Effect of energy transfer conditions on the chemical degradation of frying oil

The effects of temperature, heating time (continuous or discontinuous mode), and water steam on sunflower oil deterioration have been investigated. Oil was heated at three temperatures, 120, 150, and 190 degrees C, during a total period of 60 h to achieve a wide range of degradation degrees. At 190 degrees C, 9 h of discontinuous heating were achieved, and oil degradation was compared to the previous treatment. Water steam treatment was investigated at two oil temperatures, 120 and 150 degrees C, by frying food model samples that provided a vapor flow of 1.2 g/min during about 30 h. Energy consumptions of the fryers have been measured for each experiment. Thermooxidative degradation of heated oils was evaluated by the measurement of conjugated dienes, viscosity and total polar compounds. All these indicators showed that for the same heating time, oil degradation was more important at 190 degrees C, and most of all for an intermittent heating. However, the results showed a good correlation between oil degradation and the fryer energy consumption for all the heating experiments, continuous or discontinuous. The presence of water seemed to modify this correlation. These results highlight the complexity of oil degradation during frying, depending on oil temperature, heating mode and presence of water, which can be expressed in energy consumption.