Evaluation of stability photo-oxidative of canola oil and corn oilin the presence of synthetic antioxidants/Avaliacao da estabilidade foto-oxidativa dos oleos de canola e de milho em presenca de antioxidantes sinteticos

The stability of vegetable oils in relation to oxidation depends on the balance between the composition and the presence of antioxidants and pro-oxidants. The objective of this study was to evaluate the effect of the presence of antioxidants in vegetable oils in the protection of photo-oxidation. Samples of canola oil and corn oil in the presence and absence of the antioxidants, butylated hydroxytoluene (BHA), propyl gallate (PG) and tertbutylhydroquinone (TBHQ), were subjected to photo-oxidative stress at room temperature for 60 days. The photo-oxidation reaction followed a first order kinetics. The rate constant for the period of 20 days of photo-oxidation showed that the PG antioxidant showed greater protective effect for canola oil and TBHQ showed a greater protective effect for corn oil. The UV and H NMR data at 60 days showed that the three antioxidants had a protective effect on photo-oxidation. The UV data showed increased absorption at the 232nm band due to photo-oxidation and the formation of conjugated dienes. The reduction in intensity of this absorption band in photo-oxidation with time of exposure revealed that the protection, both for the canola oil and for the corn oil, was more effective with the PG antioxidant. The rates of [R.sub.oa] oxidation and residual fraction of diallyl, allyl and vinyl hydrogens also showed that the PG antioxidant showed the best performance in protecting both oils. Key words: vegetable oils, synthetic antioxidants, photo-oxidation, UV spectroscopy, [sup.1]H NMR. A estabilidade dos oleos vegetais a oxidacao depende do equilibrio entre a composicao e a presenca de pro-oxidantes e antioxidantes. O objetivo deste estudo consistiu em avaliar o efeito da presenca de antioxidantes sinteticos durante o processo de fotooxidacao dos oleos de canola e milho. As amostras dos oleos de canola e milho em presenca e em ausencia dos antioxidantes butilhidroxitolueno (BHA), propil galato (PG) e terc butil-hidroquinona (TBHQ), foram submetidas ao estresse foto-oxidativo. A reacao de foto-oxidacao seguiu uma cinetica de primeira ordem. A constante de velocidade no periodo de 20 dias de foto-oxidacao evidenciou que o antioxidante PG apresentou maior efeito protetor para o oleo de canola e o TBHQ para o oleo de milho. A partir dos dados de UV e RMN de [sup.1]H, no periodo de 60 dias, constatou-se que os tres antioxidantes apresentaram um efeito protetor a fotooxidacao. Os dados de UV evidenciaram aumento de absorcao da banda em 232 nm