The Evolution Of Aroma Compounds During The Process And Characteristic Aroma Compound Of Litchi Wine

The aroma component of litchi wine is an important indicator to evaluate the quality of litchi wine.However,there are a lot of losses of aroma substances,especially the terpenoids including cis-rose oxide which is the characteristic aorma in the fermentation process.Currently,the improvement of brewing process is the main way to protect the characteristic aroma compounds,but the mechanisms for the loss of aromas have not been studied deeply.Although it was reported that the losses of aromas was caused by CO₂ dispersion,there was no direct evidence to prove it.Moreover,The factors that contribute to this process still need to be further studied.With the consideration of these,the Huaizhi variety litchi was used as the research object in this study:1)to study the evolution and loss of aroma profile of litchi juice and cis-rose oxide,which is the characteristic aroma substance of litchi juice,during fermentation,processing and aging.2)to visualize the volatilization characteristics of the characteristic aroma substances by using Gas Chromatography-Ion Mobility Spectrometry?GC-IMS?;and 3)to compare the retention differences of the characteristic aroma substance in different oak barrels and bottles;and finally,4)to study the loss mechanism of characteristic aroma component in the molecular levels of physics and biochemistry by the combination of the molecular dynamics simulation with the simulation of fermentation experiment.Results are as follows:Statistical analysis of"typical","main"and"characteristic"aroma components of the litchi and related products that have been studied in previous researches,10 typical compounds were selected,including cis-rose oxide,precursors of citronellol and geraniol,phenylethyl alcohol,linalool,limonene,1-octene-3-ol,octanoic acid,ethyl octanoate,isoamyl acetate in the litchi juice and litchi wine.Afterwards,the 50/30 DVB/CAR/PDMS extraction fiber with the applicability for the terpenoid volatile components of litchi juice and litchi wine was optimized.The evolution of aroma components in the fermentation process of litchi juice was studied,and results indicated that the losses of terpenoids and cis-rose oxide mainly occurred in the main fermentation stage?4-6 days?,while octanoic acid and ethyl octanoate were formed in large amounts at this stage.The volatilization characteristics of cis-rose oxide and its precursor citronellol were studied by the GC-IMS.The results indicated that cis-rose oxide had higher volatility than citronellol,which was attributed to their cyclic and chain structure.In addition,IMS analysis showed that cis-rose oxide was a mixture of monomers and dimers,while citronellol was present as a monomer.Therefore,the regulation of the aroma components of litchi wine needed to be carried out before and after the main fermentation stage.The loss mechanism of the cis-rose oxide with characteristic aroma was analyzed by the combination of molecular dynamics simulation and fermentation experiments in the molecular level.Results from molecular dynamics simulations showed that CO₂ had the mutual electrostatic attraction and van der Waals attraction with cis-rose oxide.The intermolecular force of CO₂ was greater than the binding force of CO₂ with cis-rose oxide,and coupled with the convection of Marangoni induced by ethanol,and thus the loss of cis-rose oxide carried by CO₂ was much easier volatilization;Fermentation simulation experiments showed the rate of CO₂ formation and the loss rate of cis-rose oxide were consistent with the Slogistic1 model,which could be used for the loss speculation of the cis-rose oxide.Results from fermentation simulation experiments and molecular dynamics simulations indicated that the large loss of cis-rose oxide was affected by free diffusion,the interaction force of CO₂,physical transformation and biotransformation.Among them,the products of physical transformation were limonene,L-menthol,acetophenone and cumene,and the main products of biotransformation were limonene,ocimene and?-terpineol.It was proposed to control fermentation conditions?such as slow fermentation of yeast species,low-temperature fermentation,etc.?to decrease the volatile strength of CO₂,or to screen yeast strains that are without or low expression or knock out ATF1 and ATF2 gene fragments that can regulate the aroma components of litchi wine.The variation of aromas forms during the processing of litchi wine were studied,and 192and 65 of free and bonded aroma components were detected,respectively.The results showed that losses of terpenes and cis-rose oxide occurred mainly in the period of main fermentation?2-3 days?.After fermentation,the glycosidically bound of the typical aroma components in litchi wine were 4-1740 times than that of the free state,and among them,the maximum of glycosidically bound?6025.85?g/L?of aldehyde and ketone aroma substances were 2000 times that of the free aldehyde and ketone substances.Compared with the previous literatures,benzyl alcohol,geraniol,citronellol,myrcene,phenylethyl alcohol and linalool were common substances,and isooctanol,geranyl acetate,ethyl decanoate,n-heptanol,cis-carveol,furfural,iso-geraniol,d-carvone,cis-rose oxide,geranyl acetone,lavandulol,myrtenol,trans-rose oxide,2,4-di-tert-butylphenol,?-terpineol and acetophenone were first detected as glycosidically bounds in litchi.The main glycosidically bound compounds of litchi juice were geraniol and benzyl alcohol,while the main glycosidically bound and free compounds were geraniol and citronellol during fermented period,respectively.The glycosidically bound components were more suitable to provid precursors for the subsequent regulation of aroma components in litchi wine from the PCA and heatmap analysis.In the fermentation process,alanine was the fastest consumed in the process of amino acid metabolism,followed by glutamic acid,and then aspartic acid,serine and isoleucine,of which alanine,valine,isoleucine,leucine may be involved in the biological metabolism of the characteristic aroma component of cis-rose oxide.Based on these results,the method of adding?-glucosidase or AR2000 enzyme after the main fermentation,and adjusting the pH environment after the end of the fermentation to slow-release glycosidically bound aroma components into free aroma compounds are proposed,which can achieve the purpose of improving the aromas of litchi wine.The effects of different aging methods on the aroma components of litchi wine were also studied.A large amount of terpenoids and esters aroma components were lost during aging in different oak barrels?except ethyl acetate?.The loss mainly included linalool,cis-rose oxide,citronellol,geraniol,isoamyl acetate,ethyl octanoate and geranyl formate,citric acid,?-guaiacene,trans-nerolidol,?-cadinol,ethyl caprate,ethyl laurate.Meanwhile,the formation of alcohols and acids,of which octanoic acid,phenylethyl alcohol and isoamyl alcohol increased significantly in this process.In addition,isovaleraldehyde with fruity aroma showed that higher odour activity value in American oak barrels than that in France oak barrels?134.82>88.72?.The typical aroma compounds were lost during in bottle aging gradually,which included phenylene alcohol,linalool and geraniol,and other compounds as well as.The content of cis-rose oxide in one year bottle aging was higher than that in 8 months aging in American and French oak barrels.The acetic acid tended to enrich in the bottle aging.Results indicated that American oak barrel was suitable for the retention of terpenoids,while French oak barrel was suitable for the protection of esters.It is proposed that American oak barrels or aged at room temperature for less than one year or low temperature preservation can improve the aroma level of litchi wine.