Measurement of fat content of food with single‐sided NMR

Abstract

Fat content is one of the important parameters of quality control in many food products. Several techniques are used to determine the amount of fat in a given product. In this work, the applicability of a portable dedicated nuclear magnetic resonance (NMR) analyzer to measure the fat content in a packaged product without destruction of the material was examined. Two different low-field NMR methods, namely, a ratio method and a relaxation time method, were applied and discussed. The processed NMR signal was linearly correlated with the fat content obtained by reference methods. The linear correlation allowed the application of single-sided NMR for fat measurements.The rationale for commercial development of a bench-top pulsed NMR spectrometer was to apply the benefits of lowresolution (LR-NMR) or time-domain NMR (TD-NMR) to quality control in food products. The first application of LR-NMR was the determination of the solid fat content (SFC) in various fat compositions (1-3). For this application, the analyzer was designed to record the decay of the total NMR signal from protons in solid and liquid environments. The difference in transverse relaxation properties of solid and liquid fats was exploited to determine the relative solid and liquid contents. As a result of the success of this approach to SFC, TD-NMR was later applied to a number of applications in the food industry. For example, total fat or oil content was easily measured in seeds and other dry materials. TD-NMR methods were developed to measure parameters such as fat and water content in foods and droplet size distribution in emulsions. In these cases, commonly known NMR pulse sequences were implemented from research on high-resolution NMR systems. Current TD-NMR systems are simple and easy to use. For instance, predefined applications include routines for self-tuning and self-validation of the system. However, there are restrictions on the sample size and handling: The material to be investigated has to be filled into sample tubes with a given diameter and filling height. This restriction poses limitations for measurements on living subjects or in packaged goods.The dimension restrictions of TD-NMR are relaxed when using a single-sided NMR device. One of them is the NMR-MOUSE ® (Mobile Universal Surface Explorer, trademark of the RWTH Aachen). It is a portable NMR device used to measure NMR signals from near-surface volumes of arbitrarily shaped samples (4,5). It contains two permanent magnets mounted with antiparallel polarization on a yoke. In the gap between the magnets, a radio frequency (RF) tank circuit is positioned, which generates the RF field (B 1 ) necessary for the observation of an NMR signal. Details of the construction and use of the NMR-MOUSE were described by Eidmann et al. (4) and Guthausen et al. (5).Several publications showed promising experimental results on the use of single-sided NMR in quality control. The NMR-MOUSE ® was first used in the rubber industry for measuring the cross-link density of the polymer chains (6). The cros...

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Rapid determination of the fat content in packaged dairy products by unilateral NMR

“…The first was based on the large difference in the diffusion coefficients of the water (∼2.3 × 10 −9 m 2 s −1 ) and fat (∼9.8 × 10 −12 m 2 s −1 ) phases, 16 which was exploited to suppress the water signal. The NMR sequence employed (Fig.…”

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Portable NMR in food analysis

“…Guthausen et al investigated the applicability of a single-sided NMR sensor to measure the fat content in packaged food products [36,37]. At this aim, two methods [35] were applied.…”