| Flax is one of the most important typical oil-bearing crops in Xinjiang, and flaxseed oil(FSO) has been favored by many consumers due to its unique flavor and nutritional content. The flavor is a crucial indicator for FSO quality, and the roasting process contributes the most in forming the final flavor character of FSO. The advanced aroma compounds analysis technique provided a useful tool for making an overall evaluation of FSO, and the active flavor compounds in FSO can be further revealed. The obtained results may provide a new pathway for quality control and product development of FSO. In present study, the flavor composition of various FSO derived from different regions in Xinjiang were analyzed, and then the samples went through discriminant analysis by using multi-analysis and electronic nose. Thereafter, the fingerprint spectrum of Xinjiang FSO aroma character can be structured and verified. The aim of present research is also to explore the typical aroma developing pattern of Xinjiang FSO during roasting process, identify the critical aroma precursors of FSO and determine the enzymatic aroma production techniques and related parameters.Researching aspects and obtained results included:(1) The optimal condition for FSO aroma compounds purification and verification: the SPME extraction was operated under 50 ℃ for 40 min by using the DVB/CAR/PDMS extraction head and magnetic stirrer, and desorbed for another 4 min. There were 46 volatile substances were found in FSO by HS-SPME-GC/MS analysis, including aldehydes, ketones, alcohols, acids, esters, alkanes, heterocyclics and etc. The aldehydes, heterocyclics and alcohols had the relatively high total content, they accounted for 30.14%, 12.73% and 9.25% respectively, and it showed a satisfied experimental result.(2) There are sufficient amount of aroma substances in Xinjiang FSO, they are represented both in variety and content. The aroma character of FSO is also outstanding, and the FSO quality can be varied based on different variety of flax and processing scheme. The FSO samples from different Xinjiang flax varieties were analyzed by optimized HS-SPME-GC/MS, 60 volatile substances were identified. When combined with GC-O analysis, it was found that the FSO flavor was influenced significantly by the content of aldehydes, alcohols, heterocyclics and some acids. The multiple sensory analysis indicated that the FSO flavor concentration between three different flax varieties was YY-3>YY-4>TY-6, ranging from the highest to the lowest, and it showed significant statistical differences between almond flavor, sweet flavor, vanilla flavor, oil flavor, roasting flavor and overall flavor(p≤0.05). The three groups of FSO which were gone through different processing scheme were analyzed, there were 61 volatile substances were identified, and the sensory variance analysis indicated that the FSO flavor concentration were vary according to different processing techniques, which was HPO>CPO>EO. It also showed significant statistical differences between almond flavor, grass flavor and overall flavor in three groups of samples(p≤0.05). The FSOs from different origins were further analyzed by Principal Component Analysis(PCA), the results demonstrated that there was a correlation between different origin FSO and volatile substances, and the FSOs from different regions in Xinjiang can be identified easily by electronic nose and PCA.(3) The volatile compounds of 9 FSO samples obtained from different places in Xinjiang were analyzed by GC-MS. When combined with GC-O analysis, twenty-two aroma compounds from total different FSO samples were screened out, therefore the fingerprint which can indicate Xinjiang FSO characteristic flavor composition was set up. The validity of Xinjiang FSO fingerprint was then fully demonstrated by similarity analysis of fingerprint spectrum and PCA. The positive identification rate was 100% when the collected samples were tested by using the FSO and other oil quantitive discrimination model which was established by PLS-DA method. It was demonstrated that by using combined method of FSO fingerprint and PLS-DA, the oils including FSO can all be tested efficiently and identified accurately.(4) It causes significant changes in FSO flavor quality when the flax seeds were treated with different roasting time. There were 69 aroma compounds were identified from 6 FSO samples which were undergone different roasting time at 200 ℃. The content of volatile compounds like aldehydes, ketones and heterocyclics were increased dramatically with extended roasting time; whereas other volatile compounds like alcohols and acids decreased. The PCA analysis showed that different treating time to flax seeds contribute to different patterns of aroma compounds formation, 30~40 min was the optimal roasting time for processing FSO to release the sufficient amount of active volatile aroma compounds.(5) The partial least squares(PLS) analysis between FSOs undergone different roasting time and sensory evaluation was conducted, the results showed that there was a significant correlation between FSO flavor and sensory evaluation. There were five types of aldehydes(pentanal, E, E-2,4-pentadienal, heptanal, nonanal, E, E-2,-4-nonadienal), two types of ketones(2- hexanone, E-3-octen-2-one), three types of alcohols(butanol, nonanol, 1-penten-3-ol) and seven types of heterocyclics(2- methylpyrazine, 2,5-dimethylpyrazine, 2,3-dimethylpyrazine, trimethyl-pyrazine, 2-ethyl- furan, 2-ethyl-6- methylpyrazine, 2-methy- naphthalene) contributed to the roasting flavor, almond flavor, cereal flavor, oil flavor and vanilla flavor of FSO. While the hexanal, butanal, nonanal, E-2-nonadienal, benzaldehyde, E, E-2,4-nonadienal, hexyl alcohol, amyl alcohol, toluene and etc. were responsible for the grass flavor and sweet flavor.(6) The correlation between FSO characteristic aroma substances and the responding level of electronic nose was analyzed by PLSR method, and the results showed that there was no significant correlation, probably due to the complexity of volatile compounds and the specificity of sensor. But the overall discriminative effect was evidently. T he established radar fingerprint of electrical nose for optimal FSO and statistical quality control(SQC) analysis model can be used for discriminating the quality of FSO samples rapidly.(7) There were correlation between the formation of FSO characterist ic aroma compoundsand the content of free amino acid and soluble saccharides in flaxseed squeeze. The aroma precursors of FSO were mainly related to Asp,His,Arg,Met,Leu,Lys,glucose and fructose. It was demonstrated that the FSO characteristic aroma compounds were formed mainly through Mallard reaction of above precursors, and active substances not only can provide the characteristic aroma of FSO, but also are a good kind of antioxidant.(8) If using the degree of hydrolysis and the content of soluble nitrogen as standards, the alkaline proteases and the flavor proteases were optimally screened out as the best proteases combination. The regression model of binary enzyme substep enzymolysis was established by the quadratic general rotary regression design. The changes of hydrolyzing degrees for flaxseed squeeze were studies, and the products from simulated Mallard system were analyzed by GC-MS, the results demonstrated that 14 different substances in MPF were the corresponding substances in Xinjiang FSO characteristic fingerprint, accounted for 37.84% of total varieties. It made the standardization possible for future aroma concentration of FSO. |
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