Caracterização química de azeites português e brasileiro sujeitos a aquecimento e utilizados em fritura

Mestrado em Engenharia Alimentar - Instituto Superior de Agronomia

Olive oil is obtained by physical extraction of olives (fruit of the olive tree, Olea europea L.), retaining its flavor, aroma and unique chemical and biological properties. In addition to the balanced composition of saturated, monounsaturated and polyunsaturated fatty acids, it also has essential fatty acids, high levels of fat soluble vitamins (A, D, E and K) and natural antioxidants (tocopherols and polyphenols), providing benefits for human health. This study compared the characteristics of the portuguese Oliveira da Serra olive oil and the brazilian EPAMIG olive oil, before and after they were submitted to heating at 180 ºC on a heater or used in potato frying at 180 ºC. The potatoes were purchased on a local market. In the heating tests, 100 mL of extra virgin olive oil sample remained in the heater at 180 ° C for 10 hours. During the night they were cooled in a desiccator. This process was repeated for 20 and 30 hours of heating. For the frying test an electric fryer was used, and 500 mL of olive oil was used to fry three portions of 100 g of potatoes. Before the first frying, the olive oil was heated for 10 minutes at 180 °C. After the first 5 minutes of frying, a sample of 100 mL of olive oil was collected. The same procedure was performed for the second and third frying. The collected olive oils from heating or frying were stored in sealed glass vials, protected from light at -18 °C. Both olive oils were analysed for the acidity, peroxide value, degree of oxidation (p-anisidine and total value), fatty acid and tocopherol composition and thermal stability. According to visual observations of both olive oils, the portuguese one was clearer (more translucent) than the brazilian olive oil, which showed an intense green color, and a dark brown solid particles sediment. For both olive oils, and during the heating process, the acidity increased, 69,4% on the portuguese and 44,2% on the brazilian olive oil, in relation to the initial acidity. On the frying process, the portuguese olive oil showed an increase of 36%, but a slight decrease in the acidity of the brazilian olive oil was observed. Concerning the peroxide value determination, in portuguese olive oil it increased for up to 20 h of heating and decreased thereafter, which is caused by the degradation of hydroperoxides and secondary oxidation products formation. In the brazilian olive oil, the peroxide value stayed high after 30 h of heating, maybe because it had less initial peroxide content than the portuguese olive oil. In the frying assay, the behavior observed in both olive oils was similar to that seen in the heating trials. In frying, due to the use of potatoes, which contain naturally high levels of humidity, despite the short frying time, the process of formation and decomposition of peroxides is considerably faster. The results of p-anisidine index indicate that in the first 10 hours of heating, there was a large increase in the concentration of aldehydes in both olive oils, reaching a constant level after this. In the frying process, the index values of p-anisidine increased along the frying process on both olive oils, but once again, faster than in the heating process. The brazilian olive oil showed lower concentrations of aldehydes than the portuguese olive oil. Thus, according to this parameter the brazilian olive oil is considered to be more stable than the portuguese. In relation to the total value of oxidation, both olive oils showed a sharp increase after 10 hours of heating, reaching a plateau situation up to 20 hours of heating. A decrease in the total oxidation was observed for the portuguese olive oil after 30 hours of heating, due to the sudden decrease in the peroxide value. On the brazilian olive oil, this variation was not observed. The same result described above was seen in the frying trial with portuguese olive oil. In the third frying, the peroxide value decreased, which led to a slight decrease in the total oxidation. The brazilian olive oil is apparently more stable to oxidation than the portuguese. The fatty acid determination showed that during heating, due to degradation reactions, the polyunsaturated fatty acids decreased, resulting in the formation of saturated and monounsaturated fatty acids. This behavior is more evident in the portuguese olive oil than in the brazilian one. The levels of monounsaturated fatty acids increased over heating due to the degradation of the double bonds of polyunsaturated fatty acids. In the frying process, the same results were observed. During both processes the relative percentage of saturated fatty acids increased as the percentage of unsaturated and polyunsaturated fatty acids decreased. The α-tocopherol is the main tocopherol of the olive oil, with initial contents of 17.03 mg/kg and 17.86 mg/kg in portuguese and brazilian olive oils respectively. During heating, there was a sharp decline in the levels of tocopherols, especially during the first 10 hours. After 30 hours the brazilian olive oil had the levels of the three tocopherols ( and ) very close to zero, while the portuguese still had trace amounts of -tocopherol (1.55 mg/kg). Looking at the tocopherols profile after the first and second frying, a decrease of -tocopherol occurred, while the levels of  and -tocopherol remained unchanged. After the third frying, an unexpected value was observed: the -tocopherol content increased in the portuguese olive oil and the -tocopherol increased in the brazilian one. Concerning the thermogravimetric analysis, the portuguese olive oil indicated that it is thermally resistant up to 396,59 °C, temperature at which it begins to degrade, and its degradation maximum rate occurs at 425,14 °C. The brazilian olive oil is thermally resistant up to 385,95 °C. In conclusion, the differences observed in the values obtained in the various tests carried out for the two olive oils may have been influenced by factors such olive trees age, the olives variety, the influence of environmental and geographical process of cultivation, olive oil extraction processes or even the amounts of natural antioxidants