An opto-electronic system for in-situ determination of peroxide value and total phenol content in olive oil
Introduction
Olive oil is a vegetable lipid obtained by extraction process from olives (the fruits of Olea europaea L., family Oleaceae) highly appreciated for its beneficial effects on human health, mainly due to a high content of oleic acid and phenolic compounds (Tulipani et al., 2012). Clinical studies provide evidence that regular olive oil consumption reduces the risk of coronary heart diseases (Keys et al., 1986), oxidative damage to DNA and RNA (Machowetz et al., 2007) and Alzheimer disease (Abuznait et al., 2013, Monti et al., 2011).
Olive oil quality is related to its chemical composition, oxidative stability and sensory characteristics. Quality parameters, such as free acidity, peroxide value, UV extinction coefficients, fruity attribute, other sensory characteristics and defects, are strongly dependent on olives’ ripeness (Rotondi et al., 2004) and processing technology in the olive mills (Boselli et al., 2009). In addition, the peroxide value, defined as milliequivalent of active oxygen per kilogram of oil (meq O2/kg oil) and qualifying the oil primary oxidation, is also related to storage conditions (oxygen, light exposure and temperature) after production. Another important quality parameter is the amount of phenolic compounds that contribute to the oil sensory taste producing a distinctive bitter and a pungent perception (Gutierrez-Rosales et al., 2003). Phenolic compounds found in olive oil are principally secoiridoids (oleuropein and ligstroside isomers) and their derivatives, such as tyrosol and hydroxytyrosol, that exhibit a strong antioxidant activity: they act as free radicals traps protecting from heart disease and displaying anticancer activity (Notarnicola et al., 2011, Zanoni, 2014). Phenolic compounds are also largely responsible for the shelf-life of the oil (Lerma-Garcia et al., 2009).
The European Commission regulation No. 2568/91 and subsequent amendments define manual titration methods to measure acidity and peroxide value in olive oil (EEC 2568, 1991), to be carried out in a laboratory environment by trained personnel. Instead, no official determination is currently established for the total phenol content, usually determined using spectrophotometry or high performance liquid chromatography (HPLC), techniques requiring expensive instrumentation, a laboratory environment (IOC/T.20/Doc No 29, 2009, Tasioula-Margari and Okogeri, 2001) as well as preventive extraction of the polyphenols.
From the production point of view, the need to ship oil samples to a laboratory for analysis leads to high costs and long delays. Therefore, simple and fast techniques useable for on-site quality control are desirable, in particular for small oil mills and packaging centers. For this reason, innovative solutions have been proposed, such as: Near-InfraRed (NIR) spectroscopy (Armenta et al., 2007, Ozdemir and Ozturk, 2007) to estimate acidity and peroxide value; Time Domain Reflectometry (TDR) to determine water content (Ragni et al., 2012) and detect adulteration (Cataldo et al., 2012) in extra virgin olive oil; Rapid Fourier Transformed Infrared (FTIR) spectroscopy (Cerretani et al., 2010) and voltammetric sensors (Rodriguez-Mendez et al., 2008) to estimate total phenol content. However, all these techniques require expensive instrumentation and/or need frequent calibration for olives of different varieties, country of origin and harvest season.
As viable alternatives, amperometric and pH-metric techniques have been proposed to measure peroxide value (Kardash-Strochkova et al., 2001, Adhoum and Monser, 2008) and total phenol content (Capannesi et al., 2000), but these methods are still at research stage and have been validated only on small amounts of samples in laboratory environment. Moreover, some techniques use toxic compounds (such as chloroform) to increase oil solubility in reagents, unsuitable for use in normal working environment.
Recently, we have proposed a novel technique based on Electrical Impedance Spectroscopy to measure olive oil acidity that is fast (response time in about 30 s) and can be easily implemented in the form of a low-cost portable embedded system (Grossi et al., 2013).
To complete this work, we here present a simple and effective technique to measure peroxide value and total phenol content in olive oil that, as will be shown, is fast, accurate and can be implemented in the form of a low-cost embedded electronic system.
Section snippets
Technique
The technique used in this work is based on the creation of an aqueous emulsion between the oil sample and a chemical reagent. The optical density (OD) of such an emulsion is determined by illuminating the sample with a LED and measuring the transmitted light through the sample with a photodiode. A large set of experimental results show a good correlation between the measured OD and the quality parameters determined by reference methods. The proposed technique is suitable to be implemented in
Results and discussion
The reagents response was initially characterized with peroxide and phenolic compounds using a SmartSpec 3000 spectrophotometer. Then a set of 25 olive oil samples have been analyzed with the technique and the bench-top set-up described in Section 2.2. Finally an electronic board has been designed and fabricated to avoid the use of all bench-top instrumentation, thus demonstrating the feasibility of a simple and economical instrument for easy, fast and in-situ analysis of olive oil.
Conclusions
A novel technique to measure peroxide value and total phenol content in olive oil has been presented that is based on optical density measurements of a suitable reagent inoculated with the olive oil of interest. The technique, suitable to be realized in the form of a low-cost, embedded electronic system, has been tested using an experimental set-up built with bench-top instrumentation and the results show that it can estimate with good accuracy the peroxide value (R2 = 0.883) and the total phenol
Acknowledgements
This work has been financially supported by the CESAR Project, RIDIIT program, funded by the Ministry of Economic Development (Italy).
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