Effect Of Sunflower Seed Pretreatment On The Aroma-active Composition And Sensory Quality Of Sunflower Oils

Sunflower oil is the fourth most consumed vegetable oil in the world.Most commercial sunflower oils are refined.However,there is an increasing demand for roasted sunflower oils with rich flavor.The existing research on the flavor quality of sunflower oil are limited to the identification of volatile compounds and the mechanism of aroma formation at the molecular level has not been revealed due to the lack of identification of the aroma-active compounds.Therefore,this paper focuses on the effects of pretreatment methods of sunflower seeds(hotair roasting,oven or microwave pretreatment)on changes in the aroma-active compounds and sensory quality of sunflower oil by the means of molecular sensory science.It aimed to reveal the differences in aroma molecules that cause the sensory difference in sunflower oils.The main experiments and conclusions are as follows.(1)The differences in aroma-active composition and sensory quality of commercially hot-pressed sunflower oils were compared.A total of 514 volatile compounds were identified in 10 commercially sunflower oils,including terpenes(11.34%-54.83%),alkanes(2.57%-48.47%),heterocyclics(2.42%-41.27%),aldehydes(5.98%-30.61%)and alcohols(1.82%-20.68%).A total of 49 aroma-active compounds were identified by GC-O-MS,including 5terpenes(woody and pine-like aromas),11 pyrazines(roasted and nutty aromas),2 pyrroles(almond and sweet aromas),a pyridine(fatty aroma),furans(2,caramel-like aroma),10aldehydes(raw green and fatty aromas),4 ketones(buttery and fruity aromas)and 4 alcohols(mushroom and earthy aromas).(2)The differences in key aroma-active compounds and sensory profiles were compared between the roasted(180 ℃,20 min)and cold-pressed(unroasted)sunflower oil.A total of 13 and 50 aroma-active compounds with dilution factors(FD)ranging from 1 to 256 were identified in the cold-pressed and roasted sunflower oils,respectively.The 26 most important aroma-active compounds(roasted: FD≥8,cold-pressed: FD≥4)were quantified by the external standard method.The concentrations of aroma-active compounds were divided by the odor threshold of the aroma compounds to obtain the odor activity values(OAVs).The key aromaactive compounds in cold-pressed sunflower oil included terpenes(pine-oil like and woody aromas,OAVs=9-1857)and aldehydes(green and sweet aromas,OAVs=1-1857).The key aroma-active compounds in roasted sunflower oil included pyrazines(roasted and nutty aromas,OAVs=2-116),furans(caramel-like aroma,OAVs=1-10)and aldehydes(sweet and fatty aroma,OAVs=2-198).The key aroma-active compounds were added to the refined sunflower oil at the measured concentrations.The two aroma recombinant models successfully mimicked the characteristic aroma profiles of cold-pressed and roasted sunflower oils,respectively.(3)The aroma-active compounds and sensory quality of sunflower oils extracted from oven-roasting sunflower seeds(180 ℃,0-45 min)were analyzed.With the increase of roasting time,the surface structures of sunflower seeds obtained by SEM began to break and the contents of amino acids,total sugars and linoleic acid were significantly reduced in sunflower seeds which are the important aroma precursors.A total of 76 aroma-active compounds were identified by GC-O-MS in the 6 oven-roasted sunflower oils.With the increase of roasting time,the relative concentrations of heterocyclic compounds in sunflower oil increased from572.13 μg/kg to 105,777.25 μg/kg while the perceived intensities of roasted sunflower seed,nutty,burnt,smoky and bitter aromas increased.The relative concentrations of terpenes decreased from 77,131.08 μg/kg to 28031.28 μg/kg in sunflower oils,while the perceived intensities of raw sunflower seed,woody and green aromas were decreased.The relative concentrations of lipid oxidation products increased from 10,993.28 μg/kg(0 min)to100,821.00 μg/kg(35 min),and then dropped to 86,906.13 μg/kg(45 min)in sunflower oils.In addition,with the increased in roasting time,the oxidation induction time of sunflower oils increased from 5.21 h to 7.44 h,the total tocopherol contents decreased from 513.70 mg/kg to449.80 mg/kg,the PAHs content increased from 7.78 ng/g to 146.59 ng/g and the heterocyclic amine content increased from 0 to 246.5 ng/g.(4)Microwave pretreatment of sunflower seeds(900 W,0-7 min)had significant influences on the aroma-active compounds and sensory quality of sunflower oils.The surface structures of sunflower seeds stayed relatively complete and closed during the 7 min microwave treatment.With the increase in microwave duration,the amino acids and total sugar contents reduced dramatically in sunflower seeds,and the rate and magnitude of these reductions were higher than the oven roasting pretreatment.A total of 75 aroma-active compounds were identified in the microwave-roasted sunflower oils by GC-O-MS.With the extension of microwave pretreatment duration,the relative concentrations of heterocyclic compounds increased from 572.10 μg/kg to 241099.30 μg/kg in sunflower oils,while the intensities of roasted sunflower seed,nutty,burnt,smoky and bitter aromas gradually increased.The relative concentrations of terpenes decreased from 77131.08 μg/kg to 37218.00 μ g/kg in sunflower oils,while the intensities of woody,green and raw sunflower seed aromas gradually decreased.The relative concentrations of lipid oxidation products increased from 10993.28μg/kg to 79370.40 μg/kg.In addition,with the extension of microwave pretreatment time,the oxidation induction time increased from 5.21 h to 10.83 h,tocopherol contents decreased from513.7 mg/kg to 476.9 mg/kg,the polycyclic aromatic hydrocarbons increased from 7.78 ng/g to 109.76 ng/g,and the heterocyclic amines increased from 0 to 820.53 ng/g in sunflower oils.This study suggests that the sunflower seed roasting techniques and intensity determine the aroma-active composition and sensory quality of sunflower oils.Commercial sunflower oils have fewer aroma compounds and weaker sensory intensity likely due to a lack of sufficient seed roasting.The formation rate of aroma-active compounds from Maillard reaction in sunflower oils was higher after microwave pretreatment compared with oven roasting.The contents of lipid oxidation products in the sunflower oil extracted from the seeds that were microwaved for 7 min were lower than that from 35-45 min oven roasting.Compared with oven roasting,microwave pretreatment had weaker damage to nutrients(tocopherols and fatty acids)in sunflower seeds due to its higher roasting efficiency which significantly shorten the roasting time.The formation of heterocyclic amines and polycyclic aromatic hydrocarbons may be effectively controlled through microwave with appropriate power and time.As an emerging pretreatment technique of sunflower seed,microwave has significant advantages in improving the aroma quality of sunflower oil,maintaining its nutritional value and controlling the harmful components.The application prospects of microwave are promising.