Consumer interest in organic products has increased over the past few years because they are considered to be safer and are produced in a more environmentally compatable manner. The organic system excludes the use of synthetic fertilizers and pesticides, and is subject to controls by an EC-regulated creditation and certification system at each step of production.

Analytical controls for organic fruits and vegetables generally involve the search for pesticide residues. The absence or presence of these substances provides a criterion for discriminating between organic and conventionally grown products. However, cultivation of citrus fruits in the Mediterranean region does not generally require frequent use of pesticides. Other methods are therefore needed to authenticate organic products.

Soil fertility management in organic farming involves the use of organic fertilizers and amendments. The nitrogen mineralization rate of these fertilizers depends on climate, soil type, microbiological activity and chemical composition. Studies have shown that the ratio of stable nitrogen isotopes ¹⁵N/¹⁴N in plants depends on the kind of fertilizer used. ¹⁵N/¹⁴N ratios (expressed as δ¹⁵N values) of nitrogen-containing compounds, including proteins and amino acids in fruit juices, therefore reflect the use of organic or synthetic fertilizers, and could provide a marker for organic citrus fruits.

A study by Rapisarda et al.¹ examined the differences in quality between organically and conventionally grown Navelina and Tarocco oranges through the detection of markers linked to nitrogen metabolism. Total nitrogen and synephrine contents of juice, and the δ¹⁵N values of proteins in pulp and amino acids in juice were determined. Total nitrogen and synephrine contents were higher in conventionally grown fruits, while δ¹⁵N values were higher in organic oranges, providing useful differentiating markers.
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¹ Rapisarda, P; Calabretta, ML; Romano, G; Intrigliolo, F (2005). Nitrogen metabolism components as a tool to discriminate between organic and conventional citrus fruits. Journal of Agricultural and Food Chemistry 53 (7) 2664-2669.

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