Food products are complex mixtures consisting of naturally occurring compounds with nutritional value and contaminating substances, generally originating from technological processes, agrochemical treatments, or packaging materials. Since the exact content of a food product must be assessed before it can be put on the market, routine and/or specialized analysis protocols have been pointed out for the characterization of all compounds present in food and beverages. The main approaches involve separation methods (e.g., chromatography), spectroscopic and biologically derived protocols. By developing efficient methodology with high reproducibility and low detection limits, high quality and safety standards can be achieved to fit the developed government regulations.
1.1 Introduction
Their nutritional and health-related properties make food and beverages highly important products able to provide humans with different biologically active compounds [1, 2]. In the last few years, production, collection, storage, and distribution of food and beverages have been significantly influenced by technological and scientific developments with considerable advantages for both food quality and safety [3–5].
Food products are complex mixtures consisting of naturally occurring compounds with nutritional value (e.g., lipids, carbohydrates, proteins, vitamins, phenolic compounds, organic acids and aromas) [6] and contaminating substances (e.g., pesticides, polycyclic aromatic hydrocarbons, chlorinated and brominated compounds, veterinary drugs, toxins, mutagenic compounds, metals, and inorganic compounds), generally originating from technological processes, agrochemical treatments, or packaging materials [7–9].
The exact composition (in terms of both natural components and contaminants) must be assessed before a foodstuff can be put on the market, and several limitations are imposed by national and international control agencies in order to assure safety and quality, and to avoid frauds [10–12]. For this reason, routine and/or specialized analysis protocols have been pointed out for the characterization of all compounds present in food and beverages [13–15]. Various types of methods, including microbial methods, sensory analysis, biochemical and physicochemical methods, are used in food analysis. Spectroscopic and chromatographic methods have become very popular for separation and identification of food components due to their high reproducibility and low detection limits [16]. Similarly, biologically based assays, including polymerase chain reaction (PCR) techniques, and immunological-based methods, are also used for detection of specific targets in food samples [17].
This chapter focuses on the principal instrumental techniques proposed in food and beverage analysis.
1.2 Chromatographic Techniques in Food Analysis
Separation techniques, such as gas chromatography (GC), liquid chromatography (LC), and capillary electrophoresis (CE), have largely been used for analysis of compounds in food samples [18]. The complexity of food matrices often requires not only extensive sample preparation, but also online coupling techniques, which are used for their superior automation and high-throughput capabilities.
Many detectors with different types of selectivity can be used in gas chromatography. A first classification can be done in terms of detected compounds. Nonselective detectors are able to detect all compounds except the carrier gas, selective detectors respond to a range of compounds with a common physical or chemical property, while specific detectors are able to detect a single chemical compound. Detectors can also be grouped into concentration- and mass-flow-dependent detectors. The signal from a concentration-dependent detector is related to the concentration of solute in the detector, and does not usually destroy the sample, while mass-flow-dependent detectors usually destroy the sample, and the signal is related to the rate at which solute molecules enter the detector.
The coupling of separation techniques in tandem with mass spectrometry (MS) or high-resolution MS (time-of-flight) is a valuable tool for enhancing the selectivity and sensitivity of a detection system, giving precise information on the identity of compounds [19].
Nowadays, gas chromatography coupled to mass spectrometry (GC-MS, GC-MS/MS) with electron impact ionization is a routine technique for analysis of nonpolar, semipolar, volatile and semivolatile food compounds such as polycyclic aromatic hydrocarbons, pesticides and dioxins [20, 21].
In contrast, for polar and nonvolatile substances, LC is the technique of choice, with increased application over the last few years [22]. Detection by liquid chromatography can be carried out in different ways, highly affecting its applicability to food analysis. Ultraviolet (UV) detection has been mostly used as well as mass spectrometers or refractive index (RI) detectors [23, 24]. Mass spectrometers are sensitive and universal detectors, but they are expensive; moreover, both UV and MS detectors suffer from non-uniform responses due to differences in absorptivity and ionization efficiencies as a function of chemical structure, respectively. Most types of MS can be used to analyze food components, including triple quadrupole, quadrupole time-of-flight, LTQ Orbitrap, ion trap, and magnetic sector mass spectrometers. Others, such as RI detectors, provide a more universal response but only for moderately high concentrations. Moreover, RI detectors are relatively insensitive and incompatible with gradient elution and difficult to stabilize. In recent years, LC coupled to an evaporative light-scattering detector (ELSD) has represented a useful alternative [25]. The use of an ELSD approach for spectrophotometric derivatization (i.e., insertion of chromophoric groups) is feasible and therefore the drawbacks of derivatization (e.g., dependence on experimental parameters, incompleteness of derivatization reaction, use of salt-laden mobile phases, prolonged analysis time, additional cost for derivatization system and reagents) can be eliminated [26]. LC coupled to ELSD was successfully proposed to determine lipids and biogenic amines in different food matrices [27]. Within the past decade, the introduction of ultra-high pressure liquid chromatography (UHPLC) and rapid-scan and sensitive MS instruments has resulted in a seismic shift away from traditional chromatographic techniques towards multiclass, multiresidue methods with short injection cycle times and minimal sample preparation [28]. Comprehensive methods for some of the more important contaminant groups in residue analysis have been developed for UHPLC-MS/MS, including anthelmintics, β-agonists, steroids, quinolones and others. Expected future developments include the possibility to analyze larger numbers of classes of compounds; the use of ever higher temperatures and pressures to create more effective separation methods; and further reduction in sample preparation via online solid-phase extraction and other techniques, to increase the speed of analysis beyond current standards.
The use of supercritical fluids is another useful technology attracting increased interest from researchers in the food sector. Carbon dioxide is the most commonly used supercritical fluid, because it is nontoxic, nonexplosive, and the experimental conditions required are easily achievable, since the critical temperature and pressure are, respectively, 31 °C and 73 bar [29]. Supercritical fluid chromatography (SFC) was initially performed with pure CO2 as the mobile phase, but nowadays SFC is very often carried out under subcritical conditions because CO2 is modified with an organic modifier or additive in order to increase the solubility of polar compounds [30]. In contrast to LC, SFC allows the use of higher flow rates with lower pressure falls through the column, leading to greater efficiency in short analysis times and reduced consumption of organic solvents. This implies sharper peaks, improved resolutions and faster methods due to the shorter times for column equilibration. Moreover, it offers the possibility of analyzing thermally labile and polar compounds which cannot be analyzed by GC without derivatization. Traditionally, SFC applications have been focused on lipid compounds, which could be due to the high solubility of these analytes in supercritical CO2, while more recent studies have shown their suitability for analyzing more polar compounds such as amino acids or carbohydrates [31].
Miniaturized separation techniques, such as electromigration methods (capillary electrophoresis, CE, and capillary electrochromatography, CEC), are alternative methodologies to GC and LC with a big potential regarding analysis time and costs, and offering different advantages, e.g., minor quantities of solvents, stationary phases and samples,...
