Effect Of Vacuum Freeze Drying And Rehydration On The Key Flavor Compounds In Truffles

Black Truffle(Tuber sinensis),White Truffle(Tuber sinoalbidum)and Mother Truffle(Tuber sinoexcavatum)are three typical fresh and mature truffles in Yunnan Province of China.At present,truffles are simple and originally dug and roughly processed by most of the farmers and producers in China,but they do not know much about the selection,harvesting,sorting,transportation and storage of truffle varieties,flavor research and development,etc.In recent years,the early and indiscriminate mining of farmers has destroyed the environment of truffle,resulting in a sudden drop in the production of truffle,low quality and low price in the international market.From the perspective of flavor,this paper comprehensively studied the non-volatile compounds(soluble sugar and sugar alcohol,organic acid,free amino acid,5'-nucleotide and molecular weight distribution)and volatile compounds(aldehydes,alcohols,ketones,acids,esters,nitrogen and sulfides),and screened the key flavor substances in the three kinds of truffles.The truffles were dried by vacuum freeze-drying technology and the change trend of key flavor substances in truffles after drying and rehydration were further studied.(1)The non-volatile compunds of Yunnan fresh truffles were analyzed by hot water and ultrasonic assisted-method extraction combined with High performance liquid chromatography.The water content of Yunnan fresh truffle is 83.39-85.93%,which is rich in protein and carbohydrate,accounting for 23.62-26.46% and 63.23-64.74% of dry weight respectively.The content of soluble sugar and sugar alcohol in Black Truffle(BT)is the highest,and that in White Truffle(WT)is the lowest;the content of organic acid in Mother Truffle(MT)is the highest,and that in BT is the lowest;the content of flavor free amino acids and nucleotides,except that the TAV value of glutamate in MT is 1.4,the TAV calculated values of the other three varieties are all less than 1,which has not made much contribution to the taste.Electronic tongue and sensory evaluation were used to taste truffles flavor,which shows differences are the largest in the sweet,salty and bitter.MT is saltliest,BT is the sweetest,WT is the most bitter,which is also basically consistent with the previous test results of taste substances.(2)The volatile compounds of three kinds of fresh truffles were analyzed by Headspace Solid Phase Microextraction(HS-SPME),Gas Chromatography-Olfactometry(GC-O),Gas Chromatography-Mass Spectrometry(GC-MS)andGas Chromatography-Flame Photometric Detector(GC-FPD).51 kinds of volatile compounds were detected in three kinds of fresh truffles,among which 12 kinds of nitrogen and sulfur aroma compounds were identified.21 kinds of key aroma compounds were selected by calculating the aroma activity value(OAV)and sensory evaluation.Seven sensory attributes(mushroom,nut malt,fatty green,flower sweet,gasoline musty,baked potato and rotten cabbage sweet corn)and key aroma compunds were established Correlation.Among them,BT shows fatty green and strong sweet of corn syrup aroma;WT is more mushroom aroma,sulfur and musty aroma;MT is malt nut,flower flavor and baked potato flavor.(3)The vacuum freeze-drying method was chosen to dry the truffles because less flavor loss and better low-temperature drying effect and then rehydrated after drying.The change trend of flavor compounds of the three truffles was studied.In terms of taste,the content of soluble sugar and sugar alcohol in BT samples changed relatively little after drying and rewatering,and WT lost the most,only 18.19% of that in fresh state;the content of organic acid in BT samples increased by 23.10% after rehydration and the content of free amino acid in WT and MT samples lost 54.32% and 61.72% respectively after rehydration;the content of free amino acid in bitter taste after drying was slightly higher than that in fresh taste;the TAV value of flavor nucleotides after drying and rehydration was still less than 1,no flavor contribution.In terms of aroma,vacuum freeze-drying had different effects on the aroma of three kinds of truffles,the types and contents of acids and esters increased significantly,while some alcohols and ketones lost a lot after drying.BT sample can recover to the aroma of fresh truffle after rehydration,while WT and MT truffle have great differences after rehydration.The loss of alcohol and ketone is serious,and the increase of acid is large.Combined with instrument analysis,sensory evaluation was carried out on three kinds of truffles after drying and rehydration.The taste and aroma profile of BT samples after rehydration was similar to that of fresh ones.WT had the largest loss of taste and aroma and was irreversible after rehydration.MT samples were between BT and WT.In general,the volatile compounds in WT and MT truffles are sensitive to the drying process,and the loss is serious.However,BT truffles can maintain the stability of the main aroma compounds and basically restore the flavor fingerprints in the fresh state after drying.