Effects Of Uniform Microwave Heating And Conductive Heating On The Oxidation Of Sunflower Oil

Microwave heating has been widely used in the food industry due to fast heating and low thermal inertia.To solve the problem of non-uniform heating,a new uniform microwave heating device was built up,and the electric field distribution of the heating cavity was calculated by HFSS.The effect of uniform microwave heating and conductive heating on sunflower oil oxidation were compared,including changes in the basic physical and chemical indexes,the volatile components,and the generation of free radicals during heating.The main conclusions were as follows:(1)Standing waves were formed in the cavity of a traditional household microwave oven,which caused uneven heating.Theoretical calculations show that the heating uniformity is related to the microwave frequency and electromagnetic field distribution in homogeneous samples.The uniform microwave heating equipment used a solid-state microwave source,and a matched load was used to absorb excess microwaves and sample heated in a traveling wave manner.The electric field simulation results in the heating cavity showed that the higher the sample height,the larger the S11 reflection coefficient and standing wave ratio.The application of plexiglass diamond module improved heating uniformity and heating efficiency.(2)The uniform microwave heating system was used to heat the sunflower oil,and the conductive heating method was a conduction heating device with PID control function.In order to compare the effect of two heating methods on sunflower oil oxidation at same temperature,the temperature curve of microwave was consist with conduction.The PoV heated by microwave reached maximum at 10 minutes,and the conduction heating reached 30 minutes.There was no significant difference between the k232 and k270 heated by the two heating methods.Until the later stage of heating,the k232 of microwave heating was significantly higher than the conductive heating.Anisidine value and total oxidation value showed that the growth rate of microwave heating was higher than conduction heating,and there was no difference in MDA between the two heating methods.After heating,the content of polyunsaturated fatty acids decreased,and the content of monounsaturated fatty acids and saturated fatty acids increased.(3)HS-SPME-GC/MS separation and identification of volatile components in sunflower oil before and after heating.By optimizing the extraction conditions for the types and areas of the extracted peaks,it was found that DVB/CAR/PDMS had the best adsorption effect on the volatile components of sunflower oil,and the best extraction effect was detected at 50? for 40 min.The analysis of the original flavor substances of sunflower oil revealed 52 substances.The flavor substances were divided into alcohols,aromatic compounds,aldehydes,acids,ketones,alkanes,alkenes,heterocycles and ester.The content of flavor components increased after heating,indicating that sunflower oil has oxidized during heating process,and more small molecules of aldehydes and ketones has accumulated.And the content of flavor substances generated by microwave heating were higher than conduction heating.(4)The free radical generated during heating was determined by electron paramagnetic resonance.There was no signal of free radicals after glycerin is heated,indicating that glycerin is very stable during heating.Comparison of the detection patterns of palmitic acid and linoleic acid shows that the free radical pattern of linoleic acid is similar to that of sunflower oil,indicating that the oxidation of sunflower seed oil was mainly the oxidation of linoleic acid.The total number of spins shows that in the initial stage of heating,the number of free radicals generated by microwave heating is significantly higher than that of conduction heating.But the total number of spins in microwave heating decreases after reaching the maximum value in 40 minutes,the total number of spins in conduction heating is increasing,and exceeded microwave finally.It showed that the effects of microwave heating and conduction heating on the oxidation of sunflower seed oil were different even at the same temperature.It can be speculated that the bond energy of free radical chain reaction decreased in microwave heating.