| Pomegranate（Punica granatum L.）is a characteristic fruit of Shaanxi Province.Pomegranate juice processing has become the main strategy to extend pomegranate industry since the development of storage and preservation technology.However,the deterioration in juice flavor during traditional processing has resulted in a limited market value and thus needs to be solved efficiently.Obviously,it’s necessary to identify the key aroma compounds of pomegranate juice and demonstrate the flavor deterioration mechanism during pomegranate juice processing.In this study,four pomegranate varieties[Dahongtian（DP）,Linxuan NOl（LX）,Luyudan（LP）,Tianhongdan（TP）]from Lintong,Xi’an,were used as raw materials to prepare pomegranate juice samples.Firstly,HS-SPME-GC-MS was conducted to analyze volatile compounds in pomegranate juice,and then the characteristic aroma-active compounds were identified by GC-O,OAV,aroma recombination and omission test.OPLS-DA and PLSR were performed to explain the correlations between off-flavor compounds and different sterilization samples.Finally,omission test was conducted to identify the contribution of off-flavor compounds to allover aroma profile of pomegranate juice,and the formation mechanism of them was inferred and then verified.This study could provide further insight for pomegranate juice flavor controlled during sterilization processing.The results were as follows:（1）Volatile compounds in four pomegranate juice varieties were analyzed by HS-SPME-GC-MS,all these compounds were identified by three methods（MS,RI,St）and further quantified by external standard curve.Results showed that a total of 39 volatile compounds in different samples were identified,in which,20 compounds mainly including terpenes,alcohols,and aldehydes coexisted in four pomegranate juice samples.There were significant differences in content of volatile compounds among four pomegranate juices.Dahongtian（DP）had higher content of terpenes and aldehydes while Linxuan No1（LX）had higher level in alcohols.Six sensory properties were selected to characterize the aroma profile of different pomegranate juices by descriptive sensory analysis（DSA）.The resulted showed that it had slight differences in six sensory properties among four pomegranate juices,while Dahongtian（DP）had superior aroma properties.（2）Based on the analysis of HS-SPME-GC-MS,aroma compounds in pomegranate juice were perceived and evaluated aroma intensity by GC-O.It showed that 1-octene-3-ol exhibiting mushroom-like note highly fitted the overall aroma profile of pomegranate juice while hexanal,linalool,nonanal,limonene,Z-3-hexene-1-ol,β-pinene and naphthalene also presented strong aroma intensities.The differences in aroma compounds of four pomegranate juice samples were further evaluated based on the quantitative analysis and OAV calculation.11,10,8 and 7 aroma compounds,whose OAV were greater than 1,were identified in Dahongtian（DP）,Linxuan Nol（LX）,Luyudan（LP）and Tianhongdan（TP）,respcctively.Among which,1-octene-3-ol（mushroom-like）,linalool（floral）,limonene（sweet）,hexanol（fruity）,Z-3-hexene-l-of（grass）,hexanal（grass）,nonanal（grass）and heptanol（earthy）contributed greatly to the overall aroma properties of DP.Aroma recombination and omission test were conducted to confirm the contribution of aroma-active compounds in DP.The results showed that β-laureene,limonene,hexanol,Z-3-hexene-1-ol,1-octene-3-ol,linalool and hexanal contributed to "woody","sweet" and "grass" properties.（3）To study different sterilization methods and their effects on sensory properties and volatile compounds,Dahongtian（DP）juice samples were processed by different sterilization methods（BS,PJ,HPP,US）,and further analyzed by GC-MS/GC-O.The results indicated that there were significant differences in both aroma compound and s and sensory properties of Dahongtian（DP）juice samples.Especially,several sensory properties,such as "fruity","woody","grass","earthy/musty" and "flower",were weaken significantly in BS samples and PJ samples,however,the "sweet" was strengthen and "boiled potato" and "spicy" were perceived at the first time and had the strongest intensities in PJ sample,followed by BS sample.By contrast,HPP sample and US sample had similar sensory properties intensities with fresh Dahongtian（DP）juice.For volatile compounds analysis,it had significant changes in key aroma compounds among different sterilization samples.The contents of terpinenes,alcohols,and aldehydes in PJ and BS samples were significantly decreased,while dimethyl sulfide（boiled potato）,and 4-vinyl guaiacol（spicy）were generated with the heat-sterilization processing.（4）Multivariate analysis methods,including orthogonal partial least squares discriminant analysis model（OPLS-DA）and Partial least squares regression model（PLSR）,were performed to further analyze volatile compounds in different treatment samples.OPLS-DA model indicated that 8 compounds with VIP>1 were considered as the significant difference compounds.In which,dimethyl sulfide,furfural,α-terpineol and 4-vinylguaiacol increased significantly in heat sterilization samples and hexanal,α-terpinene,limonene,and hexanol changed slightly in non-heat sterilization samples,all these 8 aroma compounds could be used as important "targeting" compounds to distinguish between heat sterilization samples and non-heat sterilization samples.PLSR result explained the positive correlation among key aroma compounds（OAV≥1）,different sterilization samples and sensory properties.HPP and US samples showed a high correlation with 7 compounds（hexanol,limonene,linalool,β-pinene,β-Myrcene,y-terpinene,and hexanal）,which were related to 4 sensory properties（woody,flower,grass,and fruity）.Whereas,dimethyl sulfide,4-vinylguaiacol,furfural and α-terpineol had a high correlation with BS and PJ samples which presented strong "cooked-potato"and "spicy" properties.To demonstrate the formation pathway of off-flavor compounds is significant for further improving pomegranate juice flavor quality during heat-sterilized processing.（5）Off-flavor compounds with different concentration gradients were added into fresh pomegranate juice to evaluated the effect on overall flavor profile by sensory evaluation analysis（QDA）.Dimethyl sulfide（DMS）had a significant negative contribution to flavor profile of fresh pomegranate juice compared with 4-vinylguaiaco（2M4VP）,furfural and α-terpineol.When the added concentration of DMS was 170.μg/kg,the original flavor of fresh pomegranate juice changed greatly and"cooked-flavor" became the dominant flavor properties.According to the current study,methionine in pomegranate juice may be the precursor of DMS.To verify the formation mechanism of DMS,different concentration of methionine was added into the fresh pomegranate juice which then was simulated the heat-sterilization processing.The results showed a good linear relationship between the concentration of DMS and methionine.Methionine in fresh pomegranate juice may be catalyzed as S-methylmethionine（SMM）by thiopurine S-methyltransferase（TPMT）and then generated DMS by nucleophilic substitution reaction of dimethyl sulfo group in a protonated sour environment of juice during heat-sterilization processing,which leading "cooked potato" attribute strengthen in heat treatment pomegranate juice.This study lay the basis for the further analysis of off-flavor and innovation of technology in pomegranate juice flavor quality controlled. |
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