Using Nitrogen Stable Isotopes to Authenticate Organically and Conventionally Grown Vegetables: A New Tracking Framework

The demand for organic food products has increased in recent years due to them being perceived healthier, safer, and eco-friendlier by consumers, boosting the development of this industry. The higher retailing price of organic products increases the risk of fraudulent practices, making it necessary to establish control mechanisms to authenticate these products. However, the authentication of organic foodstuffs is a great analytical challenge that still requires further research. In the case of organic agriculture, regulations mainly determine the nutrient inputs that can be used by farmers, and generally prohibit the use of pesticides and/or synthetic fertilisers, aiming at maintaining soil fertility using green manures, composts, animal manures, etc. These inputs affect the final food product, and numerous analytical attempts, based on the measurement of multiple markers or complex chemical/physical profiles, have been tested over recent years. However, the high variability of these measurements due to weather condition factors reduces their efficiency and limits their use. In this sense, stable isotopes have emerged as an analytical technique with great potential for the authentication of organic agricultural products, due to their lower dependence on weather conditions and capability to reflect the origin of plant nitrogen, in the case of stable nitrogen isotopes. In this work, the feasibility was assessed using stable isotopes of bulk nitrogen for the organic authentication of four important horticultural crops (zucchini, cucumber, tomato, and pepper) produced in Almeria, southern Spain, which is the largest producing region with the highest export levels in Europe. To this end, 360 samples of vegetables were collected and their delta N-15 values were determined by combustion coupled to stable isotope ratio mass spectrometry (EA/IRMS). The results allowed an authentication framework to be established based on three ranges delimited by delta N-15 = 2 parts per thousand and delta N-15 = 5 parts per thousand, which made it possible to detect with a high degree of confidence vegetables produced under proper organic practices (delta N-15 > 5 parts per thousand), conventional practices (delta N-15 < 2 parts per thousand), and samples that should be tracked over time to be considered organically produced (middle range). The results of this study demonstrated the potential of using delta N-15 as a single measure to authenticate organic vegetables, providing official bodies with a tool to make decisions about the organic accreditation of regularly inspected farmers.