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NMR Fruit Juice Screening Ensures Quality Control

by Manfred Spraul, Bruker BioSpin GmbH, and Peter Rinke, Ph.D. and Sophie Moitrier, SGF International e.V.

The “natural” and “healthy” image of fruit juices and derived products is one of the reasons leading to their dynamic evolution in the global food sector. The fruit juice industry purchases its raw materials in a world-wide market. Despite a general trend in the food industry to install suitable traceability systems, the need for analytical control of authenticity and legal conformity has increased over the last few years. Because of the relatively large natural variability and more-and-more ingenious adulterations observed on the market, sophisticated control has become an important economic factor in the fruit juice industry.

Bruker BioSpin Avance III with a 400 MHz UltraShield Plus NMR magnet and automated Gilson liquid handler.

The main deviations and adulterations of fruit juices are the addition of sugar, water, flavor compounds or cheaper fruit types than those declared on the label. Furthermore non-permitted production techniques can also lead to quality deviations. Conse- quently, a need exists for a robust, high throughput screening method checking the products under multiple relevant aspects simultaneously. Taking into account this requirement, Spin Generated Fingerprint Profiling (SGF-Profiling) is a new approach based on 1H-NMR-techniques combined with advanced statistical evaluation. The initial intention was to create a low cost screening method that was able to pre-select samples for submission to conventional target analysis for specific quality aspects. The outcome of one SGF-Profiling experiment may be a series of analytical data.

Nuclear magnetic resonance (NMR) is one of the principle techniques used to obtain physical, chemical, electronic and structural information about one molecule or different substances in a complex mixture, such as fruit juices. If the magnetic properties of protons are used, we speak about proton-NMR or 1H-NMR. SGF-Profiling is based on a full 1H-NMR-spectrum. Bruker’s Avance III NMR spectrometer is suited for this analysis. To achieve the best compromise between information content and analytical costs, the Avance III 400-MHz spectrometer is often used with a flow injection cell and an automatic sample preparation unit. Automatic sample preparation with a Gilson liquid handler into NMR tubes is also possible, however with reduced speed and increased costs. In this case, a standard sample changer can be used to handle the NMR tubes.

Interpretation of data is carried out by a statistical comparison between a reference spectral database of authentic juices containing 1H-NMR spectral information and the samples to be reviewed. For a suitable calibration of the system, the reference database should contain spectra of representative samples covering the natural variability of the examined sample type. The quality of any analytical answer improves with the number of representative reference spectra. All spectra have to be acquired under identical conditions to ensure comparability.

As an example of the multitude of possible exploitations of one unique sample spectrum, the following possibilities are summarized. An orange juice or orange juice concentrate could be checked for the addition of different foreign fruits such as mandarin or grapefruit, the addition of different types of sugar syrup, the use of pulp wash (a low quality by-product from the juice production), any added preservatives or the presence of microbiological metabolites as indicator for product spoiling. Information about the geographical origin and/or the employed production technique could also be obtained. For each of these aspects the information in the corresponding reference spectra should be evaluated in different statistical models. Every model applies one or more dedicated mathematical approach, and each of the models is based on the same proton NMR experiment, which means all information is extracted from the same raw data. For each sample, only one proton NMR analysis is carried out. This is true for reference samples during construction of the models, as well as for the sample to be tested.

For more information, contact Manfred Spraul, Bruker BioSpin GmbH, at manfred.spraul@bruker-biospin.de or by phone at +49 721 5161 316.

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