Survey of Commercial Food Products for Detection of Walnut (Juglans regia) by Two ELISA Methods and Real Time PCR.

Raquel Madrid,Teresa García,Aina García-García,Pablo Cabrera,Isabel González,Rosario Martín

doi:10.3390/foods10020440

Abstract

Labeling of food allergens in accordance with legal regulations is important to protect the health of allergic consumers. The requirements for detecting allergens in foods involve adequate specificity and sensitivity to identify very small amounts of the target allergens in complex food matrices and processed foods. In this work, one hundred commercial samples were analyzed for walnut detection using three different methods: a sandwich enzyme-linked immunosorbent assay (ELISA) kit based on polyclonal antibodies, a direct ELISA using a recombinant multimeric scFv, and a real time PCR. The most sensitive method was real time PCR followed by sandwich ELISA kit and multimeric scFv ELISA. There was agreement between the three methods for walnut detection in commercial products, except for some heat-treated samples or those that contained pecan. The walnut ELISA kit was less affected by sample processing than was the multimeric scFv ELISA, but there was cross-reactivity with pecan, producing some false positives that must be confirmed by real time PCR. According to the results obtained, 7.0 to 12.6% of samples (depending on the analytical method) contained walnut but did not declare it, confirming there is a risk for allergic consumers. Moreover, there was one sample (3.7%) labelled as containing walnut but that tested negative for this tree nut. Genetic and immunoenzymatic techniques offer complementary approaches to develop a reliable verification for walnut allergen labeling.

Highlights

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The Walnut enzyme-linked immunosorbent assay (ELISA) kit (Biomedal) is a sandwich-type immunosorbent assay designed for the detection and quantification of walnut in food products using specific polyclonal antibodies
The limit of detection (LOD) and limit of quantification (LOQ) values calculated in this work using the standard curve were 2.2 mg kg−1 and 3.3 mg kg−1, respectively, being slightly higher than those indicated in the kit manual (0.6 mg kg−1 and 2 mg kg−1, respectively)

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Regular consumption of walnuts is increasing because they are associated with beneficial effects on human health, providing protection against the development of cardiovascular-related diseases, age-related neurological disorders, and even some cancer types [1]. Walnut (Juglans regia L.) is eaten raw or used for the manufacture of sauces, yoghurts, sweets, oils, beverages, and as an ingredient to improve the quality of food products. The presence of walnut as a hidden allergen in many processed foods constitutes a serious risk for developing adverse reactions in allergic patients, from oral allergy symptoms to anaphylaxis [2]. Sensitization to walnut storage proteins is primarily acquired during childhood, it can be acquired at a later age

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Results

Conclusion