GC-MS Fingerprint Establishing And Formation Mechanism Of Volatiles In Grass Carp (Ctenopharyngodon Idella) Soup Prepared With Frying Combined With Boiling

Fish soup is one of the most favorite traditional Chinese foods charaterized by delicious,fragrant,and rich in nutrition.However,little researches were conducted on cooking technology and evaluation standard of traditional Chinese food,and the formation mechanism of flavor compounds.In this study,grass carp(Ctenopharyngodon idella)soup was prepared with traditional Chinese cooking method,of which the flavor volatiles were identified,and the formming progress and formation mechanism of those volatiles were also investigated.This research can provide theoretical guidance for the industrialized producing flavor controllable freshwater fish products with unified standard such as fish soup and fish-like flavouring,it also can lay a foundation for developing an integrated research system of food flavor chemistry.The main results obtained were summarized as follows:(1)After single factor experiments of frying condition(power and time of frying)and boiling condition(power,time water amount of boiling),the method of radar map and UV–Vis spectrophotometery were combinated to evaluate comprehensively the flavor of grass carp(Ctenopharyngodon idella)soup,which was then used to study the cooking technology of the soup.The results showed that the method of radar map could reflect intuitively the role of various fragrance in the flavor of the soup,while the method of UV–Vis spectrophotometery could facilitate to understand the total concentration of flavor compounds in the soup.With the combination of the two methods,the evaluation of the soup flavor was conducted efficiently.Based on the analysis results of radar map and UV–Vis to grass carp soup,the optimum producing process of the soup was: 23 g soybean oil was poured into frying pan and pre-heated at 1400 W for 2 min by an induction cooker.Then 250 g fish pieces were added and fried for 2 min with each side down for 1 min.After adding 1.0 L 80-100 ? water,8.7 g garlic and 4.5 g ginger into the pan,boiling was maintained for 25 min,followed by filtering with gauze.The soup diluted to 250 g with 80-100 ? water was then used for further analysis.(2)The extracting and concentrating methods of flavor volatiles were compared after the establishing of the GC-MS analysis method for flavor volatiles of fish soup,and the volatile compounds in grass carp soup were idenfified.The result indicated that:(?)The DB-WAX column was appropriate for analyzing the volatiles in grass carp soup.The optimum chromatographic conditions were: the GC oven temperature was initially held at 40 °C for 3 min,and increased at a rate of 5 °C/min to 240 °C where it was held for an additional 10 min,and other conditions were setup according to general method;(?)Sample extracted by SDE contained more volatiles both in content and number,thus,SDE was more efficient than SPME in extracting volatile compounds from grass carp soup,and the fibers of DVB/CAR/PDMS and PDMS/DVB were more suitable than Carboxen/PDMS as extraction fiber for SPME;(?)Fractionation was more suitable for concentrating volatiles than nitrogen-blow and rotary evaporateion due to less loss of volatiles;(?)A total of 113 volatile compounds were identified from grass carp soup by combination SDE-GC-MS with SPME-GC-MS,including 33 terpenes,17 aldehydes,12 alcohols,6 ketones,11 alkenes,16 aromatic compounds,4 acids,2 esters,6 sulphur compounds,2 pyrazines,2 furans,and 2 other compounds,and terpenes and aldehydes were two most abundant flavor substances in grass carp soup.(3)Twenty model samples were analyzed with GC-MS after methodology validation of GC-MS,and seventy-eight characteristic common peaks were defined.Then the GC-MS chromatogram fingerprint was established based on the seventeen common model screened by cluster analysis of twenty model samples mentioned above with the defined seventy-eight common chromatographic peak as case.The GC-MS fingerprint contained the composition,retention time,linear retention indice and content of common peaks,which contained 9 classes of volatiles(aldehyde,alcohol,ketone,aromatic compound,alkene,sulphur compounds,furan,ester,and other compounds),which accounted for 81.8% of totall of 11 classes.The content of detected 78 common peaks accounted for above 80% of the total volatiles in amount in grass carp soup,indicating that the developed fingerprint can well reflect the composition and content of volatiles in original sample.The effectiveness of fingerprint was verified through similarity analysis,which showed that the fingerprint was workable due to its 94% and 83% of successfulrate to positive and negative samples,respectively.(4)According to volatiles identified from fish fillets or fish soup during frying and boiling,it could be concluded that lipids oxidation was the main reason for flavor volatiles formation,which was initiated and promoted by frying,and Millard reaction also occurred due to the detection of pyrazine when fish was fried.Whereas,boiling was a progress for the continuous enrichment and balance of flavor substances.A large amount of terpenes and sulphur compounds,deriving from ginger and garlic,respectively,were formed during the first stage of cooking and decreased during the last stage of cooking,while aldehydes,ketones,and alcohols were the result of lipids oxidation and were increased during cooking especially at the third stage,and all kinds of flavor volatiles achieved equilibrium at the third stage.(5)The formation mechnism of main volatiles of soup formated during preparing was proposed based on the analysis flavor precursors during frying,the effect study of spices addion on soup flvor,and model oxidation experiment of single fatty acid methyl ester.The analysis results of flavor precursors durig frying showed that flavor volatiles were mainly derived from lipids during frying of fish,of which few were formed from Maillard reaction of free amino acids.The terpenes detected in fish soup were derived from dissolution of volatiles in fresh ginger during boiling,while 2-propen-1-ol and sulfur-containing compounds were from garlic.Except for a few volitles transformed from the non-volatiles of ginger and garlic,most other flavor volatiles were mainly derived from fried fillets during boiling.The addition of spices could not promote or inhibit the formation of volatiles during boiling of fried fillets,which could be concluded that the introducing of flavor compounds from spices,mainly,was a physical process rather than a chemical process.(6)The detection of oxidation products of 6 fatty acid(oleic acid? linoleic acid?ARA?EPA?DPA?DHA)methyl esters(FAME)initiated by lipoxygenase or heating showed that the type of lipids oxidation flavor volatiles in fish soup such as aldehyde,atone and alcohol could formed from oxidation of those six fatty acids.Combination the analysis result of GC-MS fingerprint of volatiles in grass carp soup,which was that: aldehyde was the highest concentration flavor compounds which were derived from chemical reaction,in which trans,trans-2,4-decadiene and 2,4-decadiene were the two most important flavor volatiles and mainly derived from arachidonoate,the mechanism of trans,trans-2,4-decadiene and 2,4-decadiene formed by oxidation of ARA was proposed as radical mechanism,i.e.the 13-carbon of ARA formed alkyl-radical initiated by nucleophilic group or other conditions,followed by acception of a pair of oxygen atoms to form 13-carbon alkyl peroxyl radicals,and then isomerized at 11-carbon to form alkyl peroxyl radicals,which attracked and captured a hydrogen atom from adjacent molecule to form hydroperoxide.Finally,trans,trans-2,4-decadiene and 2,4-decadiene with more stable structure than other trans-and cis-decadiene conformers were formed through dissociation at O-O bond and C-C bond and molecular rearrangement.The mechanism of other aldehydes formed by oxidation of various fatty acids with the same radical mechanism but with different routes.