Desenvolvimento de metodologia analítica para determinação de compostos fenólicos totais em amostras de bebidas empregando MBTH (3-Metil-2-Benzotiazolinona Hidrazina) como reagente espectrofotométrico

Abstract

The phenolic compounds are included in the category of free radical scavengers, and effective in preventing (self)oxidation of compounds useful in organisms and foods. The phenolic compounds present in foods, are given beneficial health effects. As a result, the determination of these compounds has been widely studied. Among the known analytical methods for determination of total phenolic compounds, the Folin-Ciocalteu method is currently the most widely used, despite its limitations, as the sensitivity of selectivity, which may affect the test result. In this context, the objective of the present work was to promote the optimization of an alternative method, using the MBTH (3-methyl-2-enzothiazolinone hydrazine) as spectrophotometric reagent for the determination of total phenolic compounds. The method was applied to samples of beverages high in phenolic compounds with proven antioxidant action. The chemical and physical parameters (acid type, concentration and volume of reagents, among others), have been optimized using univariate analysis, using gallic acid as a reference standard and the sensitivity (slope of the analytics curve) was used as na analytical parameter. The spectrophotometric method was proposed based on na oxidative coupling reaction of MBTH with phenolic compounds, in the presence of na oxidant in acidic environment to generate a colored product. Therefore, the oxidizing Fe(III) diluted in H2SO4 or HCl and Ce(IV) diluted in H2SO4, were evaluated in the method. The linear range obtained for oxidant Fe(III) (0.5 to 6.5 mg L-1) and the oxidant Ce(IV) (1.0 to 18.0 mg L-1) equivalents of acid gallic where RSD values (relative standard deviation) were lower than 2.2% indicating a good analytical precision. The proposed method was applied to 18 different compounds (phenol, thiol, vitamin, reducing sugar, among others) and compared with the Folin-Ciocalteu method, applied to the same compounds. The method was applied to samples of wines (red and white), teas, infusions, coffees and sugar cane spirits and the results were compared with the Folin-Ciocalteu method and 4-AAP (4-aminoantipyrine) (methods used for determination of total phenolic compounds) and ABTS•+ (2,2'-azinobis (3-ethylbenzthiazoline-6-sulfonic acid)) and DPPH• (2,2-diphenyl-1-picryl-hydrazyl) (methods used to determine antioxidant capacity). The results were submitted to a linear correlation analysis for a range of 95% confidence, being consistent for most of the tests, they showed correlations between MBTH and FC (r = 0.9570); MBTH and 4-AAP (r = 0.9131); MBTH and DPPH• (r = 0.8566) and MBTH and ABTS•+ (r = 0.9602). A more detailed analysis was performed according to the different classes of compounds, and we observed that the results were more consistent for samples of wines, teas and infusions in relation to the MBTH with other methods. Thus, the spectrophotometric method developed was compatible for both the tests to determine the total phenolic compounds, such as antioxidant capacity, and is characterized as simple, fast, and feasible for the samples analyzed.