Analysis of the legislated metals in different categories of olive and olive-pomace oils

doi:10.1016/j.foodcont.2010.07.002

Food Control

Volume 22, Issue 2, February 2011, Pages 221-225

https://doi.org/10.1016/j.foodcont.2010.07.002 Get rights and content

Abstract

The determination in olive oils of the legislated metals (As, Cu, Fe and Pb) by the International Olive Oil Council (IOC) has been carried out by inductively coupled plasma mass spectrometry (ICP-MS) following microwave digestion with nitric acid. The method has been validated using an oil reference material and by recovery experiments, obtaining in both cases satisfactory results. The detection limits achieved are 0.9, 1.5, 3.0 and 40 ng g⁻¹ for Pb, Cu, As and Fe, respectively. These limits are much lower than the Maximum Residue Levels permitted. The four trace metals have been determined in different categories of olive oil fit for consumption by IOC (extra virgin olive oil, virgin olive oil, olive oil and pomace-olive oil) observing a high content of Cu in pomace-olive oil. To the best of our knowledge, this is the first time that the content of the legislated metals is studied in the categories of olive oil fit for consumption.

Introduction

The greatest exponent of vegetable monounsaturated fat is olive oil, and it is a prime component of the Mediterranean diet. Virgin olive oil is a natural juice which preserves the taste, aroma, vitamins and properties of the olive fruit. The beneficial health effects of this oil, including its cholesterol-lowering effect, are due to both its high content of monounsaturated fatty acids, mainly oleic acid, and its high content of antioxidative substances, in contrast to animal fats that are predominantly saturated and hence do not react readily with other chemicals, especially oxygen.

When evaluating the quality of olive oil, parameters such as free fatty acid content, peroxide value, spectrophotometric absorption at selected wavelengths (K₂₃₂ and K₂₇₀) and organoleptic characteristics are considered. Moreover, another important quality criteria is its metal content. Trace levels of metal ions, such as Fe and Cu, are known to have adverse effects on the oxidative stability of olive oil.

Other elements such as As, Cr, Cd and Pb are very important on account of their toxicity and metabolic role because they catalyse the decomposition of hydro-peroxides, aldehydes, ketones, acids and epoxides. These compounds may develop pathological effects on the digestive system and increase carcinogenic effect by reacting with other food components such as proteins and pigments.

The presence of metals in edible oils may be due to different factors: natural contamination (they arrive in the plants via deposition as well as bioaccumulation from the soil due to natural metal sources and environmental pollution), introduction of the metals during the refining process (not used for obtaining virgin olive oils), and contact with the storage material. Some of these metals may be harmful if present in the final product, even at low concentrations, having adverse effects on the oxidative stability of the oils. As a result, the International Olive Council (IOC) has established as quality criteria a Maximum Residue Level (MRL) for the content of several metals in both olive oils and olive-pomace oils: 0.1 μg g⁻¹ for As, Cu and Pb; and 3 μg g⁻¹ for Fe (IOC, 2009). Hence, it is important to monitor and control the presence of these metals in the oil.

Oils are particularly difficult to analyze for their trace metal content, as some of them are present at very low concentration levels. Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) (Anthemidis et al., 2005, Fischer and Rademeyer, 1994, Zeiner et al., 2005), Atomic Absorption Spectrometry (AAS) (Cindric et al., 2007, Karadjova et al., 1998, Zeiner et al., 2005) and Potentiometry (La Pera et al., 2002, La Pera et al., 2003, Lo Coco et al., 2003) are the most commonly used techniques for the determination of metals in olive oils up to date.

ICP-MS has also been employed for the determination of several metals in olive oils (Benincasa et al., 2007, Castillo et al., 1999, Jiménez et al., 2003), but none of the published methods focuses on determining the metals legislated by the IOC. In addition, only virgin olive oil has been studied in previously published papers. Nevertheless, there are different categories of olive oil according to IOC, where free fatty acid content expressed as percentage of oleic acid (w/w) is one of the main criteria used to establish this classification. Among them, extra virgin olive oil, virgin olive oil, olive oil and olive-pomace oil are available in the market.

In this paper, we propose an ICP-MS method for the quantitative analysis of legislated metals (As, Cu, Fe, Pb) in olive and olive-pomace oils fit for consumption, making use of microwave digestion for the sample treatment. The official methods are based on a colorimetric procedure for determining As (AOAC International, 2000), while AAS is used for the quantification of Cu, Fe and Pb (Firestone, 2005, International Organization for Standardization, 1994). These procedures are not suitable for process control purposes due to their time-consuming and mono-element analysis characteristics. For this reason, ICP-MS is an interesting tool, not only due to its high sensitivity, but also because it allows easy multi-element analysis.

The determination of these legislated metals in an oil reference material and recovery studies over different categories of olive oils have been developed in order to validate the proposed method. Finally, the method has been applied to different edible olive oils collected from Spanish supermarkets in order to evaluate the concentration of the legislated metals and the potential difference in the content of metals according to the quality of the olive oil categories.

Section snippets

Instrumentation

The quadrupole inductively coupled plasma mass spectrometer used in this work was an Agilent 7500a (Agilent Technologies, CA, USA) equipped with a Babington nebuliser, a Peltier-cooled quartz spray chamber and a standard torch (2.5 mm i.d.). Before each experiment, the ICP-MS was tuned using an aqueous multi-element standard solution (Agilent, Madrid, Spain) of 10 ng mL^–1 each of Li, Y, Co, Ce and Tl for consistent sensitivity (⁷Li, ⁸⁹Y and ²⁰⁵Tl) and minimum doubly charged and oxide species

Results and discussion

Although virgin olive oil is generically cited in most of the published papers regarding metal content in edible oils, it is worthy mentioning that there are different categories of olive oils fit for consumption. In addition, pomace-olive oil is also available in the markets of most countries. Hence, we will make a brief description of the elaboration of the different categories of olive oils taking into account the classification provided by the IOC.

Virgin olive oil is obtained solely from

Conclusions

In this paper, the determination of the legislated metals in olive oil by IOC using microwave digestion followed by ICP-MS analysis has been performed. Some advantages of the proposed method include the small amount of reagent used (5 mL of HNO₃) that significantly reduces potential environmental contamination, and shorter treatment time in comparison to traditional methods. This is the first time that the different categories of olive oil are studied and interesting results have been found.

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Analysis of the legislated metals in different categories of olive and olive-pomace oils

Abstract

Introduction

Section snippets

Instrumentation

Results and discussion

Conclusions

Analytica Chimica Acta

Analytica Chimica Acta

Microchemical Journal

Food Control

Microchemical Journal

Determination of copper, lead, and nickel in edible oils by plasma and furnace atomic spectroscopies

JAOCS, Journal of the American Oil Chemists’ Society

Official method 952, 13

Semiquantitative simultaneous determination of metals in olive oil using direct emulsion nebulization

Journal of Analytical Atomic Spectrometry