Correlação dos teores de cálcio, ferro e oxalato em plantas alimentícias não convencionais (Panc): uma avaliação analítica de diferentes métodos de preparo de amostra

Abstract

This work describes the analytical evaluation of calcium and oxalate content in species of Unconventional Food Plants (PANC), emphasizing the correlation between these constituents and the influence of sample preparation methods and analytical techniques employed, considering the nutritional potential of these plant matrices and the factors that can affect mineral bioavailability. Subsequently, iron quantification was implemented as a second mineral to expand the elemental analysis and test the capabilities of the optimized method. The plant samples were subjected to standardized preparation procedures, including cleaning, drying, grinding, and homogenization, ensuring representativeness and repeatability. Calcium determination was performed after ultrasound-assisted extraction, a technique selected for its greater efficiency and lower reagent consumption compared to conventional sample preparation methods, with quantification conducted by flame atomic absorption spectrometry and flame atomic emission spectrometry. Sample preparation optimization was conducted using a 2⁵⁻¹ fractional factorial design, in which experimental factors related to extraction conditions and instrumental technique were evaluated, allowing the identification of the most significant variables and their interactions with a reduction in the number of experimental trials. The spectrometric methods showed excellent analytical performance, with linear analytical curves throughout the evaluated working range, coefficients of determination greater than 0.99, and relative standard deviations less than 5%, indicating high precision. The limits of detection (0.7 mg L⁻¹ of Ca and 0.2 mg L⁻¹ of Fe) and quantification (2.1 mg L⁻¹ of Ca and 0.6 mg L⁻¹ of Fe) obtained were adequate for the analysis of plant matrices, and the recovery assays showed values close to the ideal range, demonstrating good accuracy and low influence of matrix effects. The comparison between the results obtained for calcium by the two spectrometric techniques revealed a strong linear correlation, confirming the analytical equivalence of the methods under optimized conditions. The calcium levels determined in the analyzed species are within significant ranges, varying according to the plant species and the preparation method (cooking), with values comparable to or higher than those reported for conventional vegetables, highlighting the potential of these unconventional edible plants (PANC) as alternative sources of the mineral. The same was observed for iron, which showed consistent behavior consistent with the levels reported in the literature for different analytical techniques. In parallel, oxalate was determined by redox titrimetry, permanganometry, and molecular absorption spectrophotometry in the ultraviolet-visible region, methods that showed good agreement with each other. The analytical curves for oxalate also exhibited high coefficients of determination, greater than 0.99, and adequate precision, with relative standard deviations of less than 5%, in addition to satisfactory statistical validation by analysis of variance, indicating significance of the regression and absence of lack of fit. The oxalate levels found varied among the analyzed species, falling within ranges that can distinctly influence calcium bioavailability, depending on the ratio between these constituents. The combined analysis of the results showed that species with high calcium levels do not always present greater effective nutritional potential, since higher oxalate concentrations can limit mineral absorption. Thus, the correlation between calcium and oxalate levels proved fundamental for a more realistic interpretation of the nutritional value of the studied unconventional food plants (PANC). Keywords: spectrometry, calcium, iron, PANC (unconventional food plants), sample preparation, experimental design.