Evaluation of the ‘antioxidant power’ of olive oils based on a FIA system with amperometric detection Saverio Mannino,* Susanna Buratti, Maria Stella Cosio and Nicoletta Pellegrini Dipartimento di Scienze e Tecnologie Alimentari e Microbiologiche, Università degli Studi di Milano, Via Celoria 2, 20133 Milan, Italy Received 15th March 1999, Accepted 14th May 1999 A new method for the evaluation of the ‘total antioxidant power’ of olive oils, based on a flow injection analysis system with electrochemical detection, is described. It represents an attractive alternative to the mostly used Rancimat method since it is based on the chemical structure of antioxidants and does not require the manipulation of several parameters, such as temperature and oxygen pressure, to accelerate oil oxidation. The proposed procedure is simple, rapid, allows a throughput of 90 samples h 21 and provides a good precision: an RSD of 3.5% was obtained for caffeic acid at the concentration level of 5 mg L 21 (n = 12). A comparison of the proposed procedure with two other methods (Rancimat method and ABTS •+ decoloration assay) was performed to investigate the applicability and limitations of the proposed method. Oxidative rancidity is one of the most critical factors affecting the shelf-life of processed food. Autoxidation of fatty acids, which takes place in the presence of oxygen, produces some unstable compounds that can modify the sensory and nutritional characteristics of foods. Natural antioxidants can protect food by preventing oxidative deterioration of lipids, and can play an important role in the prevention of certain diseases such as cancer, vascular diseases and hypertension. 1 Among natural antioxidants present in olive oils, the most important are polar compounds including phenolic acids (cinnamic and benzoic derivatives), 3,4-dihydroxyphenyletha- nol (hydroxytyrosol), 4-hydroxyphenylethanol (tyrosol) and oleosidic forms of hydroxytyrosol and tyrosol (oleuropein and ligstroside). 2 Tocopherols are also natural antioxidant com- pounds found in olive oils, although their role in the oxidative stability of oils is not clear. For instance, a-tocopherol, the best known antioxidant present in extra-virgin olive oils, has been reported to function as a pro-oxidant in purified soybean oil at concentrations above 250 ppm. 3 In another study, 4 a-tocopherol showed antioxidant activity at 100, 500 and 1000 ppm, but the lowest concentration was the most effective. Considering the interest in the use of natural antioxidants to improve the oxidative stability of food lipids, the capability of measuring the antioxidant capacity is a relevant task in food chemistry for the prediction of product shelf-life, and also for the assessment of antioxidant effectiveness. In order to evaluate the resistance of oils and fats to oxidation, the usual procedure is an accelerated stability test in which samples are subjected to various oxidising conditions and a suitable end-point is chosen to determine the oxidative deterioration. The induction period (IP), expressed as the time required to reach the end-point of oxidation corresponding to either a level of detectable rancidity or a sudden change in the rate of oxidation, is generally used as an index of resistance to oxidation. These accelerated stability tests can be classified as methods using light and metal catalysed oxidation, 5 the weight- gain method, based on an increase in weight due to oxygen absorption, 6 and high-temperature tests including oxygen bomb, the active oxygen method (AOM) and the Rancimat method. 7 Although useful for practical purposes and widely adopted, these procedures have been questioned with regard to the severity of the oxidation conditions used and published data comparing the effectiveness of various antioxidants are often difficult to interpret owing to the different test conditions. 8 In this paper, a procedure to evaluate the antioxidant capacity of olive oils based on their electrochemical properties is reported. This procedure, similarly to that recently published for the evaluation of wine antioxidant power, 9 offers a significant improvement over the commonly used methods since it is rapid, simple, does not require severe oxidative conditions and allows quantitative measurement of the antioxidant power of olive oils. Experimental Chemicals Tyrosol, caffeic acid and other phenolic acids were purchased from Sigma Chemical Co. (St. Louis, MO, USA), oleuropein from Extrasynthese (Genay, France), a-tocopherol from Sigma, chloroform and glacial acetic acid from Merck (Darmstadt, Germany) and tetrabutylammonium bromide and 2-hydroxy- 2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) from Sigma. Electrochemical method The FIA apparatus consisted in a Jasco (Tokyo, Japan) Model 880 PU pump and an EG&G Princeton Applied Research (Princeton, NJ, USA) Model 400 thin-layer electrochemical detector equipped with a single glassy carbon electrode (surface area 8 mm 2 ) operating at a potential of +0.5 V, a reference (Ag/ AgCl saturated) electrode and a platinum counter electrode. The connecting tubes were of PEEK (1.16 3 0.005 in id). Data were recorded using a Philips (Eindhoven, The Netherlands) PM 8252 recorder. Flow injection experiments were performed at room temperature using a chloroform carrier solution saturated Analyst, 1999, 124, 1115–1118 1115