The Formation And In Vivo Evaluation Of Polar Compounds Generated During Frying Process Of High Saturated Oils

As undesirable compounds generated in oils during deep-frying process differ in polarity from the non-altered triacylglycerols(TAGs),they are defined as polar compounds,including free fatty acids(FFAs),diacylglycerols(DAGs),oxidized TAGs,TAGs dimmers,and TAGs oligomers,etc.Determination of polar compounds in deep-fried oils is one of the most important method in the assessment of the quality of deep-fried oils.However,the formation mechanism of polar compounds remains unclear.The generation of total polar compounds(TPCs)and the distribution of different polar components in deep-fried oils and foods were investigated in the present thesis.Firstly,the formation process of TPCs in different kinds of oils during deep-frying was elucidated.Particular attention was paid to the distribution of different components of TPCs in deep-fried oils and frying substance.Secondly,the contents of acrylamide were determined in deep-fried foods obtained from different kinds of oils to evaluate the effects of polar components on the formation of acrylamide.Lastly,the effects of deep-fried palm oil and the TPCs isolated from deep-fried palm oil on the health and lipid metabolism of Kunming mice were investigated.The expression of genes involved in hepatic lipid metabolism was studied to elucidate their effects on lipid metabolism in liver.The present thesis provides novel insights into the formation of TPCs over deep-frying process and their effects on lipid metabolism in vivo.(1)The effects of frying oils' fatty acids profile on the formation of polar components and their retention in French fries and corresponding deep-fried oils were investigated in the present thesis,using oils with different fatty acids composition.Our analysis showed that the total polar compounds(TPCs)content in French fries was only slightly lower than that in deep-fried oils,indicating that there was no significant difference considering the amounts of TPCs in French fries and deep-fried oils.After 50 hours of deep-frying,the amount of TPCs in deep-fried palm oil(PO),refined palm kernel oil(RPKO)and refined coconut oil(RCO)was 31.65 %,38.23 % and 38.28 %,respectively.However,the amount of TPCs in corresponding French fries was 29.42 %,32.88 % and 30.54 %,respectively.Our further analysis showed that different polar components in TPCs distributed differently in deep-fried oils and oils extracted from French fries.Specifically,the level of oligomeric and dimeric triacylglycerols was higher in French fries while oxidized triacylglycerols and diacylglycerols content was higher in deep-fried oils.The different retention of TPCs components in French fries may be explained by their interactions with carbohydrates,which are shown to enhance with the increase of hydrophobic property.Chemometric analysis showed that no correlation between the polar compounds level and saturated fatty acids profile was observed.Meanwhile,the polar compounds content was highly correlated with the formation of trans-C18:1,and a highly positive association between polar compounds and C18:2 content was also observed in palm oil instead of refined palm kernel oil and refined coconut oil.(2)Oils with different fatty acids composition,including PO,RPKO and RCO,were applied in successive frying process.The formation of polar compounds during deep-frying process was studied using ~1H nuclear magnetic resonance(~1H NMR)spectroscopy.Specifically,the molar percentage of acyl groups and the concentration of aldehydes,epoxides,and alcohols were determined.Meanwhile,electron spin resonance(ESR)spectra were applied to identify the difference among different kinds of oils in radical species.Our results showed that the formation of polar compounds was dependent on the fatty acid composition of oils.The content of acyl groups in unheated palm oil was 3.04 and 5.96 times higher than that in unheated palm kernel oil and unheated coconut oil,respectively.Moreover,the content of acyl groups in deep-fried palm oil(50 h)reduced by 37.05 % while no significant difference was observed about the acyl groups levels in deep-fried palm kernel oil and coconut oil.In addition,alkyl radical was mainly observed in deep-fried palm oil instead of deep-fried palm kernel oil and coconut oil.Thus,non-polar dimeric TAGs were prone to be generated in palm oil,followed by its oxidation.In contrast,the oxidation of TAGs in palm kernel oil and coconut oil was more readily to occur,followed by the generation of polar dimeric TAGs.(3)To investigate the effects of deep-fried oils on the quality of foods,the contents of acrylamide in French fries were determined.An improved sample preparation method was developed to improve acrylamide recovery percentage in high fat foods.Chrome-Matrix C18 LC-MS/MS column(2.6 ?m,2.1×100 mm)was used for acrylamide analysis for the first time in the present study.To improve the sample preparation method,distilled deionized water is added prior to concentration process protecting acrylamide from degeneration.A good repeatability was displayed,where intraday precision(RSD)ranges from 0.04-2.38 % and interday precision(RSD)ranges from 2.34-3.26 %.Thus,the improved sample preparation method for acrylamide analysis could be highly accepted in the following studies.Further analysis on the formation of acrylamide showed that the moisture extent and free fatty acids content were related to its formation in French fries at the beginning of frying.In details,the moisture extent was negatively associated with the formation of acrylamide,and the amount of free fatty acids was positively related to the generation of acrylamide.However,acrylamide formation in French fries was suppressed,when the oils have been deep-fried for more than 30 hours.The interaction of polar components with carbohydrates might restrict the Millard reaction;thus,the level of acrylamide,a by-product of the Millard reaction,was reduced.(4)Effects of deep-fried palm oil,specifically polar compounds generated during the frying process,on lipid metabolism and liver functions were investigated in the present thesis.Kunming mice were fed with high-fat diet combined with deep-fried palm oil or polar compounds for period of 12 weeks.Hepatic lipid profile,antioxidant enzyme activity,and serum biochemical factors were analyzed,together with the morphology of liver.Our results revealed that the consumption of polar compounds was positively related to liver lesions in rodents without lipid accumulation in the liver.This was different from the liver damage caused by high energy intake,where lipid accumulation in hepatocytes was observed.The plasma lipid profile and the transcription study of genes involved in lipid metabolism showed that polar compounds facilitated the fatty acid oxidation on peroxisomes while lipid metabolism in mitochondria was suppressed by polar compounds in the diet.Glucose tolerance test(GTT)revealed that high amount of polar compounds intake impaired glucose tolerance due to its effect on the change of glucose metabolism in vivo,rather than the accumulation of lipids in the body,which was the cause of glucose tolerance under high-fat diet.Our results provided critical information on the effects of polar compounds generated during deep-frying process of palm oil on liver functions and lipid metabolism,which is important for the evaluation of the biosafety of frying oil.