FOOD & DRINK TECHNOLOGY 31


Fig. 1. Key features of NMR.


Can be calibrated to cover a concentration range from 0.5 to 100 per cent fat Primary calibration can be produced usine a single fat sample Requires infrequent recalibration Sample measurement time is short Minimal sample preparation necessary No solvents are required Excellent for bulk measurements Insensitive to sample granularity and product additives Non-destructive, facilitating repeatability measurements


Near Infra-Red (NIR) is sometimes used, but it is generally sensitive to the surface rather than the bulk of the sample, and has substantial calibration and calibration maintenance issues.


Product granularity NIR calibration is complex because measurements are sensitive to product granularity and other physical characteristics and can be affected by additives such as seasoning, making it difficult to maintain accurate calibrations on a large variety of product types. Tis gives NIR limited applicability for the quality control of fat content in foodstuffs.


In contrast to the standard wet chemistry methods and various secondary techniques, low field nuclear magnetic resonance (NMR) provides a fast, direct and user friendly method for determination of the fat and oil content in foodstuffs.


Te technique is based on measurement of the NMR response obtained from fat in the product, and quantification of the fat content by simple and direct calibration without the use of chemometrics. Te instrument is extremely easy to operate and does not require the use of skilled chemists or NMR specialists.


NMR can be calibrated to cover a concentration range from 0.5 to 100 per cent fat. Te user can


produce a primary calibration using a single sample of fat. NMR is very stable over the long-term, and therefore requires infrequent recalibration.


Sample measurement time is short, typically about 20 seconds, allowing a high throughput of samples and efficient laboratory operation. Minimal sample preparation is required because the entire sample is normally loaded into a tube and measured directly, and there are several different size tubes available.


With NMR, no solvents are required since the sample is analysed in its natural state. Te instrument facilitates bulk measurement; the signal is generated from the whole sample, ensuring that the result embodies everything inside the


sample, not just on the surface. NMR is virtually insensitive to sample granularity and such additives as spices, flavors, colors and salt.


Finally, unlike Soxhlet, the NMR technique is non-destructive, so any required repeatability measurements can be made easily.


Throughput challenges After reviewing the potential alternative solutions to bottlenecks associated with fat measurements, the laboratory contacted Oxford Instruments, which offers a benchtop NMR instrument widely used in industry to measure the oil content in foodstuffs and oil seeds. Oxford Instruments recommended its powerful but compact MQC benchtop NMR analyser for this


application because it offers the analyst the benefits of accurate quantitative results, combined with sampling ease and convenience.


To verify whether the MQC NMR instrument would meet the laboratory’s needs, applications specialists from Oxford Instruments collected and tested samples of some of the foods the lab typically analyses and compared NMR measurements with fat measurement values obtained with the Soxhlet method.


Te goals of the testing process were to analyze 80 per cent of samples using the MQC NMR instrument, achieve correlation to within 5 per cent of the wet chemistry method, and achieve a repeatability of within 5 per cent.


Applications specialists sampled a range of foods, with fat contents ranging from 2.1 to 40.2 per cent by mass, including baked cheese, muesli, milk powder, chicken powder, trifle, garlic bread, macaroni and cheese, meat, and chicken sandwich filler. Sampling was conducted using an Oxford Instruments MQC-23 benchtop NMR analyser equipped with a 26 millimetre (mm) diameter probe.


As shown on Table 1, the NMR results compared very closely to wet chemistry.


Fig. 2 shows a calibration for the Table 1 – Comparison of wet chemistry and NMR-MQC instrument Sample


Baked Cheese Muesli Milk Powder Chicken Powder Garlic Bread Macaroni Cheese Meat Sandwich Filler


Given fat content % (NMR – MQC instrument)


6.1 2.3 25


40.2 16.1


2.8 - 3.4 9.9 21.7


Measured fat content % (NMR – MQC instrument)


5.8 2.2 24.2 40.1 16.0 3.4 9.2 21.6


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