Abstract
Four different corn oil samples including stripped (SCO, Control), refined (RCO), stripped corn oil enriched with rosemary extract (SCO+ROS) and ascorbyl palmitate (SCO+AP) were exposed to microwave (MWH) and conventional heating (CVH). For both heating methods, peroxide value (PV) and conjugated dienes increased at up to 230 °C, at which temperature hexanal (HEX) and conjugated trienes started to increase instead. Kinetic analysis revealed that PV and HEX formation were first ordered and the reaction rate among the samples was as follows: Control > SCO+ROS > RCO > SCO + AP for PV and SCO + ROS > RCO > SCO+AP > Control for HEX. The unsaturated fatty acid contents of CVH and MWH treated samples showed 9.5 and 12.9% reduction in SCO, while they were 2.9 and 7.7% in RCO, 3.6 and 6.1% in SCO + ROS, and finally 4.0 and 4.8% in SCO + AP. It was concluded that MWH led to a more severe deterioration and that the antioxidant activity of ROS was superior to that of AP for both heating methods.
Highlights
Microwave heating has been used for many years in the food industry
Tan et al, (2001) indicated that the ratio of unsaturated fatty acids (UFA), such as linoleic acid, decreased during microwave heating while the sum of SFA increased in corn and soybean oils
The level of increments in peroxide value (PV), K232 and HEX were higher in microwave heating (MWH), which indicates a lower oxidative stability in microwave treatment compared to conventional heating
Summary
It has been a useful tool for improving the success of several unit operations, including extraction and filtration (Tan et al, 2001) In such operations, microwave heating has been advantageous over conventional heating due to reduced process time and improved food quality. Yoshida et al, (1992b) reported that the microwave heating of vegetable oils and fatty foods led to a loss in some vitamins, the isomerization of fatty acids and thermo oxidative degradation. All these show that microwave heating has some detrimental effects on the quality and composition of oils and fatty foods
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