Authenticity testing of environment-friendly Korean rice (Oryza sativa L.) using carbon and nitrogen stable isotope ratio analysis

Highlights

• δ¹³C and δ¹⁵N in rice samples change as a function of cultivation conditions.

• δ¹³C and δ¹⁵N allow discrimination of environment-friendly and conventional rice.

• δ¹⁵N is more useful than δ¹³C for discrimination of environment-friendly rice.

• Two-dimensional δ¹³C/δ¹⁵N plots facilitate authentication of organic rice.

• Conventional and environment-friendly rice samples lack pesticide residues.

Abstract

The increasing demand for organic foods creates, in turn, a pressing need for the development of more accurate tools for the authentication of organic food in order to ensure both fair trade and food safety. This study examines the feasibility of δ¹³C and δ¹⁵N analyses as potential tools for authentication of environment-friendly rice sold in Korea. δ¹³C and δ¹⁵N examination in different rice grains showed that environment-friendly rice can be successfully distinguished from conventional rice. No multi-residue pesticides were detected in the examined rice samples, including conventional rice. This study demonstrates the complementary feasibility of δ¹³C and δ¹⁵N analyses for the authentication of environment-friendly rice sold in Korea in cases where pesticide residue analysis alone is insufficient for discrimination of organic and conventional rice. In future, complementary analyses including compound-specific isotope ratio analysis might be employed for improving the reliability of organic authentication.

Introduction

Interest in organic agroproducts continues to increase, mainly as a result of the belief that organic farming yields healthier, safer, and more environment-friendly produce compared to conventional integrated farming (Mondelaers, Verbeke, & Van Huylenbroeck, 2009). The main difference between organic and conventional agroproducts is in the use of pesticides and synthetic fertilizers. In organic farming, pests and pathogens are usually addressed using biologically based control measures. Furthermore, crop rotation, which employs N₂-fixing plants (e.g., legumes) together with other organic fertilizers, is applied for the purpose of a stable agroproduct yield (National Organic Program, NOP, USDA, the European Council Regulation, EC No. 834/2007 and Commission Regulation, EC No. 889/2008). In contrast, conventional farming relies on the use of chemical fertilizers. Thus, substantial differences between the agricultural regimes and nutrient sources employed in organic and conventional farming systems are reflected in chemical and isotopic signatures of the resulting agroproducts.

Currently, authentication of organic produce relies predominantly on certification documents issued by inspection bodies, and on random screening for pesticide residues (Laursen et al., 2013). However, such screening does not fully cover the range of different pesticides used in agricultural systems. Furthermore, as a result of its analytical detection limits and the biological and environmental half-lives of pesticides, the random-screening method often fails to discriminate organic agroproducts from conventional ones (Hoefkens et al., 2009). Therefore, development of a reliable tool for authentication of organic produce is critical for ensuring food safety and detection of fraudulent practices that involve mislabeling of conventional produce as organic.

Analysis of carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratio is often considered to be a complementary method for organic authentication. In general, δ¹³C in agroproducts depends predominantly on the photosynthetic pathway utilized by the corresponding plants. In particular, C4 plants have higher δ¹³C (−9 to −20‰) than C3 plants (−21 to −35‰) (Badeck, Tcherkez, Nogués, Piel, & Ghashghaie, 2005). Furthermore, δ¹³C in plants is also affected by differences in water and nutrient availability, drought stress, and fertilization regimes employed in conventional and organic farming systems (Adams and Grierson, 2001, Högberg et al., 1993). In addition, the higher activity of soil microbes in organic produce caused by soil management practices utilized in organic farming can also affect δ¹³C (δ¹³C depletion) in organic biomass (vegetables). Therefore, changes in δ¹³C can facilitate differentiation of organic produce from that cultivated conventionally (Georgi et al., 2005, Santruckova et al., 2000).

Meanwhile, δ¹⁵N in agroproducts usually depends on the use of N-fertilizers (intensity and length of exposure), soil moisture, and water irrigation conditions (Bateman et al., 2005, Choi et al., 2002, Choi et al., 2003). The δ¹⁵N ratio for synthetic fertilizers commonly utilized in conventional farming ranges from −6 to 6‰. In contrast, the majority of organic fertilizers employed in organic farming exhibits a higher δ¹⁵N (1–37‰, typically >5‰) owing to the differences in nitrogen-transformation mechanisms during the composting process (Bateman et al., 2007, Mukome et al., 2013). Thus, the use of synthetic fertilizers tends to decrease the δ¹⁵N levels in agroproducts owing to comparatively lower δ¹⁵N of synthetic fertilizers (Kerley & Jarvis, 1996). Consequently, despite certain analytical limitations, several studies have reported the viability of this analysis for authentication of various organic agroproducts such as cereals (Laursen et al., 2013, Nishida and Sato, 2015, Paolini et al., 2015), vegetables (Georgi et al., 2005, Schmidt et al., 2005), and fruits (Camin et al., 2011).

In Korea, environment-friendly agricultural products are defined as the products produced without chemicals such as pesticides, chemical fertilizers, and feed additives (none or a minimum amount) to protect the environment and to provide consumers with safer agricultural products (Ministry of Agriculture Food and Rural Affairs (MAFRA), 2014). These products are classified into two classes: organic and pesticide-free. The organic class represents agroproducts cultivated without any use of organosynthetic pesticides and chemical fertilizers, while the pesticide-free group encompasses agroproducts produced without organosynthetic pesticides but using up to a third of the recommended level of chemical fertilizers. Organic and pesticide-free agroproducts accounted for 0.6% and 2.8%, respectively, of the total agroproduct produce cultivated in Korea over 2014. Their market share decreased somewhat in the past few years as a result of the decrease in the total environment-friendly agricultural produce certification area caused by the increasing strengthening of environment-friendly certification policy. The certification procedure for environment-friendly agroproducts in Korea relies mainly on pesticide residue analysis. As mentioned previously, however, this method, when used alone, can occasionally fail to distinguish between organic, pesticide-free, and conventional agroproducts as a result of its analytical detection limits and the short half-lives of pesticides.

Production of milled rice (Oryza sativa L.) reached 4327 thousand metric tons in Korea in 2015/2016 (USDA., 2016). However, to our knowledge, no studies have been reported on the authentication of environment-friendly rice in Korea using δ¹³C and δ¹⁵N analyses. Therefore, this study aims to examine the variation in δ¹³C and δ¹⁵N in environment-friendly rice samples distributed in the Korean marketplace as a function of rice type and producer. Additionally, multi-residue pesticide analysis is applied to some rice samples to provide a comparison with the organic authentication certification. Our preliminary δ¹³C/δ¹⁵N analysis results on different rice grains suggest that this technique can be developed into a valuable complementary tool for the authentication of organic produce and can facilitate the detection of incorrect or fraudulent labeling and adulteration of organic rice traded in Korea, with potential application to other countries.