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

Rationale The isotopic composition (delta N-15, delta O-18) of nitrate in fruits and vegetables differentiates organic from conventional food production practices. Organic systems do not use synthetic nitrate fertilizers high in O-18 and low in N-15 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. Methods Fresh strawberry samples were obtained from certified organic and conventional growers in Andalucia, 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-, delta N-15 and delta O-18 per 48 h. We also analyzed extracts and solids for anions and cations and for bulk delta N-15 for multivariate chemometric evaluation. Results The Ti(III)-based isotope analyses of nitrate in strawberry extracts revealed clear differentiation between organic and conventional production with mean delta O-18 and delta N-15 values of +18.3 +/- 1.2 parts per thousand and +17.6 +/- 1.2 parts per thousand versus +28.2 +/- 4.5 parts per thousand and +14.9 +/- 3.0 parts per thousand, respectively. The delta N-15 of strawberry dry mass differed slightly (+3.0 +/- 1.4 parts per thousand versus +4.0 +/- 1.4 parts per thousand) between organic and conventional samples, respectively. Chemometric analyses of nitrate isotopes and extract chemistry revealed that the delta O-18 of nitrate along with delta N-15 and Ca2+ fully differentiated organic from conventional strawberry production. Conclusions Our results show the Ti(III) reduction method provides a new low-cost and rapid analytical method to facilitate compound-specific delta N-15 and delta O-18 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.

Automated summary

The isotopic composition of nitrate in fruits and vegetables can differentiate between organic and conventional food production practices. This study introduces a new, rapid Ti(III) reduction method for analyzing the isotopes of nitrate in strawberry extracts. The results show that this method provides a low-cost and efficient way to assess nitrate fertilization practices and verify the authenticity of organic food production claims.