NMR for Food Quality Control

Michael J. McCarthy, Prem N. Cambhir, and Artem G. Goloshevsky 4.7.1 

Quality of food products and the ability to guarantee the quality of a food product is becoming increasingly important in a global economy where there are multiple sources for the food product. This need to measure, control and guarantee quality has resulted in an emphasis to develop more analytical techniques/sensors to measure a product for both external and internal quality. Consider quality evaluation of fresh fruits and vegetables. 

This chapter will describe the use of nuclear magnetic resonance and magnetic resonance imaging to characterize the quality attributes of foods and for use in process optimization, shelf-life determination and component migration. 

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The second part of the book—Chapters 9-12— presents some selected applications of chemometrics to different topics of interest in the field of food authentication and control. Chapter 9 deals with the application of chemometric methods to the analysis of hyperspectral images, that is, of those images where a complete spectrum is recorded at each of the pixels. After a description of the peculiar characteristics of images as data, a detailed discussion on the use of exploratory data analytical tools, calibration and classification methods is presented. The aim of Chapter 10 is to present an overview of the role of chemometrics in food traceability, starting from the characterisation of soils up to the classification and authentication of the final product. The discussion is accompanied by examples taken from the different ambits where chemometrics can be used for tracing and authenticating foodstuffs. Chapter 11 introduces NMR-based metabolomics as a potentially useful tool for food quality control. After a description of the bases of the metabolomics approach, examples of its application for authentication, identification of adulterations, control of the safety of use, and processing are presented and discussed. Finally, Chapter 12 introduces the concept of interval methods in chemometrics, both for data pretreatment and data analysis. The topics... 

However, the sensitivity of NMR spectroscopy is the main limitation in its application in food analysis, when compared to MS techniques. The presence of toxins or pesticides in a foodstuff cannot be investigated by NMR spectroscopy, even if the increasing developments of NMR hardware are greatly improving its sensitivity. In spite of this, NMR spectroscopy turns out to be one of the most suitable techniques for food quality control and assurance, mainly because of its reproducibility and of its ability to analyse a foodstuff sample without or with minimal chemical treatment. 

Duarte, I., Barros, A., Belton, P. S., Righelato, R., Spraul, M., Humpfer, E., and Gil, A. M. (2002). High-resolution nuclear magnetic resonance spectroscopy and multivariate statistical analysis for the characterization of beer.. Agric. Food Chem. 50, 2475-2481. Duarte, I. F., Barros, A., Almeida, C., Spraul, M., and Gil, A. M. (2004). Multivariate analysis of NMR and FTIR data as a potential tool for the quality control of beer. J. Agric. Food Chem. 52, 1031-1038. 

The study of the composition of a mixture is an extremely common problem in analytical and bioanalytical chemistry. While chromatography and solvent extraction are commonly employed to simplify the analysis prior to characterization of the constituents, NMR has provided a series of tools that help in unravelling the components of complex samples, when a previous separation of the pure compounds is not feasible or complete. Thus, TOCSY, NMR diffusometry (DOSY, among all) and heteronuclear correlation experiments are widely used to this purpose, for example, for the characterization of small molecules in biologically relevant samples, such as in metabolomics,1 plant extracts analysis,2 food quality control,3 4 to name a few cases. 

Proton magnetic resonance ( H-NMR) spectra displayed in this Handbook were recorded in two different laboratories as a double check. The spectra can be used to identify additives either pure, or as constituents of polymer extracts [5]. In the researdi project AIR-9411025 [5], it has been shown that H-NMR is a powerful tool for the quality control of most food packaging plastics, specially in an industrial framework. The materials are extracted by a suitable solvent. After ev oration to dryness, the H-NMR spectrum of the extract is recorded. This spectrum can be used as a fiiigerpiint of the material. Using the data base of this Handbook and relying on experLenoe for interpretation it is possible to decide very quickly about the presence or the absence of an additive in the material. H-NMR requires quite expensive equipment, but the essential information can often be obtained in a very short time from the spectra [13-15]. This approach therefore efficiently complements methods previously developed. 

Duarte IF, Barros A, Almeida C, Spraul M, Gil AM. Multivariate analysis of NMR and FTIR data as a potential tool for the quality control of beer. J Agric Food Chem 2004 52 1031-8. 

Advanced techniques like molecularly imprinted polymers (MIPs), infrared/near infrared spectroscopy (FT-IR/NIR), high resolution mass spectrometry, nuclear magnetic resonance (NMR), Raman spectroscopy, and biosensors will increasingly be applied for controlling food quality and safety. 

NMR spectroscopy is one of the most widely used analytical tools for the study of molecular structure and dynamics. Spin relaxation and diffusion have been used to characterize protein dynamics [1, 2], polymer systems[3, 4], porous media [5-8], and heterogeneous fluids such as crude oils [9-12]. There has been a growing body of work to extend NMR to other areas of applications, such as material science [13] and the petroleum industry [11, 14—16]. NMR and MRI have been used extensively for research in food science and in production quality control [17-20]. For example, NMR is used to determine moisture content and solid fat fraction [20]. Multi-component analysis techniques, such as chemometrics as used by Brown et al. [21], are often employed to distinguish the components, e.g., oil and water. 

A. J. Charlton, W. H. Farrington, P. Brereton 2002, (Application of 1H NMR and multivariate statistics for screening complex mixtures quality control and authenticity of instant coffee), J. Agric. Food Chem. 50, 3098-3103. 

Although phospholipids are natural components of nearly all food products, the analysis of the phospholipid composition is of importance mainly in the certification and quality control of lecithins. According to the European Analytical Subgroup of the International Lecithin and Phospholipid Society (ILPS), there is an urgent need for a standard method for the determination of the PL composition, for this would allow a better characterization of lecithin and PL products (15,16). Besides, the nonavailability of good calibration standards is a major problem when comparing analytical results between companies. In order to try to solve the latter problem, the ILPS proposes a calibration standard whose composition is certified by 31P-NMR as an absolute tech-... 

Quality control and analysis in the agro-food industry by the low resolution NMR was reviewed by D. N. Rutledge [83], in which NMR method is described as faster, more precise, simpler and cheaper technique than those traditionally used. For on-line monitoring of wine fermentation, EtOH and sugar alcohol beverage and fermenting musts were determined to a precision of 0.005-0.1% for EtOH and 4 8g/L for sugar by low resolution NMR [84]. 

In the pharmaceutical industry, the techniques are being used to examine off-target effects particularly for the early identification of toxicity. MOA can be studied through metabolomics and can also be used as a quality control tool for complex mixtures such as foods or herbal medicines. Similarly, the tools and expertise of natural products chemists are essential in metabolomics, particularly in biomarker discovery (see also Volume 9). Biomarker discovery via untargeted metabolomics can lead to metabolite signatures (nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), etc.) that are not present in current metabolomics databases. This is particularly true for plant secondary metabolism studies and nonmammalian metabolites. Structure elucidation then becomes critical to understanding the metabolomics results and for biomarker development. 

Relative to most other techniques discussed in this book, NMR has found a limited number of niche applications in food analysis. For example, the determination of oils is seeds (or fat in chocolate ), based upon low resolution, solid phase NMR is well used in quality control laboratories. Actually, most apphcations that found their way in food analysis are methods based upon the differences in relaxation times of various molecules e.g. free water molecules versus bound water molecules). Consequently, for the purpose of this particular chapter, we shall discuss only two specific applications of NMR to food analysis. These examples were chosen solely to demonstrate the broad range of applications that NMR can cover and the reader is advised that the mere fact that they were selected here should not be interpreted as a judgement of their value over other related references. 

LR-NMR is the most commonly used NMR technique to date for quality control in food science and industry, with several official quality control methods in force. The success of the technique is due to several factors, including the power of the method (in terms of information and speed), the early application of the method to foodstuffs, the ecological appeal (no longer needing polluting chemicals), and the relatively low cost of the equipment, making it a very attractive alternative to the tedious wet chemical methods. 

Solid fat content (SFC) analysis is probably the most used LR-NMR application in the food industry. The initial success of the method prompted Unilever (manufacturer of margarines and related products) and Bruker (manufacturer of NMR instruments) to start a joint venture with the goal of building a table-top LR-NMR spectrometer for solid-to-liquid ratio analysis in the fat industry. The method was developed in the early 1970s and over the years leaded to various quality control protocols for fat and oils that are by now adopted as official methods by various international and national organizations. The success story of SFC analysis opened wide the door for LR NMR methods to penetrate as routine techniques in the food industry. 

In this chapter, we report the NMR-based metabolomic approach in food analysis and display its more instructive applications in quality control in order to illustrate the set of problems related to peculiar data source, the potentiality, and the development features of main interest for chemometricians in this field. Therefore, to overview the different MS-based strategies applied to food analysis, we address the reader to the most recent comprehensive reviews [5,7]. 

Food products can generally be considered as a mixture of many components. For example, milk, cream and cheeses are primarily a mixture of water, fat globules and macromolecules. The concentrations of the components are important parameters in the food industry for the control of production processes, quality assurance and the development of new products. NMR has been used extensively to quantify the amount of each component, and also their states [59, 60]. For example, lipid crystallization has been studied in model systems and in actual food systems [61, 62]. Callaghan et al. [63] have shown that the fat in Cheddar cheese was diffusion-restricted and was most probably associated with small droplets. Many pioneering applications of NMR and MRI in food science and processing have been reviewed in Refs. [19, 20, 59]. 

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