Análise comparativa entre oito métodos de esterificação na determinação quantitativa de ácidos graxos em óleo vegetal

Abstract

To perform the analysis of fatty acids through gas chromatography, it is necessary to apply esterification procedures in which fatty acids are converted into more volatile compounds, such as the fatty acids methyl esters (FAME). Esterification methods are usually subdivided into two categories: acid catalysis and base catalysis. In acid catalysis, the reagents mostly used are the hydrochloric acid (HCI), the sulfuric acid (H2SO4) and boron trifluoride (BF3) in methanol; whereas the ones used in base catalysis are the sodium hydroxide (NaOH) or potassium (KOH) in methanol, and sodium methoxide (NaOCH3) in methanol. Due to the possibility of obtaining different results regarding the concentration of fatty acids for the same sample, in function of the esterification method used, the efficiency of 8 different esterification methods that involve acid and base catalysis, in the quantitative determination of FAME, was verified in tive vegetable oils (soybean, canola, f1axseed, olive and palm oil). The methods of acid catalysis analyzed were described by Metcalfe et al., 1966 (MET); Bannon v, 1982a (BAN); Joseph and Ackman, 1992 (JAC); Hartman and Lago, 1973 (HLA); whereas the base catalysis methods analyzed were Jham et al., 1982 (JHA); ISO 5509,1978 (ISO); Bannon et al., 1982b (BBA) and Schuchardt and Lopes, 1988 (SLO). Results showed the efficiency of the esteritication methods for the main saturated fatty acids (C16:0 and C18:0) that were present in the vegetable oils analyzed. MET and BAN methods presented close results, though inferior, when compared to other methods for the unsaturated fatty acids, possibly due to the heating up period used in esterification. The SLO method has also presented inferior concentration values for the unsaturated fatty acids. That may have occurred due to the molecule size of the catalytic unit and by distortions in the chain of unsaturated fatty acids, as well as by the heating time used in the method. Among the main unsaturated fatty acids, which were present in the oils analyzed, JHA and HLA methods presented inferior concentration values for methy oleate and methyl linoleate, respectively. However, it should be taken into consideration that those methods use reagents with low cost, lower toxicity but larger availability, if compared to methods that use BF3. The most efficient methods for esterification of unsaturated fatty acids in the oils analyzed were JAC, ISO and BBA. Nevertheless, the BF3 reagent in methanol, used in JAC method, is extremely toxic, besides presenting high cost and limited useful life. Thus, when the oil to be analyzed presents low acidity, the method of base catalysis ISO, which uses NaOH in methanol and BBA (NaOCH3 in methanol) can be used to substitute JAC method, once they use reagents presenting lower toxicity and lower cost. Results obtained showed that the method to be chosen for performing the analysis of fatty acids depend also on the composition of the oil to be studied.