Study Of The Formation Of Glycerol Core Aldehydes During Oil Heating Oxidation

Glyceryl ester core aldehydes（GCAs）are a group of triglycerides containing at least one aldehydic acyl group（up to three）,which are widely present during frying and are potentially harmful to human body.Although there are reports involving the formation of GCAs during frying or heating of oils,the effects of iron,water and oxygen on the formation of GCAs,which are the main factors affecting the oxidation of oils,are still blank.This study explored the effects of air,water and ferric ions(Fe³⁺)on the formation of GCAs during the heating high-oleic sunflower oil（HOSO）.And the HOSO was heating at frying and cooking temperatures.The main research contents are as follows:Firstly,the GCAs in the oil heated at 180°C were determined,and the influences of the oxygen in the oil on the GCAs formation during the heating process was explored.The oxygen content in the heating oil was regulated by static heating with different exposure area of oil,air pumping,or stirring.Four main GCAs were detected,including 8-oxo-acyl triglycerides,9-oxo-acyl triglycerides,8-decenoic-acid acyl triglyceride and 9-undecylenic-acid acyl triglyceride.The first two were saturated and the last two unsaturated.The total amount of GCAs was the highest during stirring heating,followed by pumping air,and the least during static heating.The total GCAs reached 4.86±0.04 mg/g after stirring at 180°C for 10 h and was3.30±0.03 mg/g when pumping air with 120 m L/min flowrate.Also,the total GCAs was less than 1 mg/g during static heating.The proportion of the four GCAs was also changed.GCAs（9-oxo）accounted for the largest proportion,and as the oil and air contacted more and more fully,the proportion gradually decreased,and finally stabilized at about 45%,while the proportion of the other three GCAs increased.The proportion of the two kinds of unsaturated GCAs was similar,and finally stabilized at about 25%,and the other saturated GCAs finally stabilized at about 6%.Secondly,the effect of water on the GCAs formation was investigated when the oil was stirred at 180°C by adding 2%～8%water content of oil weight.The results showed that adding small amount of water would increase GCAs content.Adding 2%and 4%water,the total GCAs increased by 23%and 11%,respectivly after heating 10 h compared with control.But too water had not affected the formation of GCAs.Lower water content（2%）increased the accumulation of saturated GCAs,and higher water inhibited the accumulation.The accumulation effect of unsaturated GCAs aggravated with the increase of water.Water only had a significant effect on the saturated GCAs（8-oxo）proportion,and the higher the water content,the more significant the GCAs（8-oxo）proportion decreased.When adding 8%of water and heating for 10 h,its proportion decreased by 30%compared with control.Thirdly,the effect of Fe³⁺on the formation of GCAs with pumping air during heating at120～180^oC was studied by adding 0～0.001mol/L Fe³⁺to the oil.The higher the temperature,the more serious the cumulative effect of Fe³⁺intensifying saturated GCAs.When adding0.0005 mol/L Fe³⁺and heated at 180^oC,the saturated GCAs content reached 2.25 mg/g increased by 97%compared with control.When temperature increased from 120°C to 140°C,Fe³⁺inhibited the unsaturated GCAs accumulation,and only when temperature was further increased an aggravating effect was occurred.The increase of Fe³⁺concentration also changed the composition of GCAs that increased the proportion of saturated GCAs and reduced the unsaturated GCAs proportion.Temperature also affected the composition of GCAs,the average proportion of saturated GCAs were significantly lower（p<0.05）at 120^oC than those at 140^oC,160^oC,and 180^oC,the former accounted for 52.78%without Fe³⁺and the latter three stand at between 61.41%and 68.01%.Furthermore,zero-order,first-order and pseudo-first-order kinetic equations were used to fit the GCAs formation,and the formation rates of the four GCAs were compared.The results showed that the GCAs formation during the heating process was more in line with the pseudo-first-order kinetics,and the R² of the fitting equations were all greater than 0.98.GCAs（8-oxo）formation rate was the smallest,between 8.96E-06 and 6.54E-05;GCAs（9-oxo）was the largest,between 7.61E-05 and 5.42E-04;the two unsaturated GCAs were in the middle and the values were close that between 1.96E-05 and 1.19E-04.The prediction equation of GCAs was established using multiple linear regression analysis to quantitatively evaluate the effects of air,moisture and Fe³⁺on GCAs,and the equation was:total amount of GCAs=0.754（heating time）+0.093(Fe³⁺concentration)+0.059（temperature）+0.138（surface area/volume of oil）+0.283（air velocity）+0.198（water content）.Finally,the acid values,peroxide values and p-anisidine values and total oxidation values of the oil samples were determined,the pearson correlation coefficients（PCC）between the four GCAs and these values were analyzed to explore the internal relationship of the four GCAs,as well as the relationship between GCAs and lipid hydrolysis,oxidation primary products,and secondary products.The results showed that the correlation among the two saturated aldehydes and among the two unsaturated aldehydes were high,with PCC of 0.972 and 0.996,respectively.The saturated GCAs content had a strong relation with the content of unsaturated GCAs with PCC ranging from 0.719 to 0.756.GCAs were weakly correlated with peroxide value,but strongly correlated with acid value,p-anisidine value and total oxidation value,indicating that the deep oxidation and hydrolysis of oil were closely related to the generation of GCAs,while the relationship between primary oxidation of oils and GCAs was not significant.