Study On Changes Of Fatty Acid And Volatile Components Of Vegetable Oils During The Oxidation

Vegetable oils are the primary source of necessary fatty acids because of its rich unsaturated fatty acid. Particularly, some vegetable oils have large amounts of Linoleic acid which can contribute to reduce the level of blood press and cholesterol. In addition, there is a high content of Linolenic acid, which has the function to enhance intelligence and immunity in other vegetable oils. However, it is extremely rapid to happen that the unsaturated fatty acid will react with oxygen and degrade after the oils are exposed in the air. On one hand, it will cause the decline of nutrition of vegetable oils. On the other hand, the off-odor from the degradation will affect the aroma of vegetable oils. What is more, some poison compounds will be produced such as trans-fatty acid and hydro peroxide which will significantly damage the health of human beings after eating. The fatty acid and volatile components of some common edible vegetable oils were analyzed and identified through this study in order to figure out the relation among the composition of fatty acids, volatile components and primary volatile components. Experimental data for quality management of vegetable oils were provided by this study and data references for the volatile components generation pathways were offered in this study. In addition, through the accelerated oxidation experiment, an ideal method was established to evaluate the degree of oxidation and seek out the source of off-odor during soybean oils’long-term storage at the room temperature. What is more, it is helpful to establish the methods to evaluate the oxidation of other vegetable oils and prevent people from eating the deteriorated oils to damage their health. The main research contents were as follow:Firstly, fatty acid composition of six common fresh vegetable oils were analyzed by gas chromatography (GC) and compared including soybean oil, corn oil, sunflower seed oil, olive oil, rice bran oil and linseed oil. As a consequence, the basic experimental data for the volatile components generation pathways were acquired. Then, the comparative analysis of the volatile components of six fresh vegetable oils was worked out by headspace solid phase micro-extraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS). The relative odor activity value (ROAV) was applied to determine the primary volatile components of different samples. After that, fresh soybean oil was studied as the raw material through the accelerated oxidation with stable temperature to seek out the variation and correlation among the sensory smell, peroxide value (POV), the contents of trans-fatty acid and characteristic volatile components which contributes to establish an ideal analysis method for vegetable oil oxidation. Lastly, the method established by the accelerating oxidation could help to estimate the oxidation metamorphic degree of fresh soybean oil during the long-term storage at room temperature after opening. The major results were as follow:(1) The fatty acid composition of the six samples was analyzed by GC.15fatty acids were detected in all the six samples, including7saturated fatty acids,4monounsaturated fatty acids and4polyunsaturated fatty acids without any trans-fatty acid. The composition of soybean oil, com oil and sunflower oil was extraordinarily similar. On the contrary, the constitution of fatty acid of olive oil and linseed oil was totally opposite as well as from the other four oils.(2) The volatile components of the six samples were analyzed by HS-SPME-GC-MS.78volatile compounds were acquired in all the six samples, including19aldehydes,13ketones,12hydrocarbons,11aromatics,10alcohols,6acids,4esters and1furan. In addition,2-Tertbutyl-2,5-cyclohexadiene-1,4-dione, Pentanal,(E)-2-Heptenal, Hexanal and Nonanal were the compounds with high relative content in soybean oil; Hexanal,(E)-2-Heptenal, Pentanal, Nonanal and Heptanal were the compounds with high relative content in corn oil;2-Tertbutyl-2,5-cyclohexadiene-1,4-dione, Hexanal,(E)-2-Heptenal, Pentanal and Nonanal were the compounds with high relative content in sunflower seed oil; the compounds with high relative content in olive oil include Nonanal,1-Tridecene,(E)-2-Heptenal, Hexanal, Isopropenyl cyclohexane and Octanal; the compounds with high relative content in rice bran oil consist of Hexanal, Pentanal,(E)-2-Heptenal, Nonanal and Heptanal; the coupounds with high relative content in olive oil contain Pentanal,(E,E)-3,5-Octadien-2-one, Hexanal,3,5-Octadien-2-one and (E,E)-2,4-Heptadienal. What is more, the volatiles constitution was determined by the composition of fatty acid so that the constitution of soybean oil, corn oil, sunflower seed oil and rice bran oil were similar and olive oil and linseed oil were extremely different as well as from the other four oils.(3) ROAV method was applied to evaluate the contribution of the primary volatile components of six vegetable oils. The primary volatile components of soybean oil were composed of Decanal,(E,E)-2,4-Decadienal,(E)-2-Decenal, Nonanal, Pentanal, Octanal, Hexanal,2-Pentyl-furan,(E)-2-Heptenal, Heptanal,(E)-2-Octenal, Ethyl acetate and Methoxyphenol; the primary volatile components of corn oil were consisted of Decanal,(E,E)-2,4-Decadienal, Nonanal, Hexanal, Octanal,(E)-2-Decenal,2-Pentyl-furan, Heptanal,(E)-2-Heptenal, Pentanal,(E)-2-Octenal, Ethyl acetate and2-Methoxy-4-vinyl phenol; the primary volatile components of sunflower seed oil were constituted of (E,E)-2,4-Decadienal, Decanal, Nonanal, Hexanal,(E)-2-Decenal, Octanal,2-Pentyl-furan, Heptanal,(E)-2-Heptenal,(E)-2-Octenal and Pentanal; Nonanal,(E)-2-Decenal, Octanal,(E,E)-2,4-Decadienal, Decanal, Hexanal,2-Pentyl-furan,2-Methoxy-4-vinyl phenol.(E)-2-Heptenal, Heptanal,(E)-2-Octenal,(E,E)-2,4-Heptadienal and Ethyl acetate were the primary volatile components of olive oil; Decanal,(E,E)-2,4-Decadienal, Nonanal, Octanal, Hexanal,(E)-2-Decenal,2-Pentyl-furan, Pentanal, Heptanal,(E)-2-Heptenal and (E)-2-Octenal were the primary volatile components of rice bran oil;(E,E)-2,4-Decadienal, Decanal, Hexanal, Octanal, Pentanal,(E,E)-2,4-Heptadienal,(E)-2-Decenal,2-Pentyl-furan, Heptanal,(E)-2-Octenal,2-Methoxy-4-vinyl phenol and Methoxyphenol were the primary volatile components of linseed oil. Furthermore, the constitution of the primary volatile components was determined by the composition of fatty acid so that the constitution of the primary volatile components of soybean oil, corn oil, sunflower seed oil and rice bran oil were similar and olive oil and linseed oil were extremely different as well as from the other four oils.(4) The changes of some indexs including POV-value, trans-Linoleic acid and12volatile components which extremely effect smell in soybean oil were analyzed. The measured results show that:the content of Hexanal, Nonanal and trans-Linoleic acid and peroxide value in the soybean oil went up while the accelerated oxidation time increased. The rise of two compounds shows a high correlation with trans-Linoleic acid, POV and smell index. According to these results, these indexes together were available to evaluate the oxidation in soybean oil objectively which could be referenced to evaluate the oxidation for vegetable oils.(5) The indexes including smell index, peroxide value, the content of trans-Linoleic acid, Hexanal and Nonanal were applied to evaluate the oxidation during the room-temperature long-term storage. The measured results show that: with the time of storage went, the content of trans-Linoleic acid, Hexanal and Nonanal increased while the rising rate of the content of Hexanal and Nonanal went up initially but decreased afterwards as well as the peroxide value, which indicated that the degree of oxidation of soybean oil deepened. To be specific, the peroxide value was38.6meq/kg and the content of trans-Linoleic acid, Hexanal and Nonanal were1.75%,2.62μg/g and1.96μg/g respectively when the rancid smell appeared in the deteriorated soybean oil after6-month storage. Eventually, the peroxide value was20.6meq/kg and the content of trans-Linoleic acid. Hexanal and Nonanal were5.72%,7.16μg/g and5.26μg/g separately after15-month storage. As the increasing rate of the content of Hexanal and Nonanal would drop until stable, the smell would tend to be steady differently from the oxidation deepening.