A novel photometric method for evaluation of the oxidative stability of virgin olive oils.(FOOD COMPOSITION AND ADDITIVES)(Report)

The Oxitester method, a novel, simple, and fast photometric method for the evaluation of the antioxidant capacity of olive oils, was validated and compared to the official oil stability index (Rancimat) method. The Oxitester method appeared to be a good alternative to the Rancimat method with adequate correlation for a wide range of virgin olive oil samples, including extrissima virgin olive oils (correlation coefficient 0.88), and extra virgin olive oils of increased acidity (free fatty acids >0.45%, correlation coefficient 0.89). Other quality factors (flavor, free fatty acids content, specific absorbance at 270 and 232 nm, peroxide value, and content of oleic, linoleic, and linolenic acids) were also measured and correlated to the antioxidant capacity values of the Oxitester and Rancimat methods. The Oxitester method, in contrast to the Rancimat method, was indicative of the flavor characteristics of the olive oils and the content of linolenic acid.

Virgin olive oil (VOO), themajor edible vegetable oil of Mediterranean countries, is totally obtained from the olive fruit (Olea europaea L.) solely by mechanical means (i.e., water washing, filtration, pressure, decantation, or centrifugation), without other further treatment (1). In recent years, as a result of the growing body of evidence of VOO beneficial health effects, the VOO-rich Mediterranean-style diet is highly promoted. In conjunction with its great gastronomic characteristics, VOO has been established as a widespread food product of great value (2, 3).

The quality of VOO is determined by flavor criteria, as stated by the International Olive Oil Council (IOOC), and by several physical and chemical parameters, as indicated by the European Community Codex (EC-2568/91 and its amendments). The highest grade is designated as extra-VOO, which corresponds to the oil that is produced in controlled mild temperature, with limited oxidative or other degradation (1, 4).

Apart from the above standards, which suggest or enforce limits for quality and authenticity criteria, the overall quality of VOO is further highly dependent on its stability over time (shelf life). The evaluation of VOO shelf life seems to be a rather complicated issue, as long as it is a multivariable dependent parameter. The stability of VOOs is mainly attributed to their characteristic fatty acid composition (low unsaturation), but in order to estimate the overall stability behavior, the determination of several stability factors, such as the free fatty acid content, the peroxide value, the specific absorbance, the organoleptic score, and the phenolic compounds, is required (5, 6).

The abundance of oleic acid (56-84%), a monounsaturated fatty acid (18:1, n = 9), is a unique feature of VOO among other vegetable oils. Linoleic acid, a diunsaturated fatty acid (18:2, n = 6), is present in concentrations between 3 and 21%, and the linolenic acid (18:3, n = 3) between 0.4 and 0.9% (7, 8). The main antioxidants of VOO are carotenes and phenolic compounds (polyphenols), including lipophilic and hydrophilic phenols (9). Although the lipophilic phenols, among which are tocopherols, can be found in other vegetables oils, the profile of VOO hydrophilic phenols is unique, with a well-established antiradical activity (10, 11).

Polyphenols is a broad term used in the natural products literature to define substances that possess a benzene ring bearing one or more hydroxy groups, including functional derivatives (12). Phenolic acids with the basic chemical structure of [C.sub.6]-[C.sub.1] (benzoic acids) and [C.sub.6]-[C.sub.3] (cinnamic acids), such as caffeic, vanillic, syringic, p-coumaric, o-coumaric, protocatechuic, sinapic, and p-hydroxybenzoic acid, were the first group of polyphenols observed in VOO (13-15). The prevalent phenols in VOO, however, are the secoiridoids that are characterized by the presence of either elenolic acid or elenonic acid derivatives in their molecular structure. These compounds (e.g., oleuropein, demethyl oleuropein, ligstroside) are derivatives of the secoiridoids glucosides of olive fruits. The breakdown products of the 2 major phenolic constituents of the olive fruit, oleuropein and ligstroside, form the majority of the phenolic fraction (16).

For the prediction of the overall olive oil resistance to oxidation under storage conditions, the oil stability index (OSI) method [commonly called Rancimat method, American Oil Chemists' Society (AOCS) Cd 12b-92] is the most widely used. In the OSI method, purified air is passed through the heated sample. The effluent air from the oil sample is then bubbled through a vessel containing deionized water, in which the conductivity is continuously monitored. The OSI is defined as the time, expressed in hours, that is needed to reach the maximum change of conductivity (due to volatile organic acids swept from the oxidizing oil). Other available methods are the Swift test (International Union of Pure and Applied Chemistry 2.506), the Active oxygen method (AOCS Cd 12-57), the Oven storage test for accelerated aging of oils (AOCS Cg 5-97), the Accelerated light exposure of edible oils (AOCS Cg 6-01), and several others of minor use (17, 18). Apart from the Rancimat method, none of the previous methods is supported by a commercially available instrument, and therefore a significant variance is observed among independent laboratories or analysts. Furthermore, a common drawback of the previous methods is the prolonged analysis time and the elevated temperature of the test (accelerated degradation), which may result in deviations from the ambient-shelf mechanism of autoxidation (19).

A different way to estimate the olive oil shelf life is the determination of individual classes of antioxidants. Most …