| Aroma is an important sensory character of raisins.The evolutionary pattern of volatiles in raisins is determined by both grape variety and drying method.The volatiles of furans(furfural,5-methyl-furfural,2-acetyl furan and 2-pentyl furan)and pyrazines(2-ethyl-6-methyl pyrazine,2,6-diethyl pyrazine and 5-ethyl-2,3-dimethyl pyrazin)generated during grape drying give raisins specific aromatic characteristics are called characteristic "raisin" note.The earlier studies focus on the generation of these volatiles under the reaction with the temperature over 100?,while few study reports the their source in the real drying conditions.This work identified volatiles during grape drying and raisin storage in 2012 and 2014 via HS-SPME GC-MS.The evolutionary pattern of volatiles during grape drying and raisin storage is researched by applying chemical modelling and metabolomics.The influence of drying condition on such volatile compounds is clarified.This research aims to provide academic support and practical techniques for manipulation of raisin flavor.The results are as following:A total of 107 volatile compounds were identified in different raisins by using gas chromatography-mass spectrometry(GC-MS).The majority of raisin volatiles are derived from three sources:fresh grapes,the oxidative degradation of unsaturated fatty acids,and the Maillard reaction process.Raisin made by different dring methods share similar aroma characters.The characteristic aroma of air-dried raisins is mainly derived from fresh grapes,and the characteristic aroma of sun-dried raisins is not only derived from fresh grapes,but also derived from the Maillard reaction and the oxidative degradation of unsaturated fatty acids.High temperature and strong sunlight in air-drying method provided favorable conditions for the Maillard reaction and the oxidative degradation of unsaturated fatty acids.Floral note was the main aroma character of Thompson Seedless raisins,followed by fruity,fatty and vegetable aromas.Floral and fruity notes were the most outstanding aroma characters of Centennial Seedless raisins and other aromas were balanced.Through the analysis of the change of aroma substances during raisin making in 2012 and 2014,we provided the theory basis for the source of the characteristic aroma substance studied by Maillard simulation reaction.Different drying method has a great influence on the generation of new aroma substances during raisins storage process.Pretreatment increased the aroma originating from UFAO but inhibited the aroma generated from MR.The use of dry-promoter decreased the influence of drying method and package on aroma variation during storage.The effects of processing factors on the raisins storage aroma intensity is:drying method was greater than dry-promoter,packaging materials was weakest,and different drying method and packing material had no effect on the kinds of determined aromatic compounds,but had effect on the intensity of them in different stages.During grape drying and raisin storage,the content of esters,terpenes and alcohols that originates from grape berry gradually decreased,while the content of pyrazines and furans from MR and acids from UFAO increased and they contribute to the characteristic raisin notes which are different from grape berry.Simulation of Maillard reaction is applied to study the origin of characteristic volatiles generated during grape drying which is not the perfect condition for Maillard reaction and the relevant pathways are clarified.2-ethyl-6-methyl-pyrazine is formed from the condensation reaction of aspartic acid,threonine,proline and glucose.2,6-diethyl-pyrazine is generated by the condensation reaction of threonine,proline and glucose.Isoleucine,glutamic acid and histidine are the potential precursors.5-ethyl-2,3-dimethyl pyrazin is formed by the reaction of proline and glucose and isoleucine,glutamic acid,threonine and histidine are the potential precursors. |
PDF Full Text Request