Desenvolvimento de procedimentos de decomposição em sistema fechado para análise elementar de queijos brancos.

Abstract

Cheese is the most consumed dairy product, and its popularity is attributed to its flavor, versatility of use, price, wide variety, and nutritional value. Recently, consumers have preferred white cheeses over yellow ones because of the lower caloric content. However, studies on determining its essential elements are still scarce in Brazil. In this sense, this work aims to propose analytical strategies for determining the total content of Ca, Cu, K, P, Na, and Zn in white cheese samples using inductively coupled plasma optical emission spectrometry (ICP OES) in order to contribute to the establishment of their nutritional contents. The following acid decomposition procedures were proposed: i) by conductive heating in a closed digester block; ii) assisted by microwave radiation. In both, the decomposition parameters, HNO3 concentration, decomposition time, and temperature were optimized, employing two-level full factorial designs, ensuring maximum digestion efficiency. For this purpose, 6 mL of HNO3 was added to samples of white cheese (m ≈ 0.250 g) in the presence of 2 mL of 30% H2O2 (w w-1). The decomposition efficiency was evaluated by visual appearance and dissolved carbon content, in addition to using multiple responses. The optimal condition for decomposition in the digester block was established: HNO3 concentration of 2.0 mol L-1 and heating at a temperature of 180 ºC for 80 minutes. For the microwave oven procedure, the optimal condition was: HNO3 concentration of 2.0 mol L-1 and irradiation for 30 minutes at a temperature of 140 ºC. Under the optimized conditions, the DOC contents determined in digested with the digester block and microwave oven were 1.35 ± 0.11 and 1.04 ± 0.06 g L-1. The proposed procedures were validated by determining the figures of merit as recommended by ANVISA. They showed good accuracy with concordant results (p<0.05) with the certificates in the certified reference material (SRM NIST 1549a – Powdered milk). Furthermore, good precision was found (RSD < 10%), with acceptable linear working ranges and good linearity (R≥ 0.999), and no matrix effect. Limits of detection ranged from 2 (P) to 16 μg g-1 (Ca) (MW procedure) and 0,3 (Cu) to 14 μg g-1 (Na) (closed block). The validated procedures were applied to analyze nine white cheese samples of different types (buffalo mozzarella, minas fresh cheese, ricotta, curd, and cottage cheese). It was observed that the highest concentrations were for Ca, followed by P, Na, and K. The lowest concentrations were observed for the microelement Cu, followed by Zn. It was found that the type of cheese influenced the variation in nutrient content. Furthermore, for all types of cheese, a discrepancy (p<0.05) was observed between concentrations for at least one analyte, confirming the lack of uniformity in nutritional composition. The recommended daily intake (RDI) evaluation showed that all samples of buffalo mozzarella cheese and Minas fresh cheese analyzed could be considered sources of Ca, P, and Zn. Regarding sodium, only the ricotta of F brand had concentrations below the recommended by law (>80 g 30 g-1 ). Additionally, these contents were significantly higher than those declared on the labels.