Quality Changes Of Flaxseed Oil And Palm Olein During Deep Frying Of Dough Stick And Their Effects On Intestinal Health

Frying is a traditional way of cooking food,and fried foods are widely favored by the public because of their unique flavor.However,long-term repeated frying process at high temperature will deteriorate the quality of edible oil and pose a threat to human health.In this study,the deep frying experiment was conducted using flaxseed oil and palm olein at a temperature of 180±5 ℃,where the end point of deep frying was fixed as the total polar compounds reached 27%(National Health Standard for edible oils).The quality changes of two edible oils were explored during deep frying process,and the two oils before and after frying were used to feed the animals by gavage method in order to compare the changes in apparent index,blood lipid levels and oxidative stress index of rats with different oils intake.At the same time,changes of intestinal tissue structure,inflammation levels,barrier proteins,intestinal flora and alimentation function in rats were explored combining with the results of physical and chemical properties of frying oils above,which comprehensively analyze the effects of different oils on the intestinal health.The main contents are as follows:(1)Changes of physicochemical indicators like acid value,total oxidation value,total polar compounds,triglyceride polymer,oxidation stability,and fatty acid composition were analyzed during the whole frying process.The results showed thatthe acid value,total oxidation value,total polar compounds and triglyceride polymer contents of two vegetable oils increased with the increase of frying time in the continuous deep frying process.However,the frying stability of flaxseed oil was better than palm olein within 4 hours,the retention rate of linolenic acid could reach97% and the total polar compounds was lower.Compared to palm olein,the content of polyunsaturated fatty acids in flaxseed oil especially α-linolenic acid gradually decreased when the frying time exceeded 4 hours.The total polar compounds of flaxseed oil reached 27.78±0.08% after 13 hours of frying,and the total polar compounds of palm olein reached 27.27±0.29% after 18 hours of frying,which exceeded the national standard limit(27%).In addition,the induction time of flaxseed oil oxidation before and after frying was much lower than that of palm olein,indicating its poor oxidation stability.However,the atherosclerosis index(AI)and thrombosis index(TI)of flaxseed oil were also much lower than that of palm olein.Through a series of physicochemical index comparisons,it was proved that flaxseed oil was suitable for short-term(<4 hours)frying,while palm olein showed better stability during long-term frying.(2)Feeding Sprague-Dawley(SD)rats for 8 weeks by gavage method with dosage of 5g/kg according to the body weight of rats,the changes of intestinal tissue structure,inflammatory factor expression level and mucosal barrier protein content were analyzed to explore the effects of different oil intakes on the health of intestinal tissues.The results showed that the intake of frying oils resulted in abnormal blood lipid levels,the frying flaxseed oil was more likely to cause obesity.In addition,the oil intake triggered the body’s oxidative stress response,where the frying oil made this reaction more obvious.From the structural changes of duodenum,jejunum,ileum,and colon,the tissue structures of four intestinal segments from the frying oil group were damaged,intestinal epithelial cell structure was disordered accompanying with inflammatory cell infiltration and the diffused end of intestinal villi.However,the intestinal tissue structure of flaxseed oil group was relatively stable,the intestinal villi were clearly arranged without any changes.Furthermore,the intake of frying oils significantly increased the expression level of pro-inflammatory factors,the reduced expression level of anti-inflammatory factors also promoted the inflammatory response of intestinal tissues,and the frying oil intake could significantly reduce the content of intestinal mucin(MUC2)and some tight junction protein,resulting in the damage of mucosal barrier of intestinal tissues.Nevertheless,the intake of flaxseed oil increased the content of these proteins,which was good for intestinal tissues to form mucosal barriers,and had a beneficial effect on maintaining intestinal health.(3)The effect of different oil intake on the environmental homeostasis of intestinal flora and alimentation function was explored by analyzing changes of intestinal flora composition,colon contents and the content of nutrients in rat feces.The results showed that flaxseed oil diet could increase the abundance of some probiotics such as Lachnospiraceae family,Peptostreptococcaceae family,Allobaculum genus and Romboutsia genus,which are beneficial to prevention and treatment of diabetes,metabolic disorders and Crohn’s enteropathy.Moreover,the intake of flaxseed oil reduced the abundance of Firmicutes phylum and increased the abundance of Bacteroidetes phylum,which is good to improve obese conditions.The intake of deep frying oil increased the abundance of pathogenic bacteria like Bacteroidetes phylum,Proteobacteria phylum,Desulfovibrionaceae family,and Erysipelotrichaceae family,and decreased the abundance of beneficial bacteria like Actinobacteria phylum,Lactobacillaceae family,Lachnospiraceae＿NK4A136＿group genus.In frying flaxseed oil group,the water content of colon contents,water-soluble protein and reducing sugar content in feces were higher than those in normal flaxseed oil group.The Na and K elements in feces of the frying oil group were significantly higher than those in other groups.Thus,the intake of frying oil,especially frying flaxseed oil,would disorder the intestinal flora structure,reduce the abundance of beneficial bacteria and the absorption effect of nutrients,which affects intestinal health.