The food industry is subject to intense scrutiny throughout the supply chain due to the vital requirement to verify the safety and authenticity of foods. Many traditional analysis techniques are limited in their capabilities, and in a high-throughput environment like a food testing laboratory, rapid methods for non-specific analysis are required.
Nuclear magnetic resonance (NMR) has long been a preferred method for organic compound analysis, but it’s quantitative NMR (qNMR) that’s making waves in a field that has so far been reliant upon chromatography for its quantitative analysis requirements. Although NMR has a quantitative performance in principle, it has previously been considered big, expensive, low-sensitivity, and altogether complicated when compared to chromatographic methods. However, that’s all changing, with qNMR attracting attention from a variety of fields for the reliability of the results it can achieve.
qNMR Catches On
Traditionally a research method, NMR is increasingly becoming an analytical tool that has particular merit in the food industry. NMR methodology enables primary and secondary metabolites to be identified and quantified, delivering high-throughput spectroscopic and structural information on a wide range of metabolites simultaneously.
A key benefit of using NMR for food testing is that samples can be analyzed either in solid or liquid state, negating the requirement for complicated sample preparation, and facilitating efficient sample screening for multi-component mixtures (i.e. foods).
A major feature of qNMR is that it does not require a standard reference material (RM) that is identical to the analyte. Hydrogen within the molecules can be observed and measured with NMR, so even if the molecules are different, the presence of hydrogen means that quantitative analysis is possible. This is extremely useful for quantification of new compounds and means that calibration curves are not required for this analysis.
False Food Claims
Food characterization is becoming increasingly important due to food fraud scandals around the world and changing consumer attitudes towards what’s in their food. As a result of consumers’ rising interest in this, there’s also increased regulation regarding substantiation of health benefits in order to police false claims. The two together have led to much more scrutiny of labels, and consumers are becoming more confident in reading food packaging, understanding it, and what it means for their health.
Food testing labs are validating that the claims manufacturers are making on their packaging are correct, and the technology available is making this process much easier than it has been in the past.
Traceability is crucial to the global food supply chain as companies are under mounting pressure to understand and implement ways to track and trace their products throughout the food chain and prove authenticity and place of origin. This pressure comes in a bid to improve food safety, but also to ensure security and avoid a public health disaster or negative economic impact.
RMs are indispensable for accurate analysis of hazardous substances in food, however, regulators have not been able to keep pace to provide RMs for the growing number of organic compounds that might require analysis.
In collaboration with JEOL Ltd, the National Metrology Institute of Japan improved the 1H NMR method to perform precise comparisons of signal quantities from protons at different chemical shifts. This enabled quantitative analysis at an acceptable level of uncertainty for a variety of organic RMs by using a primary RM for protons.
NMR thus allows for accurate (and rapid) quantification of analytes derived from natural sources when it is difficult to obtain RM for quantification. Tartary buckwheat contains large amounts of rutin as a functional flavonoid. Quercetin, a flavonoid, can be detected as a degradation product of rutin in samples because there are rutin degrading enzymes in Tartary buckwheat noodles. An NMR system was used to quantify the amount of quercetin found in a sample of noodles, using hexamethyl disilane as an internal reference standard. It was determined that the content of quercetin was 1.58 ± 0.14 mg per gram of tartary buckwheat. Figure 1 shows the 1H NMR spectrum of methanol extract of the sample.
The Challenge of Counterfeiting
The USP (United States Pharmacopeia) Food Fraud Database lists hundreds of incidents of economically motivated adulteration, substitution, counterfeiting/mislabeling of food products, e.g. olive oil and milk powder, and some adulterants such as melamine. Techniques like chromatography can provide a detailed profile of food but require a lot of sample prep and manual processes, which means they’re expensive and time-consuming. The food industry needs quick methods for non-specific analysis.
NMR reveals multiple components of food on a single spectrum, with high throughput analysis. Measurement of the ratio of components can address the issue of adulteration—with qNMR, it is possible to confirm the proportion of a target component in a sample and to determine the absolute amount of the component of interest.
Figure 2 shows a 1H-NMR spectrum of apple juice, with the signals of ethanol and sugars clear. In this sample, the signals can be viewed separately, allowing for the extraction of both quantitative and qualitative information in an efficient manner, without having to change measurement conditions.
NMR analysis, together with chemometrics, has allowed some important characteristics of food such as geographical origin, genotype-phenotype relation, quality, and authenticity to be investigated. Olive oil is a good example of this application. Complex interactions between the variety of olives, pedoclimatic conditions, fruit ripening, and agronomic factors make up the composition of extra virgin olive oils. NMR can characterize them in terms of geographical origin, genetic origin, authenticity, and quality.
The Future of Food Testing
Because foods are so diverse and complex, with different compounds and chemical structures, concentrations, solubility, and nutritional values, the technique used for detailed analysis is crucial. With so many challenges around proving the authenticity of food, it’s clear that qNMR is a valuable tool for analysis and, importantly, a time-saving technique for food testing laboratories. As food research and development progress and become ever more innovative, the need to prove authenticity, safety, and quality of foods grows in importance.
Although it might be new to many in the food industry, qNMR isn’t a new tool for analysis; rather an under-used technique that is becoming increasingly popular among analytical chemists due to its growing reputation for making analysis easier.
As industries employing NMR techniques invest in technology capable of ever more complex research and analysis, it’s entirely possible that NMR will become a standard method for quantitative analysis in the future.
Dr. Frey, an analytical instruments product manager for JEOL USA, Inc., has worked in a variety of NMR areas including software development and NMR spectrometer R&D. Reach him at firstname.lastname@example.org. Dr. Suematsu, an NMR applications chemist at JEOL Resonance Inc., is qNMR technical advisor for Accreditation System of National Institute of Technology and Evaluation and a committee member of Japanese Industrial Standards for qNMR. Reach her at email@example.com.