Formation Mechanism Of Aldehydes And Ketones During Thermal Processing Of Edible Oil

Vegetable oil is an indispensable edible oil for our daily cooking.It is very popular because it has large amounts of unsaturated fatty acids.However,unsaturated fatty acids are extremely unstable.People have to worry about the oxidative rancidity of vegetable oil.It is known that triglycerides are decomposed into glycerol and free fatty acids under the influence of oxygen,moisture,light,heat and microorganisms.Free fatty acids can undergo oxidative rancidity.Further,hydroperoxides and free radicals have been formed.These oxidation mechanisms have been basically studied clearly.But,few literatures of secondary oxidation which is produced by the continued decomposition of hydroperoxides were reported.Among them,most are research on lampblack,and aldehydes and related carbonyl compounds remaining in edible oil have rarely been reported.Therefore,six kinds of common edible oils were selected as research objects(camellia seed oil,olive oil,corn oil,suriflower oil,linseed oil and perilla oil).Camellia seed oil and olive oil are representatives of high oleic acid oil,corn oil and sunflower oil are representatives of high linoleic acid oil,and linseed oil and perilla oil are representatives of high linolenic acid oil.The six edible oils are heated by an oil bath to accelerate the oxidation of the oil.The heating temperatures were set to 120?,150?and 1 80?,respectively.Firstly,the aim was to find out the relationship between the oxidation degree of edible oil(acid value,peroxide value,anisidine value,total oxidation value and aldehyde and ketone content),the type of fatty acid,tocopherol content and the ability to scavenge DPPH free radical.Then we can conduct a comprehensive assessment of the degree of oxidation of edible oils.Finally,we expect to deduce the most likely formationlnechanism of carbonyl compounds remaining in edible oils.The following are the main results and conclusions of this paper:1.The acid value,peroxide value,p-anisidine value and total oxidation value of the edible oil after heating and oxidation were determined by the national standard method.It was found that any single indicator can not accurately show the degree of oxidation of oil.In general,the total oxidation value was more accurate for the evaluation of the oxidation degree of most edible oils.But,there were some deviations in the evaluation of the degree of oxidation of high oleic acid oil.After heating at 180? for 24h,the oxidation degree of the 6 edible oils from high to low was perilla seed oil>linseed oil>sunflower oil>corn oil>camellia seed oil>olive oil.2.The fatty acid content and changes of 6 fresh edible oils were determined by gas chromatography.Olive oil and camellia seed oil were rich in oleic acid(79.08%and 74.43%);corn oil and sunflower oil were rich in linoleic acid(54.94%and 60.68%);linseed oil and perilla seed oil were rich in linolenic acid(56.57%and 55.24%).3.The contents and changes of tocopherols in six fresh edible oils were determined by liquid chromatography.It was found that the ?-T content from high to low was sunflower oil>olive oil>corn oil>camellia seed oil,?-T content from high to low was followed by perilla seed oil>corn oil>linseed oil>sunflower oil>olive oil.4.The DPPH method was used to determine the in vitro antioxidant capacity of six fi esh edible oils.The order of DPPH free radical scavenging ability from high to low was:corn oil>olive oil>sunflower oil>linseed oil>perilla seed oil>camellia seed oil.5.A comprehensive analysis of vitro antioxidant capacity(DPPH),tocopherol content,fatty acid composition and oil oxidation degree showed that the degree of oxidation of oil is affected by many aspects.The effect of in vitro antioxidant capacity(DPPH)on the degree of oxidation of oils contained the effect of tocopherol type and content,but the effect of fatty acid composition was bigger than that in vitro antioxidant capacity.And the effect of vitro antioxidant capacity was greater than that in tocopherol type and content.At the same time,it was also found when the content of tocopherol in vivo exceeded the optimal antioxidant concentration,it will promote the oxidation.In addition,the fatty acid composition will affect the optimal antioxidant concentration of tocopherol and will affect the degradation rate of tocopherol.6.There were 21 aldehydes and 6 ketones detected in the heated edible oil by LCMS-IT-TOF technology.They were formaldehyde,acetaldehyde,acetone,propanal,butanone,butanal,valeraldehyde,pentanone,hexanal,hexanone,heptanal,octanal,nonanal,nonanone,decanal,decanone,acrolein,crotonaldehyde,pentenal,hexenal,heptenal,octenal,nonenal,decenal,undecenal,heptadienal and decadienal.Then,the main aldehydes were quantitatively analyzed by HPLC.Principal component analysis was carried out by selecting 12 kinds of aldehydes and 3 octadecyl fatty acids.The result was that the production of these aldehydes was closely related to the type of initial unsaturated fatty acids in the oil sample.The formation of nonanal,2-decenal and 2-undecenal was closely related to oleic acid;the formation of hexanal 2-heptenal and 2,4-decadienal was closely related to linoleic acid;the formation of propanal and 2,4-heptadienal was closely related to linolenic acid.These aldehydes can be used as an oxidation indicator for each fatty acid oil sample during high temperature heating.In addition,this paper speculates the possible formation mechanism of these 8 aldehydes in the thermal processing of edible oils,and clarifies the formation mechanism of aldehydes and ketones in edible oil during processing.