Assessment of rapid low‐cost isotope (δ¹⁵N, δ¹⁸O) analyses of nitrate in fruit extracts by Ti(III) reduction to differentiate organic from conventional production

Abstract

RationaleThe isotopic composition (δ15N, δ18O) of nitrate in fruits and vegetables differentiates organic from conventional food production practices. Organic systems do not use synthetic nitrate fertilizers high in 18O and low in 15N and thereby help reveal producers’ fertilization claims. Isotope analyses of nitrate extracted from fruits and vegetables are done by bacterial reduction which is costly and by specialized laboratories. Rapid, low‐cost methods are needed to promulgate nitrate isotope analyses of food products to support organic food product certification and to verify the authenticity of production claims.MethodsFresh strawberry samples were obtained from certified organic and conventional growers in Andalucía, Spain. We applied a new, rapid, one‐step Ti(III) reduction method to convert the nitrate from strawberry extracts to N2O gas for headspace isotope analyses using isotope‐ratio mass spectrometry. Using the Ti(III) reduction method, 70 samples, controls and references were prepared and analyzed for NO3−, δ15N and δ18O per 48 h. We also analyzed extracts and solids for anions and cations and for bulk δ15N for multivariate chemometric evaluation.ResultsThe Ti(III)‐based isotope analyses of nitrate in strawberry extracts revealed clear differentiation between organic and conventional production with mean δ18O and δ15N values of +18.3 ± 1.2 ‰ and +17.6 ± 1.2 ‰ versus +28.2 ± 4.5 ‰ and +14.9 ± 3.0 ‰, respectively. The δ15N of strawberry dry mass differed slightly (+3.0 ± 1.4 ‰ versus +4.0 ± 1.4 ‰) between organic and conventional samples, respectively. Chemometric analyses of nitrate isotopes and extract chemistry revealed that the δ18O of nitrate along with δ15N and Ca2+ fully differentiated organic from conventional strawberry production.ConclusionsOur results show the Ti(III) reduction method provides a new low‐cost and rapid analytical method to facilitate compound‐specific δ15N and δ18O isotope analyses of nitrate in selected fruit types, and likely other food products, for the purposes of assessing nitrate fertilization practices of organic versus conventional production claims and to support authenticity investigations.

Innovative Tools for Nitrogen Fertilization Traceability in Organic Farming Products: A Cauliflower Case Study

“…All the corymb samples were subjected to 13 C/ 12 C, 15 N/ 14 N, and 34 S/ 32 S analyses using an isotope ratio mass spectrometer (Delta plus XP Thermo Finnigan, Bremen, Germany) equipped with an elemental analyzer (EA Flash 1112 Thermo Finnigan). The values of the isotopic ratios were expressed in δ ‰, calculated against working in-house standards (mainly atropine), and calibrated against international reference materials including NBS-22 oil (IAEA) for δ 13 C and glycine (USGS 64) for δ 15 N measurement.…”

“…The freeze-dried samples of the leaves were weighed in tin (2.5 mg for 13 C/ 12 C, 15 N/ 14 N, 34 S/ 32 S) or silver capsules (0.25 mg for 2 H/ 1 H and 18 O/ 16 O) on a microbalance. The dried soil samples and the freeze-dried fertilizers were weighed in tin capsules (10 mg) for 15 N/ 14 N and 34 S/ 32 S analyses. The comparative equilibration method was used for the analysis of 2 H/ 1 H. The samples and standards were left in the laboratory's air moisture for a minimum of 96 h and then placed in a desiccator with P 2 O 5 under N atmosphere.…”

“…The comparative equilibration method was used for the analysis of 2 H/ 1 H. The samples and standards were left in the laboratory's air moisture for a minimum of 96 h and then placed in a desiccator with P 2 O 5 under N atmosphere. The 13 C/ 12 C, 15 N/ 14 N, and 34 S/ 32 S isotope ratios were all measured using an isotope ratio mass spectrometer (Vision, Elementar, Langenselbold, Germany) via a continuous flow inlet system to a Vario Isotope Cube Elemental Analyser (Elementar, Germany). Both the 2 H/ 1 H and 18 O/ 16 O isotope ratios were analyzed using an isotope ratio mass spectrometer (Finnigan DELTA XP, Thermo Scientific, Bremen, Germany) after complete pyrolysis in an elemental analyzer (Finningan DELTA TC/EA, high-temperature conversion elemental analyzer, Thermo Scientific).…”

“…2018/848) that is currently in place [7] only requires documentary evidence of the organic management applied to a product; no official analytical method is recognized or employed as means to support the eradication of food fraud in this specific sector. In an effort to give an answer to these specific needs, several research studies, over almost the last two decades, have focused on the discrimination of organic vs. conventional foods [8][9][10][11], including fruit [12][13][14][15], vegetable [16][17][18][19][20][21], meat [22][23][24], dairy [25][26][27], and seafood [28] products. The nitrogen fertilization traceability in organic products still remains a matter of great relevance and interest, since synthetic mineral fertilizers are not allowed in organic cultivation, and the need to identify their fraudulent use is pressing.…”

“…To achieve this goal, six different treatments were adopted and compared, including conventional, organic, and mixed treatments at different % of mineral fertilizers, also comprising the application of agronomic methods for the management of soil fertility, such as the introduction of agroecological service crops. The variations in the 15 N/ 14 N isotopic ratio induced by the combination of different fertilization treatments together with the use of agroecological service crops in the organic production system were quantified, thus evaluating how soil management based on an agroecological approach affected the 15 N/ 14 N isotopic ratio in the final product, in a comparison between the organic management method based on the full substitution of mineral fertilizers and conventional agriculture approaches.…”