Fast and Accurate Determination of Olive Oil Acidity by Electrochemical Impedance Spectroscopy

Abstract

Virgin olive oil is an important ingredient of the Mediterranean diet highly appreciated for its beneficial health effects due to the presence of mono-unsaturated fatty acids and antioxidant properties. The olive oil quality is defined by many parameters, among which acidity and peroxide index and according to international regulations, these two determinations are carried out in laboratory environment, by means of manual titration. This kind of method, however, cannot be used on oil production sites, where, instead, it would be desirable to know the main characteristics of the oil for proper product classification.In this paper a new technique to measure olive oil acidity by electrical conductance is presented that allows for fast, costeffective and on-site measurements. Such a technique has been validated with a set of 55 olive oil samples. Two different oil emulsions have been used and compared for the measurements: based on a hydro-alcoholic solution (60% ethanol 40% distilled water) and distilled water, respectively. The data show that the electrical conductance of the emulsion with hydro-alcoholic solution is a strong function of the olive oil acidity, that can be estimated with good accuracy (R 2 = 0.9308). The experiments with distilled water, instead, exhibit no significant correlation between oil acidity and emulsion conductance that, however, is found to provide information about peroxide index, polyphenols content and filtration technique.Furthermore, using both types of emulsions to compensate for conductance variations other than free fatty acids the accuracy in acidity determination can be improved reaching R 2 = 0.9786.

Electrical impedance spectroscopy (EIS) for biological analysis and food characterization: a review

“…A different approach was used by Grossi et al (2014a) to measure the acidity of olive oil samples (Valli et al, 2016). In this case, the SUT was mixed with an hydroalcoholic solution (60 % ethanol, 40 % distilled water) to create an emulsion.…”

“…In the experiments, 55 olive oil samples with free acidity in the range 0.2-2.3 % were tested and a good correlation (R 2 = 0.9308) was found with the acidity values obtained with the reference titration Figure 7. Procedure to estimate the free acidity in olive oil using the technique discussed in Grossi et al (2014a). method. This technique was also implemented in the form of a portable electronic system suitable for in situ measurements in oil mills and packaging centers (Grossi et al, 2013b(Grossi et al, , 2014b.…”

“…Electrical impedance spectroscopy (EIS) is a powerful technique (Barsoukov and Macdonald, 2005) that can be used in a broad range of applications, such as microbiological analysis (Ramirez et al, 2008), food products screening (Grossi et al, 2011a(Grossi et al, , 2014a, corrosion monitoring (Zhu et al, 2016), quality control of coatings (Amirudin and Thieny, 1995) and cement paste (Christensen et al, 1994), characterization of solid electrolytes (Rafiuddin, 2016) and human body analysis (Clemente et al, 2013).…”

Time resolved study of hydroxyl radical oxidation of oleic acid at the air-water interface

“…Being surfactants, they tend to exist at oil-water or air-water interfaces. They widely occur in many kinds of oils 1,2 and living organisms. 3,4 Due to this ubiquity, fatty acids such as oleic acid (OA) are found on the surfaces of atmospheric aerosols formed by sea spray [5][6][7][8] and result from anthropogenic activities like cooking.…”

Determination of the acid values of edible oils via FTIR spectroscopy based on the OH stretching band

“…To overcome the major disadvantages of this chemical method, such as being time-consuming and labour-intensive, requiring large amounts of organic solvents, and difficulty in distinguishing the end-point with samples containing coloured substances, some improved methods have been developed, including automatic photometric titration (Crispino & Reis, 2013), flow injection analysis Saad, Ling, Jab, & Lim, 2007), and potentiometric titration (Osawa, Goncalves, & Ragazzi, 2006). Various instrumental methods have also been used to determine the AVs of edible oils, including gas chromatography (Tan, Ghazali, Kuntom, Tan, & Ariffin, 2009), high-performance liquid chromatography (Li et al, 2011), capillary electrophoresis (Balesteros et al, 2007), electrochemical impedance spectroscopy (Grossi, Lecce, Toschi, & Ricco, 2014), ultraviolet-visible spectroscopy (Zhang et al, 2015), Raman spectroscopy (Muik & Lendl, 2003), 1 H NMR (Satyarthi, Srinivas, & Ratnasamy, 2009;Skiera, Steliopoulos, Kuballa, Diehl, & Holzgrabe, 2014), infrared spectroscopy, and near-infrared spectroscopy (Adewale, Mba, Dumont, Ngadi, & Cocciardi, 2014;Cantarelli, Funes, Marchevsky, & Camiň a, 2009;Ng, Wehling, & Cuppett, 2007).…”