| Jujube belongs to the Rhamnaceae family and is recognized as the most important zizyphus species for fruit production.According to the statistical data of Xinjiang Statistical Yearbook,the annual yield of jujube fruit in Xinjiang increased year by year,and in 2016,Xinjiang produced 39.61%jujube fruit in China,which was 3.264 million tons.Drying is the main processing wasy of jujube fruit.However,bitterness is easy to be perceived in dried jujube fruit,and it can reduce the commondity value of jujube fruit.Therefore,the present study aimed to identify the species of bitter compounds and to reveal the mechanism of their formation.Firstly,the bitter compounds of jujube fruit were identified using the techonogies of sensory-guided fractions techniques and LC-MS/MS.Secondly,the key periods and bitter-compounds formation and the underlying mechanisms were studied.The main contents and results were summarized as follow:1.Seperation and identification of bitter compounds from jujube fruitUnder the guidance of sensory evalution,jujube fruit was in turn extracted with ethanol and n-butanol,and purificated using macroporous resin,and then these bitter fractions were analyzed using the technique of LC-MS/MS.Fourty one copounds,including 7 nitrogen conpounds,4 phenolic acids,15 flavonoids,12 saponins,2 fat acid oxides and one heterocycle compound(HMF),were intentatively identified from these bitter fractions.HMF,protocatechuic acid,chlorogenic acid,catechin,epicatechin,p-coumaric acid,rutin,p-hydroxybenzoic acid and quercetin were known bitter compounds in foods.Additionally,quercetin glycosides and saponins were also commonly reported as bitter compounds in foods.This suggests that endogenous phytochemicals were the main source of bitter taste in jujube fruit.Jujube fruit is rich in reducing sugars and amino acids,so Maillard reactions are likely to happen during drying and storage.Maillard reactions products are commonly recogniesed as one source of bitte taste in food,and HMF and 6 nitrogen compounds were tentatively identified from the biter fraction of F-Ⅱ-1-MR2.These suggest that Maillard reactions may be the sources of bitterness in jujube fruit.2.Effect of harvest,drying and storage on bitteness and physico-chemical qualities of jujube fruitHarvest had significant influences on bitterness,moisture,sucrose,Vc,catechin,p-hydroxybenzoic acid and rutin(P<0.05).The contents of moisture,Vc,catechin,p-hydroxybenzoic acid and rutin were significantly high in fresh jujube fruit,which was harvested at the beginning of October,but its bitterness score and contents of sucrose and p-hydroxybenzoic acid were low.After drying,the contents of moisture,Vc and free amino acids of jujube fruit decreased significantly(P<0.05),while its bitterness score increased significantly(P<0.05).The contents of moisture,glucose,VC,free amino acids,chlorogenic acid and rutin in dried jujube fruit decreased significantly after storage.Meanwhile the bitterness scores and contents of titratable acid,total phenols,total flavonoids,Pro,Tyr,epicatechin,caffeic acid and protocatechin increased during storage,and with the extension of storage time,the contents were increased.From bitterness scores,drying and storage are key periods of bitterness formation in jujube fruit.The bitter taste thresholds of polyphenols are generally in the range of 20-250mg/kg.The total phenol content of jujube fruit was 10.52g GAE/kg DW,which was much higher than the bitter threshold of polyphenols.It suggests that polyphenols are the main bitter comounds of jujube fruit.The composition of jujube fruit is very complex,because it is not only rich in bitter polyphenols,but also rich in sweet sugars.Due to the mask effect of high sugars and the dilution effect of high moisture,bitterness of fresh jujube fruit was extremely low.However,the concentation of bitter compounds could be increased due to moisture evaporation during drying and storage,consequently,bitterness of jujube fruit increased.Additionally,contents of free amino acids decreased during drying and storage,which suggested that the degree of Maillard reactions was aggravated.Maillard reactions products are often bitter,so the bitterness of jujube fruit could be aggravated during drying and storage.3.Development of HMF determination method and drying kinetic analysis of jujube fruitDichloromethane was the best solvent for sample pretreatment,because HMF wasn’t only capable of fully extracting from jujube sample,but also most of polar chemicals were capable of screening out to reduce disturbance.A suitable method was developed for HMF determlination of jujube fruit.It was validated according to the parameters,including specificity,linearity,precision,accuracy,LOD and LOQ.The kinetic results showed that HMF formation is highly dependent on the drying condition:drying temperature and drying duration.The HMF values were increased linearly with drying time,and higher values were found at higher drying temperature.In this experiment,HMF reached its maximum value of 2.172 mg/kg DW in the sample dried at 60 ℃ for 48 h.From the present results,HMF content of jujubes is too low to result in health risk,however,HMF is still reliable index of the intensity of heat treatments or Maillard reactions.4.Study on the distribution and change mechanism of polyphenols of jujube fruitAdopting the technique of LC-MS/MS,forty seven compounds,including 5 nitrogen conpounds,8 phenolic acids,20 flavonoids and 14 saponins,were tentatively identified from the free polyphenol extract of jujube fruit.Meanwhile,twenty one compounds,including 6 phenolic acids and 15 flavonoids,were tentatively identified from the bound polyphenol extract of jujube fruit.The distribution of polyphenols was unbalanced in jujube fruit.The content of free polyphenols of jujube peel was much higher than those of jujube flesh and kernell.Protocatechuic acid,chlorogenic acid,epicatechin,p-hydroxybenzoic acid and rutin were the main free polyphenols of jujube fruit,and protocatechuic acid,catechin and p-coumaric acid were the main bound polyphenols of jujube fruit.The content of free chlorogenic acid,p-hydroxybenzoic acid and rutin decreased 10-20%after storage,while the contents of protocatechuic acid,epicatechin,ellagic acid and caffeic acid increased after storage,especially the content of epicatechin increased about 40 times after storage for 12 months.The contents of free total phenol and total flavonoids also increased nearly 50%after storage.The content of bound protocatechuic acid,catechin,p-coumaric acid,rutin,total phenol and total flavonoids decreased in varying degree after storage.It suggested that bound polyphenols degraded and transformed into free polyphenols,consequently resulted in increase of free polyphenols of jujube fruit after storage.5.Correlation analysis of shrinkage,bitterness and physico-chemical qualities of jujube fruitAt present,the transverse diameter of jujube fruit is generally used as standard for fruit grading,but there is a shortage of this method that it couldn’t separate shriveled fuits from full fuits.Based on this phenomenon,the effect of shrinkage on bitterness andphysico-chemical qualities of jujube furit was studied.The experiment results showed that the bitterness of full jujube was extremely low,while that of shriveled jujube fruit was high.The content of titratable acid,total penol and total flavonoids of shriveled jujube fruit were 22.85 mg/g DW,13.75mg GAE/g DW and 1.39mg RE/g DW,respectively,which were much higher than those of full jujube fruit.Meanwhile the content of total sugars of shriveled jujube fruit was 451.37mg/g DW,which was much lower than that of full jujube fruit(725.99 mg/g DW).It can be seen that the content of bitter polyphenol in shriveled fruit was high,but the sugar content is low,so the bitter taste of shriveled fuit was heavy.It suggests that shrinkage is also responsible to the bitterness of jujube fruit. |
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