Study On The Aroma Production Mechanism Of Fragrant Rapeseed Oil During Roasting

China is the traditional vegetable oil production and consumption country.Rapeseed oil(RO)was one of the main edible vegetable oil,accounting for about 21.6% of edible oil consumption in China.There are many verieties of rapeseed oil in market to meet the consumer demand.Fragrant rapeseed oil(FRO)is obtained from rapeseed as raw material by roasting,physical pressing and moderate refining,which retain the nutrition and flavor to the maximum extent.FRO is popular with consumers due to its health,nutrition and flavor.However,the characteristic flavor of FRO is vary greatly and the flavor stability is poor during processing.It is not clear how the processing technology affect the formation of FRO flavor,which has become a major bottleneck for the development of FRO in oil processing market.Therefore it is of great significance to explore the formation mechanism of FRO flavor.In this study,the rapeseed from five main rapeseed producing regions(Hanzhong,Qinghai,Sichuan,Dali and Gansu)were selected as raw material and roasted in different time to produce the oil samples.The flavor of oil samples were analyzed by sensory evaluation,e-nose technology(E-nose)and headspace solid-phase microextraction/ gas phase-mass spectrometry technology(HS-SPME/GC-MS).The main precursor substances content in rapeseed were determined and combined with the results of volatile compounds to explore the formation mechanism of FRO flavor.The main results are as follows:(1)The flavor changes of rapeseed oil during roasting were determined by sensory evaluation,HS-SPME/GC-MS technology and E-nose technology.Results showed that the sensory evaluation of RO with different roasting time changed significantly.Rapeseed oil obtained by cold pressing without roasting processes were similarly characterized by seed-like/green and weak musty sensory attributes.With the prolong of roasting time,the grass flavor of rapeseed oil decreased and the oxidized oil,nutty,roast,and pungent sensory attributes gradually increased until bitter coffee-like flavor appeared.The flavor of rapeseed oil from different regions were different.A total of 162 main volatile compounds were detected by HS-SPME/GC-MS.Aldehydes,alcohols and alkanes accounted for 64.40%-82.49% of the total volatile compounds in cold pressed RO.With a prolonged roasting time,a large number of new volatile compounds,including nitriles,pyrazines and other nitrogen-containing compounds,as well as sulfur-containing compounds,emerged and increased,which were the main volatile compounds of FRO and gave FRO distinctive oxidized oil flavor,pungent flavor,baking flavor and bitter almonds flavor.Results of principal component analysis(PCA)of E-nose and HS-SPME/GC-MS showed that PCA could differentiate RO with different roasting time.(2)The glucosinolates(GSL)content in rapeseed during roasting were determined by high performance liquid chromatography(HPLC)to explore the effects of GSL degradation on FRO flavor.The results showed that during roasting,the GSL contents of rapeseed were gradually decreased,with the largest decrease(30.8%-84.02%)in 50-60 min.The negative correlation analysis of volatile compounds with GSL content indicated thermal degradation of GSL could produce nitriles and isothiocyanates.As the key volatile compounds of RO,nitriles and isothiocyanates gave oil pungent flavor.According to the structures of different GSL,nitriles and isothiocyanates,the thermal degradation pathway of GSL was preliminarily predicted.The thermal degradation of GSL has an important effect on the FRO flavor.(3)Based on the quantitative analysis of the precursor compoundes,reactant intermediates and products of maillard reaction and the establishment of the solid-phase maillard model,the effects of maillard reaction on the FRO flavor during roasting were studied.The results showed that during roasting process,the content of amino acid(lysine,aspartic acid,glutamate,proline,leucine)and reducing sugar(glucose,maltose,fructose)in rapeseed were gradually reduced,the browning degree of rapeseed was deepened,5-hydroxymethylfurfural(5-HMF)and melanoidins contents were increased.The key amino acids in the solid-phase maillard model are lysine,aspartic acid and glutamate.Maillard reaction model could produce a variety of volatile compounds with the same as FRO,mainly including 5 pyrazines,5 esters,4 furans,21 aldehydes,8 acids,11 alkanes,2 olefins,14 ketones and 7 alcohols.Therefore,lysine,aspartic acid,glutamic acid and glucose were the main precursors of marillard reaction during rapeseed roasting.(4)The effect of lipid oxidation on FRO flavor was investigated by analyzing the fatty acid composition change,peroxidation value,acid value of rapeseed and the lipid oxidation characteristic aroma substances in RO during roasting.Results showed that with the prolong of roasting time,the content of oleic acid,linoleic acid and linolenic acid were decreased,saturated fatty acid content,the acid value and peroxide value of RO were fluctuated.It indicated that fatty acid oxidation reaction were occurred during rapeseed roasting.The main volatile compoundes were octanal,4-nonenal,2,4-nonadienal,2-heptenal,2,4-decadienal,1,5-hexadien-3-ol,1-pentanol,3-methyl-1-butanol,acetic acid-methyl ester,2-ethyl furan and so on,which were the degradation products of oleic acid,linoleic acid and linolenic acid and gave FRO special flavor.