Stady On The Responses And Changes Of Edible Vegetable Oil In Thermal Environment

Under heating process, a series of chemical reactions in vegetable oil will lead to quality changes of oil including acid value,peroxidevalue, carbonyl value and the content of trans-fatty acids and polar substances. By studying the changes of the representative indicators aforementioned, and then we can analyze the quality of the vegetable oil under heating and other factors that might affect the vegetable oil’s quality.The most ordinary edible soybean oil was selected as a representative of the vegetable oil, then different heat treatment processes are designed by means of orthogonal test. After being treated under different heat processes, the sensory quality of oil, the basic physical and chemical indices of the oil and the conductivity of the oil have been measured. The study systematically investigates the effects of heating temperature, time, addition amount of edible salt and monosodium glutamate on quality of oil by using iron pot and non-stick pan as two kind of heating medium respectively. The results showed that time has a most significant effect on acid value, carbonyl value and the conductivity of the oil, while the addition amount of slat and monosodium glutamate have lest effect on these indicators of oil compared with time and heating temperature. Generally, the acid value and carbonyl value of non-stick pan are larger than that of iron pan, indicating that heating medium affect the quality of oil and stable quality of oil is obtained under iron pan environment.FTIR analytical test of the oil samples in various heating processes has been conducted, the results showed that an obvious infrared absorption band appears in the vicinity of wavelength, where trans-fatty acid(TFA) can be easily observed, while all of these detected TFA are harmful to human health, they are a kind of non-conjugated trans-fatty acid.There are no absorption peaks appeared at the range of wavelength 3700-3500cm-1below temperature of 150℃,after the oil being heated for 20 min in the iron pan, there is no hydroperoxide or secondary oxidative products are observed during the process. While peaks appear when the non-stick pan is heated 20 min,which shows that the heating medium will affect the deterioration rate of the oil and the non-stick pan affects the quality of oil significantly,that is, the oil heated in the non-stick pan has a higher deterioration rate. The conclusion drawn is agree with that in the second chapter of the work, which concluded that the acid value and carbonyl value had similar trend with the oil quality. When the heating temperature increased to 180 ℃, heating the oil 20 min,peaks observed at the range of wavelength 3700-3500cm-1, indicating that the increase of the heating temperature will accelerate the degradation of the unsaturated components of the oil, hence the temperature of the oil should be controlled below 150℃ as possible.Then, the components and quantity of the fatty acids of the edible oil sample treated in different heating process has been studied by GC analytical technology, the results showed that afterhigh temperature heating treatment, the contents of main fatty acid C16:0 and C18:0 in Soybean oil increased with the extension of heating time, and in contrast to the two kinds of saturated fatty acidsabove,the contents of unsaturated fatty acids C18:1, C18:2 and C18:3 reducedwith the extension of heating time.The content of total fatty acids of the oil is gradually reduced with the extension of heating time, the quality of oil deteriorated gradually. The content of trans fatty acids showed a trend of increase with the extension of heating time, among them, the growing rate of trans C18:3 isthe largest,the growth of trans C18:2 is larger, with the iron pan medium, after 4h heating under 210 degree, the contents of the two kinds of main trans fatty acids are 0.021% and0.025% in the initial and increased to 1.901% and 1.533% in the end, achieve54.60%and44.03% of the total amount of trans fatty acidsrespectively; with the non-stick pan, after 4h heating under 210 degree, the contents of trans C18:3 and trans C18:2 respectively increased to 1.915% and 1.564%,theincreased level of trans C18:1 is the minimum. Compared with the iron pan medium,non-stick pan has more effect on the content of fatty acids of oil,the quality of oil in non-stick pan deteriorated seriously and faster.