Prediction of NaCl, nitrate and nitrite contents in minced meat by using a voltammetric electronic tongue and an impedimetric sensor

Abstract

A method for predicting levels of sodium chloride, sodium nitrite and potassium nitrate in minced meat by using a combination of two different electrochemical methods; namely an electronic tongue (ET) based on pulse voltammetry and electrochemical impedance spectroscopy measurements is proposed here. The measurements with the voltammetric ET were carried out on both saline solutions (brines) and minced meat, whereas the impedimetric sensor was used only in minced meat. The addition of the salts was performed following an experimental design in which a system of three compounds/three levels was established. Multivariate analysis including Cross validation and Partial Least Square (PLS) techniques were applied for data management and prediction models building. A very good prediction of the concentration of chloride was achieved, whereas the prediction for the concentration of nitrate and nitrite can be considered as moderate.

Introduction

The preservation of meat has been a constant and important concern for human race from the starts of its existence. The preservation of most of the meat products can be made by combination of both physical and chemical methods, being the salting–curing–drying as one of the most frequent combinations in the Spanish meat industry, yielding products such as ham and cured loin.

Before or simultaneously to the salting stage, usually a nitrification process occurs by the addition of nitrites, nitrates or both of them, which are added with the aim of giving a red coloration to the product as well as to inhibit the microbial growth (Marriott, Graham, Shaffer, & Phelps, 1987). Additionally, it has been reported that the effect of sodium chloride in the meat tissue depends on the salt concentration (Ruiz-Ramírez, Arnau, Serra, & Gou, 2005). For instance, the quantity of “retained” water by proteins depends of the salt concentration. It is well-known that for salt concentrations between 45 and 60 g/L the meat tissue exhibit a high swelling (Offer & Trinick, 1983), meanwhile at concentration between 100 and 200 g/L the volume of the fiber diminishes, the tissue leaks water and the proteins precipitate with the subsequent changes of the matrix (Arnau, Serra, Comaposada, Gou, & Garriga, 2007). The important role of NaCl in the technological and sensory characteristics of meat product has lead to the extensive study of the development of fast and non-destructive measurement techniques (Bertram et al., 2004, García-Breijo et al., 2008, Vestergaard et al., 2004).

In the 1950s, the Fleischverordnung (meat regulation) limited the residual amount to 100 mg sodium nitrite/kg in ready-to-eat meat products. In raw hams 150 mg NaNO₂/kg were permitted. Also nitrate restrictions were applied. This regulation was followed by the European Parliament and Council Directive 95/2/EC on food additives other than colorings and sweeteners (Directive, 1995) where indicative ingoing amount no-higher than 150 mg nitrite/kg and 300 mg nitrate/kg were permitted in practically all meat products.

In accordance with the importance of having under control the use of salt content and nitrificant salts concentrations in derivates products from meat, our aim here is to propose a rapid, simple and low-cost method for predicting levels of salts in minced meat. The method consists in the simultaneous use of different electrochemical sensors namely an electronic tongue based on pulse voltammetry and a coaxial needle electrode used for impedance spectroscopy measurements. Quantification of additive levels is expected to be achieved by the combination of electrochemical measurements with multivariate analysis.

Impedance spectroscopy allows the analysis of the properties of materials and systems by applying to them alternate electric signals of different frequencies (voltage or current) and measuring the corresponding electric output signals (current or voltage) (Bard and Faulkner, 2001, Barsoukov and Macdonald, 2001). The impedance spectroscopy has proved to be a powerful analysis tool in many fields. For instance, it has been applied before in medicine to the detection of breast cancer (Kerner, Paulsen, Hartov, Soho, & Poplack, 2002) or to the assessment of the effectiveness of Alzheimer’s disease therapeutics (Szymanska, Radecka, Radecki, & Kaliszan, 2007). It has also been broadly applied in food analysis. Characterization of bread dough fermentation process (Toyoda, Ihara, Tamaki, & Ohta, 2007), evaluation of meat quality and ageing (Chanet et al., 1999, Damez et al., 2008, Guerrero et al., 2004), prediction of lamb carcass composition (Altmann, Pliquett, Suess, & von Borell, 2004) and yogurt processing (Kitamura, Toyoda, & Park, 2000) are some of the applications of the electrochemical impedance spectroscopy (EIS) in the field of food engineering.

On the other hand, voltammetry represents an extensively used technique in analytical determinations due to its high sensitivity. The term voltammetry comprises a large range of techniques including lineal voltammetry, cyclic voltammetry, differential pulse voltammetry, stripping voltammetry, etc. Additionally voltammetric techniques have recently become popular for the design of electronic tongues using arrays of electrodes suitable for voltammetric experiments with very fine results (Holmin et al., 2001, Winquist et al., 1997). In fact voltammetry is an especially appealing technique to be used when studying systems containing redox-active chemical species. Thus, for instance, electronic tongues based on voltammetry have been applied in food quality studies, namely milk (Winquist et al., 1998, Winquist et al., 2005), wines (Arrieta et al., 2003, Parra et al., 2006), fruit juices (Gutés, Ibáñez, del Valle, & Céspedes, 2006), among other liquid samples but very few experiments have been conducted on solid samples (Rodríguez-Méndez, Gay, Apetrei, & De Saja, 2009).

Following our interest in the design of electronic tongue devices for their application in a wide range of problems (Gil et al., 2008, Labrador et al., in press, Martínez-Máñez et al., 2005) we report here the development of an electronic tongue that combine electrochemical measurements using metallic electrodes and electrochemical impedance spectroscopy, and that combination was used for the determination of the content of sodium chloride, potassium nitrate and sodium nitrite in minced meat by using a simple undemanding device. Multivariate analysis including Cross validation and Partial Least Square (PLS) techniques have been applied to treat the collected data in order to build suitable management and prediction models.