Abstract
The prices of walnuts vary according to their geographical origin and, therefore, offer a financial incentive for adulteration. A reliable analysis method is required to quickly detect possible misdeclarations and thus prevent food fraud. In this study, a method to distinguish between seven geographical origins of walnuts using Fourier transform near-infrared (FT-NIR) spectroscopy combined with chemometrics as a fast, versatile, and easy to handle analytical tool was developed. NIR spectra of 212 ground and afterwards freeze-dried walnut samples, harvested in three consecutive years (2017–2019), were collected. We optimized the data pre-processing by applying and evaluating 50,545 different pre-processing combinations, followed by linear discriminant analysis (LDA) which was confirmed by nested cross-validation. The results show that in the scope of our research minimal pre-processing led to the best results: By applying just multiplicative scatter correction (MSC) and median centering, a classification accuracy of 77.00% ± 1.60% was achieved. Consequently, this complex model can be used to answer economically relevant questions e.g., to distinguish between European and Chinese walnuts. Furthermore, the great influence of the applied pre-processing methods, e.g., the selected wavenumber range, on the achieved classification accuracy is shown which underlines the importance of optimization of the pre-processing strategy.
Highlights
With over 2.5 million tons produced per year, Juglans regia L., the English or Persian walnut, is one of the most economically important nut crops [1]
1260 runs) of French walnuts are identified as German
Addressed comprehensively: (i) Chinese walnuts are currently traded at relatively low prices [1,10], can be distinguished from all other walnut origins with an accuracy of 95.7%. (ii) Likewise, the Conclusions more expensive French walnuts can be distinguished from Chinese, Hungarian and US-American conclusion, the present study clearly that NIR
Summary
With over 2.5 million tons produced per year, Juglans regia L., the English or Persian walnut, is one of the most economically important nut crops [1]. Walnuts are widely consumed as a raw, toasted, or pickled snack, used as an ingredient in pastries, ice cream, muesli, and for the production of liquor or oil. Their broad acceptance occurs due to their special organoleptic characteristics, and because of their high nutrition value that is associated with health benefits, such as the improvement of serum lipid profiles and a reduction of the risk for coronary heart disease [2,3,4]. Some walnuts from smaller harvesting areas such as France are, economically significant because of their particular sensory characteristics, which are closely linked to their geographical origin and are subject to special protection by labels
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