable during 12 frying operations. The reduction in their original concentration was much smaller than that for hydroxytyrosol and its derivatives and showed a roughly linear relationship with the number of frying operations. The antioxidant activity of the phenolic extract measured using the DPPH test rapidly diminished during the first six frying processes, from a total antioxidant activity higher than 740 micromol of Trolox/kg down to less than 250 micromol/kg. On the other hand, the concentration of polar compounds, oxidized triacylglycerol monomers (oxTGs), dimeric TGs, and polymerized TGs rapidly increased from the sixth frying operation onward, when the antioxidant activity of the phenolic extract was very low, and as a consequence the oil was much more susceptible to oxidation. The loss of antioxidant activity in the phenolic fraction due to deep-fat frying was confirmed by the storage oil and oil-in-water emulsions containing added extracts from olive oil used for 12 frying operations.

Genesis and development of DPPH method of antioxidant assay

“…The modified method introduced of Brand-Williams et al (1995) has been extensively used (Gomez-Alonso et al 2003;Yepez et al 2002). This suggested the oversimplification of the interpretation by Blois (1958) and that because of the complexity of the reactions that follow the Eq.…”

“…The 1 max , used for the absorbance measurements are 515 nm (Gomez-Alonso et al 2003;Lebeau et al 2000), 516 nm (Schwarz et al 2001), 517 nm (Lu and Foo 2000;Zhu et al 2002), 518 nm (Leitao et al 2002) and 520 nm (Kim et al 2002). A reaction time of 30 min was followed by Kim et al (2002).…”

Oleocanthal: A Naturally Occurring Anti-Inflammatory Agent in Virgin Olive Oil

“…In general, research has shown that olive oil phenolic compounds are subject to degradation upon the application of heat during cooking (Brenes et al, 2002, Gomez-Alonso et al, 2003. However, oleocanthal has demonstrated to be relatively stable upon heating when the EVOO contains a considerable quantity of this compound initially.…”

Effect of frying on the rheological and chemical properties of palm oil and its blends

“…Fats and oils are largely responsible for the pleasant creamy taste of foods; therefore, its physiological characteristic is important (Belitz et al 2009). Frying causes oil to undergo a wide range of reactions, namely hydrolysis, oxidation, isomerization, and polymerization due to high temperature and absorption of water and oxygen (Go'mez-Alonso et al 2003;Gloria and Aguilera 1998). The deteriorated oil has nutritional and toxicological implication on human health since the toxic substances may accumulate in oil through the food stuff fried in it; final products fried in deteriorated oil possess unpleasant taste, odor and texture (Tena et al 2009).…”