Avaliação da qualidade e estudo cinético de óleo de gergelim sob estresse termoxidativo por espectroscopia de UV e RMN de H1 e C13

Abstract

Volumetric techniques commonly used in quality control of vegetable oils are laborious and require time, using large amounts of sample. Thus research has been conduced in order to implement new techniques faster and more efficient. In this context, spectroscopic techniques such as UV and NMR appear as alternatives to conventional process. This study aimed to monitor by ultraviolet spectroscopy and nuclear magnetic resonance, the quality of sesame oil under heat stress and to investigate the thermoxidation kinetics. The samples of sesame, commercial sesame and soybean oils were subjected to heat stress (180 C) for 120 h in a discontinuous way and sampled every 8 hours and analyzed by volumetric and spectroscopic techniques. The results showed that sesame oil has higher thermal stability than the commercial sesame and soybean oils. Data obtained from H1 NMR spectra and kinetic data indicated that the hydrogen more susceptible to thermoxidation were: bisallyl, vinyl and allyl respectively. A satisfactory Multiple correlation was obtained between H1 NMR data and acidity values (R2 = 0.7940; r = 0.89; n = 15) explaining 80% of the correlation. Good simple correlation was obtained between acidity value and UV absorbance at 230 nm (R2 = 0.8976; r = 0.9474; n = 11) explaining 90% of the correlation. Monitoring the concentration of hydrogen bisallyl, vinyl and allyl, the kinetics of thermoxidation, the first order, showed the following rate constants (mol L-1.h-1). Sesame: 0.0088; 0.0052 and 0.0037; Sesame commercial: 0.0169; 0.0101 and 0.0074 and soybeans: 0.0191; 0.0133 and 0.0097. The results indicated that the UV, H1 and C13 NMR spectroscopic techniques, are effective in controlling the quality of vegetable oils under heat stress and may be used in routine analysis.