Applications of Nuclear Magnetic Resonance Technique in Purity Check of Food Items

Kishan Kumar* and S S Verma ; Department of Physics, S.L.I.E.T., Longowal; Distt.-Sangrur (Punjab)-148 106

2021-07-28 15:57:29



Nuclear magnetic resonance (NMR) spectroscopy is the study of molecules by recording the interaction of radiofrequency (Rf) electromagnetic radiations with the nuclei of molecules placed in a strong magnetic field. Nuclear Magnetic Resonance (NMR) was first detected experimentally at the end of 1945, nearly concurrently with the work groups Felix Bloch, Stanford University and Edward Purcell, Harvard University. The first NMR spectra was first published in the same issue of the Physical Review in January 1946. Bloch and Purcell were jointly awarded the1952 Nobel Prize in Physics for their research of Nuclear Magnetic Resonance Spectroscopy.  

Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a frequency characteristic of the magnetic field at the nucleus. This process occurs near resonance, when the oscillation frequency matches the intrinsic frequency of the nuclei, which depends on the strength of the static magnetic field, the chemical environment, and the magnetic properties of the isotope involved; in practical applications with static magnetic fields up to ca. 20 tesla, the frequency is similar to VHF and UHF television broadcasts (60–1000 MHz). NMR results from is widely used to determine the structure of organic molecules in solution and study molecular physics and crystals as well as non-crystalline materials. NMR is also routinely used in advanced medical imaging techniques, such as in magnetic resonance imaging (MRI). specific magnetic properties of certain atomic nuclei. Nuclear magnetic resonance spectroscopy is possible, but the roost commonly used by organic chemists are proton nuclear magnetic resonance (PMR or 1H NMR) spectroscopy and carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy techniques. Various applications of NMR in food science and safety are coming and few of them for specific food items are briefly mentioned here.

Preventing food fraud: Nuclear magnetic resonance (NMR) spectroscopy is a non-invasive, high throughput technique that has recently been shown to be able to distinguish between fresh and frozen-thawed fish by measuring the metabolic products formed from enzymatic reactions in the extracellular fluid. Using a Bruker Advance 600-Mhz spectrometer, researchers performed 1D 1H-NMR analysis on fresh and frozen-thawed Atlantic Salmon samples, firstly from the same fish and secondly from fish in different packages. The frozen-thawed samples were frozen overnight (≤ –20 °C) before thawing and storing at 4 °C for up to 18 days after slaughter. The fresh samples were kept at 4 °C throughout. 1D 1H-NMR was performed at regular intervals starting from the day after the frozen samples were thawed. Fresh and frozen-thawed samples from the same fish, and those from different packages, were found to have different levels of aspartate (2.81–2.84 ppm). In the frozen-thawed samples, the formation of aspartate was observed until it reached a maximum concentration of 3.6–3.8 mg per 100 g on the third day after thawing and then decreased to zero. In comparison, no aspartate formation was observed in the fresh samples. This difference is thought to be due to an increase in mitochondrial aspartate aminotransferase in frozen-thawed fish. This enzyme catalyses the conversion of 2-oxoglutarate to L-aspartate, explaining the increase of aspartate observed in frozen-thawed samples, while no formation was seen in fresh samples. This study shows NMR spectroscopy can readily distinguish between fresh and frozen-thawed fish, and the non-invasive nature of this technique makes it highly desirable for use in food fraud analytics.

Recognizing honey adulteration: The food industry, as a whole, has seen its fair share of controversies, but very few have matched the severity that is associated with honey. Now more than ever, food monitoring programs have an increased need for more sensitive and highly accurate tools that can identify adulterated honey quickly and effectively nuclear magnetic resonance (NMR) imaging is one of the most promising tools for identifying food adulteration, especially for honey. the NMR approach can measure different compounds and provide researchers and food monitors with information about the structure of these compounds. one commercially available tool, the NMR food-screener from bruker, features a honey-profiling module that can efficiently identify honey adulteration in the lab the food-screener provides NMR fingerprint that is specific to the honey sample and compares it to a larger honey sample database to see if there’s a match. just one experiment with NMR produces results that are repeatable and reproducible, ensuring a quality finding every time. also, NMR experiments are typically fast, low cost, and do not destroy the sample. In a recent study from china, researchers used a 600 Mhz bruker ascend NMR spectrometer to analyse 90 authentic chinese honey samples and 75 adulterated honey samples. a metabolically similar cluster of total honey samples was found, suggesting that the adulterated honey samples were designed to look similar to real honey in terms of the natural carbohydrate profile. the NMR experiment classified true honey and fake honey with high accuracy the NMR data used in this study were also able to identify contributing compounds that differentiated fake honey from true honey. these components included things like proline, xylobiose, URIDINE, Β-glucose, melezitose, Tura nose, and lysine. researchers from this study suggest that these markers, in combination with NMR, can help in the development of a tool   that rapidly separates fake honey from real honey. While future research is needed, the current data suggests that NMR may be a promising approach for monitoring adulterated food. the increasing use of NMR and other high-throughput technologies may bolster the confidence among consumers while improving the strength of true honey industry at large. As a general rule, the NMR honey test should be made mandatory for all types of honey and honey brands. before they advertise and sell their honey to the customers in India or abroad. it matters a lot due to the overall health impact it has on customers. check the NMR certified honey in India NMR passed honey in India before you buy it

Milk authenticity identification: Milk is an important food component in the human diet and is a target for fraud, including many unsafe practices. For example, the unscrupulous adulteration of soymilk into bovine and goat milk or of bovine milk into goat milk in order to gain profit without declaration is a health risk, as the adulterant source and sanitary history are unknown. A robust and fit-for-purpose technique is required to enforce market surveillance and hence protect consumer health. Nuclear magnetic resonance (NMR) is a powerful technique for characterization of food products based on measuring the profile of metabolites. In this study, 1D NMR in conjunction with multivariate chemometrics as well as 2D NMR was applied to differentiate milk types and to identify milk adulteration. Ten metabolites were found which differed among milk types, hence providing characteristic markers for identifying the milk. These metabolites were used to establish mathematical models for milk type differentiation. The limit of quantification (LOQ) of adulteration was 2% (v/v) for soymilk in bovine milk, 2% (v/v) for soymilk in goat milk and 5% (v/v) for bovine milk in goat milk, with relative standard deviation (RSD) less than 10%, which can meet the needs of daily inspection.

The metabolic profile of aqueous extracts of kiwifruit: Kiwifruit are an important horticultural crop with a characteristic flavour, fragrance and health properties which are due to the fruit chemical composition. Knowledge of the metabolic profile of kiwifruit is extremely important for fruit commercial value and also for the industries which extract specific compounds from the fruit to obtain specific additives. The metabolic profile of aqueous extracts of kiwifruit (Actinidia deliciosa, Hayward cultivar) as well as the water content of the whole fruit was obtained by NMR methodologies2. The knowledge of the metabolic profiling of kiwifruits at different development stages may have an important role in the determination of the most suitable time of harvesting as well as in the quantitative determination of nutrients at different growth times. The metabolic profile has been monitored from June to December showing ripening associated changes of some com – pounds such as sugars, organic acids and amino acids. The water content, also monitored over the seven-month period, was determined directly on the intact fruit, measuring the spin-spin relaxation time by a portable unilateral NMR instrument that was fully non-invasive. Clear trends of the relaxation time have been observed during the monitoring period, providing information on the state of kiwifruit growth and ripeness.

Determination of free fatty acids in vegetable oils: 1H NMR method to quantitatively determine FFA in vegetable oils, animal fats and bio diesel. It is based on the integration of the signal corresponding to the α-carbonyl methylene protons of FFA (the methylene group directly adjacent to the COOH group) and the α -carbonyl-CH2 signal of esterified fatty acids. In vegetable oil and biodiesel, α-CH2 peaks of fatty acids appear at chemical shift (δ) values higher than those of the ester, as a consequence of the stronger deshielding effect of the carboxylic group with respect to the ester group. Hence, one of the peaks of the triplet of FFA is visible outside the α-CH2 region of the ester, while the other two peaks overlap with those due to the ester. This means that a sample of vegetable oil or biodiesel containing both FFA and ester shows a pseudo-quartet signal in the α-CH2 region of the proton NMR spectrum and that the intensity of the peaks depends on the content of FFA in esters. Different strategies reported in the literature based on 1H, 13C and 31P NMR spectroscopy for the determination of free fatty acids in vegetable oils have been reviewed and compared. Two 1H NMR methods, based on the integration of the α-carbonyl methylene protons or of the carboxyl proton signal of FFA (free fatty acid) were reported. The main drawbacks are related to (i) the narrow spectral width and the following risk of signal overlap, (ii) sensitivity issues in the first method, and (iii) the need for some effort in the sample preparation in the second method. In cases of severe overlap in the proton spectrum, 13C methods may be useful, as they rely on the identification and quantification of signals in the carbonyl or the aliphatic region. The main limitation of 13C NMR spectroscopy is the long acquisition time required to obtain a spectrum with a proper signal-to-noise ratio. To solve the problem and shorten the experimental time, it is possible to add relaxation reagents to the samples. All NMR approaches give reliable results in agreement with conventional methods, and can represent, therefore, a non-invasive, non-destructive and quantitative analytical toolbox for the determination of free acidity in vegetable oils, including waste cooking oils. Typical limitations of all NMR-based techniques are their sensitivity and limits of detection, which could be relevant issues, especially for commercial edible oils or pharmaceutical products. However, impressive progress was made in this respect over the years and, nowadays, the availability of ultra-high magnetic fields and new generation probe heads make it possible to incredibly reduce the noise and push the NMR detection limits in terms of absolute sensitivity.

Verification of cocoa authenticity: Cocoa is the raw product used in the manufacture of chocolate. The quality and processing of the cocoa bean can significantly impact the quality and taste of the end-product. The quality of cocoa is mainly determined by the location in which it is grown, but can also be affected by the way it is processed. Chocolate created from cocoa that has been cultivated in particular geographical regions can thus justify higher prices than chocolate containing cocoa from other regions. In one study, an analytical evaluation of cocoa samples using a range of spectroscopic techniques allowed cocoa to be classified according to the three major cocoa varieties (Forastero, Criollo and Trinitario) and to be linked to a particular region of cultivation With numerous advances in proton nuclear magnetic resonance (1H NMR) technologies over recent years and the success of its application to authenticity screening of honey and wine, proton NMR is now being explored for determining the origin of cocoa. Proton NMR simultaneously evaluates all the components in a sample, and so the spectral fingerprint will show that the compounds characteristic of the origin in question are present and that there are no additional unwanted ingredients. A comparison of the spectra from unverified samples with those of known samples provides a simple and rapid means of verifying authenticity. This approach has successfully differentiated between cocoa samples of differing levels of fermentation, but it has proved more challenging to determine geographical origin using spectroscopic analysis. This is largely due to the changes in metabolite profile that occur during fermentation which obscure relevant information. A recent study profiled 48 cocoa samples from 20 countries using a combination of stable isotope-ratio mass spectrometry (IR-MS) and proton NMR. Proton NMR spectra were obtained using a Bruker 400 MHz food screener spectrometer equipped with a BBI probe head with z gradients. Chemometric analysis of the IR-MS data and the proton NMR spectra resulted in good separation of the different cocoa samples and enabled better classification rates compared with the techniques used individually. No fractionation effects of fermentation were observed.