Food analysis detection

M. Cai eii and A. Mangia, Multidimensional detection methods for sepai ations and their application in food analysis . Trends Anal. Chem. 15 538-550 (1996). 

On-line coupled LC-GC methods have been developed in food analysis for several reasons, i.e. lower detection limits can be reached, the clean-up is more efficient, and large numbers of samples can be analysed with a minimum of manual sample preparation in shorter times. 

Fluorescence is much more widely used for analysis than phosphorescence. Yet, the use of fluorescent detectors is limited to the restricted set of additives with fluorescent properties. Fluorescence detection is highly recommended for food analysis (e.g. vitamins), bioscience applications, and environmental analysis. As to poly-mer/additive analysis fluorescence and phosphorescence analysis of UV absorbers, optical brighteners, phenolic and aromatic amine antioxidants are most recurrent [25] with an extensive listing for 29 UVAs and AOs in an organic solvent medium at r.t. and 77 K by Kirkbright et al. [149]. 

Immunosensors have been developed commercially mostly for medical purposes but would appear to have considerable potential for food analysis. The Pharmacia company has developed an optical biosensor, which is a fully automated continuous-flow system which exploits the phenomenon of surface plasmon resonance (SPR) to detect and measure biomolecular interactions. The technique has been validated for determination of folic acid and biotin in fortified foods (Indyk, 2000 Bostrom and Lindeberg, 2000), and more recently for vitamin Bi2. This type of technique has great potential for application to a wide range of food additives but its advance will be linked to the availability of specific antibodies or other receptors for the various additives. It should be possible to analyse a whole range of additives by multi-channel continuous flow systems with further miniaturisation. 

HPLC analysis of food proteins and peptides can be performed for different purposes to characterize food, to detect frauds, to assess the severity of thermal treatments, etc. To detect and/or quantify protein and peptide components in foods, a number of different analytical techniques (chromatography, electrophoresis, mass spectrometry, immunology) have been used, either alone or in combination. The main advantages of HPLC analysis lie in its high resolution power and versatility. In a single chromatographic run, it is possible to obtain both the composition and the amount of the protein fraction and analysis can be automated. 

Vitamin D2 and D3 exhibit identical UV absorption spectra and they do not possess fluorescence. Electrochemical detection is limited and only few methods are applied in food analysis [530,533], MS detection has been applied achieving satisfactory detection limit (10 mol/mL) [534,535],... 

Caboni, M.F. and Rodriguez-Estrada, M.T. 1997. High-performance liquid chromatography coupled to evaporative light scattering detection in lipid analysis Some application. Seminars in Food Analysis 2 159-169. 

Detection of amino acids is typically by UV absorption after postcolumn reaction with nin-hydrin. Precolumn derivatization with ninhydrin is not possible, because the amino acids do not actually form an adduct with the ninhydrin. Rather, the reaction of all primary amino acids results in the formation of a chromophoric compound named Ruhemann s purple. This chro-mophore has an absorption maximum at 570 nm. The secondary amino acid, proline, is not able to react in the same fashion and results in an intermediate reaction product with an absorption maximum at 440 nm. See Fig. 5. Detection limits afforded by postcolumn reaction with ninhydrin are typically in the range of over 100 picomoles injected. Lower detection limits can be realized with postcolumn reaction with fluorescamine (115) or o-phthalaldehyde (OPA) (116). Detection limits down to 5 picomoles are possible. However, the detection limits afforded by ninhydrin are sufficient for the overwhelming majority of applications in food analysis. 

In general it has to be stated that molecular species analysis of phospholipids is not frequently applied in food analysis most of the studies involving molecular species are instead found in the fields of biochemistry and nutrition. Thus, in the recent reviews by Bell and by Olsson and Salem, special emphasis has been given to the characterization of biological tissue samples (83,84). However, the molecular species composition has been shown to affect the accuracy of the quantification of phospholipid classes and hence is important in food analysis too (47,52). In the vast majority of published methods, isocratic elution has been used. In our opinion, this should be ascribed mainly to the fact that the traditional UV detector remains. Keeping account of the inherent problems associated with UV detection of underivatized phospholipids, it is astonishing that ELSD has hardly been exploited in this subdomain. As far as the stationary phase is concerned, nearly all methods prefer octadecyl-coated stationary phases. 

In food analysis, sensitivity is not the only requirement for analytical method development. Besides confirmation of the identity of pesticides, the identification of nontarget analytes is also important. One powerful tool is LC/MS, especially when it is combined with appropiate sample-treatment procedures it allows one to obtain detection limits adequate for trace-level analysis. Liquid chromatography-MS has demonstrated that it is an effective way to obtain both qualitative and quantitative information. 

Various HPLC methods that have been reported for the determination of NOC in foods can be divided into three categories (a) those relating to the development of methodology, (b) those actually using such methods for food analysis, and (c) those of a confirmatory nature in which an HPLC method has been used to confirm GC-TEA findings. Weston in New Zealand used HPLC-TEA for the determination of NDMA in milk powder (87) and malt and beer (88). Dennis et al. (89) and Sen et al. (90) also used HPLC-TEA methods for the determination of V-nitrosamino acids, NMA, and other NOC in Icelandic smoked mutton. More recently, Sen et al. (85) used both HPLC-TEA and an HPLC-postcolumn chemical denitrosation technique (method h in Table 2) for detecting the presence of two V-nitroso-tetrahydro-/3-carbolines in several nitrosated samples... 

The first edition of Food Analysis by HPLC fulfilled a need because no other book was available on all major topics of food compounds for the food analyst or engineer. In this second edition, completely revised chapters on amino acids, peptides, proteins, lipids, carbohydrates, vitamins, organic acids, organic bases, toxins, additives, antibacterials, pesticide residues, brewery products, nitrosamines, and anions and cations contain the most recent information on sample cleanup, derivatization, separation, and detection. New chapters have been added on alcohols, phenolic compounds, pigments, and residues of growth promoters. 

Stability, duration, sensitivity, interference, and availability of substrates to contact enzymes are the criteria for the success of an enzyme sensor. These criteria depend on sources of enzymes, immobilization techniques, and transducers used. Food matrices are much more complicated than the clinical samples, hence, these criteria become extremely important for the application of the enzyme sensor in food analysis. An extensive list of the response time, detection limits, and stability of biosensors was summarized by Wagner (59). 

However, it should be mentioned that there is a flexible hand-held electrochemical instrument on the market, which can be programmed to be used in a variety of voltammetric/amperometric modes in the field [209]. Although the majority of biosensor applications described in this review were for single analyte detection, it is very likely that future directions will involve development of biosensor arrays for multi-analyte determinations. One example of this approach has been described in an earlier section, where five OPs could be monitored with an array of biosensors based on mutant forms of AChE from D. melanogaster [187]. This array has considerable potential for monitoring the quality of food, such as wheat and fruit. Developments and applications of biosensors in the area of food analysis are expected to grow as consumer demand for improved quality and safety increases. Another area where biosensor developments are likely to increase significantly is in the field of environmental analysis, particularly with respect to the defence of public... 

The microstructure of bread and other microporous foods can be conveniently studied by applying synchrotron radiation X-ray microtomography (X-MT) (Falcone et al., 2004a Maire et al., 2003) to centimeter- or millimeter-sized samples (Lim and Barigou, 2004). X-MT application only requires the presence of areas of morphological or mass density heterogeneity in the sample materials. The use of this technique for food microstructure detection is of recent date. It was traditionally used for the analysis of bone quality (Peyrin et al., 1998, 2000 Ritman et al., 2002). 

Similar to tumor markers discussed above, a number of hazardous proteins are not immediately linked to an accompanying nucleic acid and therefore are prime targets for IPCR. These are interesting examples of how the clinical importance of IPCR is not limited to the diagnosis of diseases. IPCR was reported as a useful tool for the prevention of intoxications or infections because of highly sensitive detection of potentially dangerous compounds, especially in food analysis. 

As sample extraction and sample handling are of general consideration for the more exotic biological matrices often found in food analysis, the application of IPCR in the research project MYCOPLEX [89], founded by the European Union, promises interesting new developments. This project is dedicated to the detection by IPCR of ochra- and aflatoxins in milk and coffee, focusing on sensitivity and simplified antigen extraction by dilution of the samples. 

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