Can stable isotopes separate organic from conventional bananas?

Similarly to the measurement of pesticide residues to ensure that the specifications or regulations and certifications have been met, it is essential to have methods to differentiate fruits that received organic fertilization from those that received fertilization with synthetic fertilizers. The nitrogen-15 isotopic signatures of synthetic fertilizers (resulting from fixation of nitrogen of the air) and of organic fertilizers (coming from organic matter) are very different. The objective of this study was to test whether it was possible to differentiate bananas that had received organic fertilization from those that had received conventional fertilization. We studied the isotopic signature of bananas from seven production regions in Africa and French West Indies; in each we analysed an organic and a conventional couple. Importantly, we were sure that in the organic situations the fertilization was only done with organic matter. In each of the 14 situations studied, we analysed 17 fruits on average. On each fruit, we analysed with an isotopic mass spectrometer the isotopic abundances of 15N (reported in δ notation). The δ15N values of banana fruits ranged between -1.25 and 8.91‰, with a highly significant effect of the type of fertilization on δ15N values. Among countries, the average difference between the two types of fertilization ranged between 2.18 and 4.77‰ with a mean value of 3.09‰. These significant differences in mean δ15N values indicate that these measures reliably segregate banana fruits grown organically from those grown in conventional systems. The next step will be to establish a baseline of δ15N values of fruit from broad origins. To date, measurements of δ15N on banana fruit appear to be effective information for detecting fruit that has potentially failed to meet organic specifications and thus guide controls by certification authorities.