Study On The Aroma Characteristics And Mechanisms Of Change And Formation Of Key Aroma Compunds Of Fragrant Rapeseed Oil

The fragrant rapeseed oil(FRO)is produced from rapeseed roasting and pressing,which is favored by consumers for its strong fragrance.Aroma is one of important quality attributes of FRO.At present,FRO research still mainly remain in the level of volatile substances and aroma profile.The types,sources and formation mechanisms of the key aroma compounds that constitute the characteristic aroma of FRO are still unclear.Therefore,this article systematically studied and compared the flavor characteristics and substance composition of commercial FRO products by means of modern flavor chemistry(e.g.,Sensomics approach),analyzed the key aroma substances and their formation and changes during the process of seed roasting,investigated the thermal degradation and aroma generation mechanisms of the key flavor precursor glucosinolates(GLS)in rapeseed,the main contents are as follows:First,for the problem of large difference in composition and flavor of commercial FRO products.The composition and flavor characteristics of 33 representative commercial FROs in China were systematically analyzed via liquid chromatography(HPLC),gas chromatography(GC),gas chromatography mass spectrometry(GC-MS),headspace solid phase microextraction(HS-SPME),flash gas chromatography electronic nose,and free sorting sensory analysis method.Results showed that the erucic acid contents of oil products were 0.07%-20.52%,with an average value of 6.13%.The erucic acid content of all oil products from Sichuan province exceeded 3.0%.A total of 51 major volatile flavor substances were identified,including 21 carbonyl compounds,17 heterocyclic compounds,2 S-containing compounds,8 nitriles,3 hydroxylated compounds.The GLS degradation products(including nitriles and isothiocyanates)accounted for the largest proportion(24.22%?81.63%),which was significantly positively correlated with the content of erucic acid in the oil(R2=0.796,p<0.01).Free sorting sensory analysis combined with the hierarchical cluster analysis(HCA)divided the oil products into three categories(fatty,original and roasted aroma types).The average erucic acid content of roasted aroma type oils was 9.31%.Seventy-seven percent of roasted aroma type oils contained more than 6%erucic acid.Seven out of the 8 oil products in Sichuan were classified into the group of roasted aroma type due to their strong roasted,nutty,fatty,cabbage-like and pungent flavor.Secondly,in view of the unidentified key aroma substances of FRO,based on the data of volatile substances,Sensomics approach including HS-SPME with gas chromatographyolfactometry-mass spectrometry(GC-O-MS),aroma extraction dilution analysis(AEDA),and aroma reconstruction method were used to analyze and identify the key aroma compound of representative FRO products.Results indicated that 24 volatile substances had aroma activity,including 9 aldehydes,1 ketone,3 acids,1 alcohol,2 sulfur compounds,1 nitrile,3 furans,and 4 pyrazines.The aroma compounds with the highest dilution factor(729)were 4isothiocyanato-1-butene(pungent flavor)and dimethyl trisulfide(cabbage-like and sulfury flavor).The key aroma compounds of FRO were hexanal,3-butenenitrile,1-octene-3-one,2,5dimethylpyrazine,dimethyl trisulfide,2-ethyl-6-mthylpyrazine,nonanal,trimethylpyrazine,(E)-2-octenal,3-ethyl-2,5-dimethylpyrazine,acetic acid,4-isothiocyanato-1-butene,(E,E)-2,4heptadienal,5-methyl-2-furanaldehyde,butyric acid,(E)-2-decenal,2-furanmethanol,(E,E)2,4-nonadienal,and(E,E)-2,4-decadienal.Both aroma profiles of oil sample and the corresponding reconstituted sample showed high similarity proving the successful identification and quantitation of the key odorants of FRO.Thirdly,for the problem that the seed roasting process has a large impact on the volatiles,and the change mechanism is still not clear.HPLC and HS-SPME/GC-MS were used to study the effects of the conditions of the roasting on the GLS and volatiles contents of rapeseed.The results showed that the degradation rate of GLS in rapeseed increased with the intensification of the roasting conditions.When the seeds were roasted at 150? for 60 min,the degradation rate of total GLS in rapeseed reached 85%,and it increased to 94%at 170? for 40 min.Almost all GLS in rapeseed were degraded after roasting at 170? for 60 min.Aldehydes,acids and alcohols were the main volatile substances in cold-pressed rapeseed oil,accounting for 24.56%,28.77%and 25.40%,respectively.Nitriles and heterocyclic compounds had the largest increase over time under roasting at 150?.After roasting for 60 min,contents of nitriles and heterocyclic compounds accounted for 58.13%and 19.20%of the total volatiles,respectively,becoming the main volatile compounds.Volatiles exhibited first an increasing trend and then decrease with increasing roasting temperature and time.Hexanal,3-butenenitrile,2,5dimethylpyrazine,dimethyl trisulfide,2-ethyl-6-methylpyrazine,nonanal,trimethylpyrazine,(E)-2-octenal,3-ethyl-2,5-dimethylpyrazine,acetic acid,4-isothiocyanato-l-butene,(E,E)-2,4heptadienal,5-methyl-2-furanaldehyde,(E)-2-decenal,2-furanmethanol,and(E,E)-2,4decadienal were detected in roasted rapeseed oil,most of which showed a similar change trend to that of volatiles.Sensory evaluation suggested that the roasting of seeds at 160? should not exceed 50 min,and that at 170? should not exceed 40 min.Roasting at higher temperatures(180,190,and 200?)should not exceed 30 min.At last,in view of the unclear mechanism of the thermal degradation of the key flavor precursor GLS in rapeseed,the main GLS(progoitrin,PRO)in rapeseed(Brassica napus)was studied by HPLC and gas chromatography time-of-flight mass spectrometer(GC-TOFMS).Thermal degradation behavior,pathway,and volatile products of PRO in liquid phase(phosphate buffers with different pH),sea sand,and rapeseed meal(after deactivation of the myrosinase activity and removal of the GLS).The degradation rate of PRO under high temperature(>150?)decreased in the following order:pH9.0>sea sand>rapeseed powder>pH7.0>pH5.0.2,4-Pentadienenitrile was the major nitrile,and thiophenes were the major sulfur-containing volatile compounds formed.Possible formation pathways of main sulfurcontaining and nitrogen-containing volatiles were proposed.In summary,A total of 19 key aroma substances were identified,representing 7 categories in FRO.The main pattern of change of flavor substances during roasting.Thermal degradation and aroma generation pathways of key flavor precursor GLS in rapeseed were proposed.This article could provide a basis for the production of flavor-oriented fragrant rapeseed oil.