Origin Determination of Walnuts (Juglans regia L.) on a Worldwide and Regional Level by Inductively Coupled Plasma Mass Spectrometry and Chemometrics

https://doi.org/10.3390/foods9111708

“To counteract food fraud, this study aimed at the differentiation of walnuts on a global and regional level using an isotopolomics approach. Thus, the multi-elemental profiles of 237 walnut samples from ten countries and three years of harvest were analyzed with inductively coupled plasma mass spectrometry (ICP-MS), and the resulting element profiles were evaluated with chemometrics. Using support vector machine (SVM) for classification, validated by stratified nested cross validation, a prediction accuracy of 73% could be achieved. Leave-one-out cross validation was also applied for comparison and led to less satisfactory results because of the higher variations in sensitivity for distinct classes. Prediction was still possible using only elemental ratios instead of the absolute element concentrations; consequently, a drying step is not mandatory. In addition, the isotopolomics approach provided the classification of walnut samples on a regional level in France, Germany, and Italy, with accuracies of 91%, 77%, and 94%, respectively. The ratio of the model’s accuracy to a random sample distribution was calculated, providing a new parameter with which to evaluate and compare the performance of classification models. The walnut cultivar and harvest year had no observable influence on the origin differentiation. Our results show the high potential of element profiling for the origin authentication of walnuts.”

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2.2. Sample Preparation

A total of 237 reference samples of relevant, market-available walnuts from three years of harvest (2017, 2018, and 2019) were collected and analyzed in this study. The walnut samples originated from ten countries and were purchased as shelled or in-shell goods. Thanks to the cooperation with regional producers and project partners who work according to our internal guidelines to ensure the authenticity of the reference material (e.g., by applying the HACCP guidelines, FSSC 22,000, or providing the structure meta date), authentic walnut samples could be acquired. See Table S2 (Supporting Information) for detailed information and Figure 1 for a visual illustration. On arrival, walnut samples were frozen and stored at −80 °C until further processing could take place. Element patterns are less influenced by storage than other profiling levels, such as the metabolome [27]. At the applied storage conditions of −80 °C, enzyme activities are inhibited—i.e., cell lysis is also inhibited [28]. One German walnut sample (harvest year 2018, Hesse) was selected as a quality control (QC) sample.

Figure 1. Overview of the 237 walnut samples with regard to their origin and harvest year.

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