Abstract
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.
Highlights
Walnuts are the seeds of the Juglans tree, of the English or Persian walnut treeJuglans regia
The element pattern is considered suitable for determining the geographical origin of walnuts, since walnut kernels grow inside the shell and are protected from the environment—i.e., practically unaffected by perturbations such as anthropogenic aerosols and soil dust [2,20]
Walnuts are mostly traded as shelled goods: in 2018, the percentages of shelled walnuts imported to Europe and Germany were 62% and 81%, respectively [40]
Summary
They are appreciated for their high level of polyunsaturated fatty acids as well as their high tocopherol and potassium content. Consumers are increasingly interested in products made with selected ingredients. The interest in sustainable and regional food is growing correspondingly. Consumers are willing to accept higher prices for products with a specific geographical origin [4]. The annual financial damage by food fraud is estimated at 40 billion dollars, and there are health risks with lethal consequences [5]. The omics disciplines are suitable for authentication by creating a fingerprint of the examined food to prevent food fraud [4]. DNA-based methods for food authentication, which are only able to determine the genotype, are inevitably limited
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