Determination of Nitrite in Milk- and Soy-Based Nutritional Ingredients by Derivatization with 2,3-Diaminonaphthalene and Fluorescence Spectrometry

Abstract

Abstract Nitrite (NO2–) is an inorganic anion that can be found in various powdered milk- and soy-based nutritional ingredients as an incidental contaminant. Reliable determination of NO2– in nutritional ingredients is of paramount importance to ensure the safety of finished products. The derivatization reaction of NO2– with 2,3-diaminonaphthalene with the formation of fluorescent 2,3-naphtotriazole has been adapted to milk- and soy-based nutritional ingredients. The sample preparation consisted of protein precipitation with Carrez solution, simple pass-through cleanup of extracts utilizing a carbon black-based cartridge and derivatization, followed by batch fluorometry. The method was validated in six representative ingredient matrixes—i.e., whole-milk powder, nonfat dry milk, milk protein concentrate, whey protein concentrate, sodium caseinate, and soy protein isolate. Recovery values were 82–109%, whereas within-day and intermediate precision were 0.6–5.2 and 3.6–11% (RSDs), respectively. The method LOQ was 0.1 or 0.2 µg/g sodium nitrite (NaNO2), depending on the ingredient matrix. Surveyed NO2– concentration levels in 25 lots of 10 types of nutritional ingredients ranged from between less than 0.1 to 29 µg/g NaNO2. This method is proposed as a more sensitive and rugged alternative to the widely used ion chromatographic and colorimetric approaches.